file: idl-simple-manual.txt = introduction to IDL basics
last: Jun 16 2021  Rob Rutten  Deil

SIMPLE IDL INSTRUCTION FOR ASTRONOMY STUDENTS

Robert J. Rutten

Lingezicht Astrophysics Deil
Institutt for Teoretisk Astrofysikk Oslo

This compact IDL tutorial is a beginner's introduction to IDL, showing
how to do simple calculations, make plots, write IDL programs.

It consists of a didactic sequence of IDL commands that you should try
out on the IDL command line.  It starts after an extensive
introduction with general information and weblinks.

There are parallel txt, pdf, and html versions of this manual at

This manual was written in the early 1990s for second-year astronomy
students at Utrecht University doing the "Stellar Spectra" exercises at
https://robrutten.nl/Exercises.html  I irregularly add more IDL fads and fallacies that I stumble upon.

===============================
INTRODUCTION TO THIS INSTRUCION
===============================

Why use IDL?
------------
IDL is an interactive programming language with the following advantages:
- programming language, not a package: make up your own stuff, experiment
- interactive "interpreter": test statements and tricks on the command line
- array notation: c = a + b handles multi-dimensional arrays (images, movies)
- journaling: keep a log of all trials, then pick out what worked best
- save/restore: store a complete session to share with others

Although IDL is commercial and licenses are excessively expensive, it
was the mainstay in astronomical image processing - but public Python
is taking over.  IDL was indispensable in solar physics through the
extensive SolarSoft library at
http://www.lmsal.com/solarsoft   butSunPy is on its way to replace this.  (I haven't tried Fawlty Language
= a public IDL replacement.)

My habits
---------
I run ancient IDL 6.4 (2007) under Ubuntu linux in the emacs IDLWAVE shell.
I often use SolarSoft routines from
https://www.lmsal.com/solarsoft  I sometimes use Coyote Graphics "cg" routines from
http://www.idlcoyote.com/documents/programs.php  I habitually swear at IDL because:
- it has far to many counter-intuitive idiosyncracies
- its figure layout differs hardware-dependently between screen and ps
- its figure annotation remains a hassle even with textoidl
- it has confusing plot parameter choices between graph area and plot area
- it counts my fingers 0 to 9
- its array notation [column,row] describes images, not matrices
- its CNTRL d is not next-character-delete as in Emacs but kills the session
- its CNTRL c does not stop program execution but may kill the session
- it does not have command-line tab completion (except in IDLWAVE)
- it does not have a comprehensive !! system parameter reset
- it started prefering square brackets for array indices far too late
- its error messages are primitive and often bewildering
- my "life-long license" is nearly impossible to re-activate

Other IDL manuals
-----------------
The online help (type ? in an IDL session) is reasonably complete but
most examples are too simplistic.  The IDL 6.4 help GUI is primitively
browser-like.

IDLWAVE accesses the IDL help files by keystrokes on procedure names.
Extensive manual (but assuming nontrivial knowledge of emacs) at
http://www.gnu.org/software/emacs/manual/html_mono/idlwave.html  A searcher that can also search IDL Google groups resides at
http://www.physics.emory.edu/~weeks/lab/searchidl.html
Explanatory comment blocks:
Many user-supplied routines (functions, procedures, full programs), as
those in the SolarSoft, Astronomy, and Coyote IDL libraries, start
with explanatory comment blocks between ;+ and ;- lines.
You can read these by typing
doc_library, 'routinename'
at the IDL prompt, but it may be more convenient to produce a html help
tree that you can inspect with your prefered html browser with, for example:
mk_html_help,'~/idl/coyote','~/idl/help/coyote.html'
(the Coyote library contains this as file: program.documentation.html).
IDLWAVE opens such ;+...;- comment blocks with keystrokes.
I prefer to use my misclib sp.pro ("show program") to open them in a
separate editor window.

Numerous url's for astronomical IDL are collected at
http://idlastro.gsfc.nasa.gov/other_url.html  Books:
David Fanning: "Traditional IDL graphics" (2011)
David Fanning: "IDL Programming Techniques, 2nd Edition" (2000)
Lilian Gumley: "Practical Idl Programming"
Ken Bowman:    "An Introduction to Programming with IDL"

IDL routine libraries
---------------------
David Fanning's coyote library, including 2011 cg routines used below:
http://www.idlcoyote.com/documents/programs.php  textoidl.pro: get the version under pro/plotting in the Sloan library at
http://idlastro.gsfc.nasa.gov/homepage.html  SolarSoft = "ssw" = solar physics IDL library:
https://www.lmsal.com/solarsoft  IDL startup
-----------
IDLWAVE for Emacs
Recommended modus of IDL operation, offering many keystroke shortcuts
and debugging options:
http://www.gnu.org/software/emacs/manual/html_mono/idlwave.html    The IDLWAVE settings in my own .emacs file are shown at
https://robrutten.nl/Recipes_linux_unix.html    My setup defines hyperkey+mouse-middle-click to call my misclib sv.pro
("show variable') to diagnose the variable content as print or plot or
movie.

Solarsoft startup
In my Ubuntu linux I use a shell script "idl" to always run ssw:
#!/bin/csh
setenv SSW /usr/local/ssw                      # if ssw stuff sits here
setenv SSW_INSTR "sot aia hmi trace ontology"  # select instruments
source $SSW/gen/setup/setup.ssw sswidl IDL startup code to resolve library clashes SolarSoft took Coyote routines long ago and changed them without name change. The worst clasher is "linkedlist__define.pro". The remedy is to make IDL search the coyote library before the ssw libraries. SolarSoft puts its ssw libraries before any others, so this cannot be done in a .login file or a shell resource (.bashrc, .cshrc) file, but needs the following use of Coyote's "addtopath.pro" in your "idlstartup.pro": cd, '/home/usr/idl/coyote',current=thisdir ; adapt to your coyote path addtopath cd,thisdir cd,current=workdir ; repeat for your actual working dir addtopath,workdir ; routines in your workdir now override any others NB: in "idlstartup.pro" I also have, following page 47 in Fanning 2011: device,retain=2,decomposed=0 ; indexed colors (255 only) window,xsize=10,ysize=10,/pixmap,/free ; initializing window wdelete,!d.window ; to avoid empty white window Format of this instruction -------------------------- IDL executes on the command line when you hit return ("interpreter"). This makes it easy to try new statements and statement sequences. The up cursor arrow brings back earlier commands. The main body of this instruction consists of a didactic sequence of command-line entries. Simply enter the IDL statements consecutively on the IDL> command line (type or copy-paste). Predict their action before you enter them! Many are goodies but some will surprise you negatively. The end of the instruction describes program structure, parameter passing, session saving, etc. Enjoy! =============================== START OF THE ACTUAL INSTRUCTION =============================== IDL MATH BASICS =============== help ---- ? [search term] ; IDL's help: inspect some IDL routines and concepts number games ------------ print,3*5 ; semicolon = comment, IDL skips the rest of the line a=3*5 ; no variable declaration needed a = 3 * 5 ; add spaces as you like help,a ; show nature and value of this variable help,A ; IDL is case-insensitive, shows variables in caps whatever_name_you_like$like_this_perhaps = a       ; _ and $are permitted print,whatever_name_you_like$like_this_perhaps     ; no spaces, +, -, *
d=32767               ; "short" integers run from -32768 to + 32767
print,d+1             ; did you predict this value?
print,d+1.            ; IDLWAVE: SHIFT mouse2 = print variable under cursor
print,2^15            ;
print,2.^15           ; why is the integer word length not 16 bits?
? integer             ; check the other number formats
print,3276700l        ; long integer, sign+31 bits
print,3276700ul       ; unsigned long integer, 32 bits
print,3276700ull      ; unsigned long long integer, 64 bits
print,3/5
print,3/5.            ; operation with one float makes the result a float
print,2^15.
a=[1,2,3,4,5,6]       ; IDL variables can be 1-8 dimension arrays
a=[0,a,7]             ; lengthen this 1D "vector" by adding value(s)
print,a,1E6*a         ; single precision: 6 significant digits, < 10^38
print,a,1D6*a         ; double precision: 16 significant digits
print,a,1/a           ; divide by 0 gives error message without stop
print,a,1./a
print,a,a^2
print,a,alog10(10^a)                  ; NaN = Not a Number
print,a,alog10(10^float(a))
a=1.*a                                ; convert into float
print,a,alog10(10^a)
print,a,alog(exp(a))
print,a,acos(cos(!pi/a))*180./!pi     ; !something is a system variable
print,!dpi                            ; double precision value of pi
print,!dtor                           ; so what is this?
print,a,a mod 2
print,fix(!pi)        ; fix = entier to short integer
print,long(!pi*1E8)   ; long = entier to long integer
b=sqrt(a)             ; type of b is defined through its assignment
a=3
if (a=1) then print, 'yes, a=',a else print,'no, a=',a      ; IDL quirk
a=3                                                         ; try again
if a eq 1 then print, 'yes, a=',a else print,'no, a=',a     ; better
if (a eq 1) then print, 'yes, a=',a else print,'no, a=',a   ; nicer
if ~(a eq 1) then print, 'yes, a=',a else print,'no, a=',a  ; ? ~ operator
help                  ; help without variable shows all variables

string manipulation
-------------------
print,'b=',b          ; 'something' is a string
pathfile='rootdir/homedir/ownerdir/workdir/todaydir/thisfile.txt'
print,strmid(pathfile,strpos(pathfile,'/',/reverse_search)+1)  ; IDL...
print,file_test('path/file')           ; check file exists
fileonly=file_basename(file)
print,str_match(file,'substring')      ; does filename contain substring?
newstring=str_replace(string,'-','.')  ; replace all - by .
print,'b = ',string(b,format='(f5.2)')                 ; ancient Fortran
print,'b = ',strmid(string(b,format='(f5.2)'),1)       ; IDL...
print,'b = ',strmid(string(b+1e3,format='(f7.2)'),1,6) ; with zero padding
print,'b = ',ntostr(b)                 ; that's easy!  Google ntostr.pro
print,'b = ',ntostr(b,format='(f5.2)') ; better spaces removal
print,'b = ',trim(b)                   ; SSW alternative
print,'b =',trimd(b,3)                 ; my own number printer, 3 decimals
c=!pi^50                               ; make a large number
print,c,c,c,c,c,c,c,c,c                ; wide printout
print,ntostr([c,c,c,c,c,c,c,c,c],format='(20E10.3)')  ; compact printout
print,ntostr([c,c,c,c,c,c,c,c,c],format='(G15.5)') ; chooses float or exp

one-dimensional arrays
----------------------
a=bytarr(100)         ; define a as byte array a,..,a=0
a=intarr(100)         ; define a as integer array a,..,a=0
a=fltarr(100)         ; define a as floating number array a,..,a=0.0
a=dblarr(100)         ; double-precision float array = 0.0000000
a=a+1                 ; now they are all 1.0000000
for i=0,19 do a[i]=i  ; remember that IDL starts counting at 0
a=indgen(20)          ; same thing: a=[0,1,....,19] without a[] declaration
print,a,a      ; always mind the virtual startoff finger
print,a[10:19]
print,a[*]            ; same as print,a and as print,a[0:19]
print,moment(a)       ; mean, variance, skewness, kurtosis (set /double?)
b=sqrt(a)             ; check that b is a float array - why?
print,a+b
c=b                   ; define float array the same size as a and b
for i=0,19 do if (b[i] gt 3) then c[i] = a[i] + b[i] else c[i] = a[i]
print,c
print,a+b*(b gt 3)       ; the same, processes faster, needs no declaration
print,a+b>3              ; beware: gives 3 or a+b where (a+b)>3
print,a+(b>3)            ;         gives a+3 where b<=3, a+b where b>3
print,a+(b gt 3)         ;         gives a, adding 1 where b>3
print,a+b gt 3           ;         gives 0 for (a+b)<3, 1 for (a+b)>3
print,a+b[where(b gt 3)] ;         gives b[10:19] added to a[0:9]
print,max(1,2,3)         ; did you predict the answer?
print,max([1,2,3])

two-dimensional arrays
----------------------
ar = [[1,2,3],[4,5,6]]   ; integer [3,2] array
print,ar                 ; 1st index = column number, "runs fastest"
; 2nd index = row number
print,ar,ar[0,0]      ; mind the virtual finger
print,ar[0,*]            ; * = all values of this index
print,n_elements(ar)     ; predict all these
print,total(ar)          ; for large arrays set /double
print,shift(ar,-1)
print,transpose(ar)
print,reverse(ar)
print,invert(ar)         ; needs square array
ar=ar+1                  ; add 1 to each array element
ar=temporary(ar)+1       ; idem but in place requiring less memory
vec1=[1,2]
vec2=[3,4]
ar=[[vec1],[vec2]]             ; simple 2x2
print,ar
print,ar*vec1                  ; f*g  = f[i,j]*g[i,j]
print,ar#vec1                  ; f#g  = columns x rows (IDL habit)
print,ar##vec1                 ; f##g = rows x columns = transpose(f#g)
print,ar#reverse(ar)           ; predict or check manually
print,ar##reverse(ar)          ; predict or check manualy
print,invert(ar)#ar            ; unit diagonal, OK
ar=[[1,2,3],[4,5,6],[7,8,9]]       ; now 3x3 without virtual finger
ar=indgen(3,3)+1                   ; the same
print,invert(ar)#ar                ; should be unit diagonal but isn't
arinv=invert(ar,status,/double)    ; try again
print,arinv#ar                     ; as bad in double precision
print,status                       ; status=1: singular, so invalid

three-dimensional arrays
------------------------
ar=indgen(3,4,5)+1          ; let's say 3x4 px frames in a 5-frame movie
print,ar                    ; successive indices run slower
ar3=ar(*,*,2)               ; third movie frame
print,total(ar)             ; sum all elements
print,total(ar,1)           ; (4,5) row sums = sum over other dimensions
print,total(ar,2)           ; (3,5) column sums
print,total(ar,3)           ; (3,4) frame sums
sizear=size(ar)
print,sizear       ; nr dims, dim1, dim2, dim3, type (integer), nr elements
mean=total(ar,3)/sizear(3)  ; temporal mean of this movie
xslice=ar[*,0,*]            ; distill (x,t) timeslice at y=0
help,xslice                 ; oops, still 3D array
xslice=reform(xslice)       ; reform removes degenerate dimensions
help,xslice                 ; 2D array now
br=[[[ar]],[[ar]],[[ar]]]   ; what is this?
help,br

; more of the same / soortgelijks / und so weiter / ibid
ar=indgen(6,5,4,3,2)+1
print,ar
print,size(ar)

free array to regain memory space
---------------------------------
undefine,arra,arrb,arrc,..  ; regain memory anywhere (cg program)
delvar,arra                 ; regain memory but only in main part
ar=0                        ; doesn't regain memory but leaves a hole

GRAPH PLOTTING
==============

basic plot
----------
x=findgen(100)          ; float array x=0., 1., ...., 99.
plot,sin(x/10)          ; 10 doesn't have to be 10. since x is float
y=sin(x/5.)/exp(x/50.)  ; but I like float specification for safety
plot,y                  ; plot,x,y uses array index for x if not given
plot,alog10(x),y        ; x and y may differ in array size
oplot,alog10(x),y^2     ; over-plots in existing graph
plot,alog10(x),y^2+10   ; too much emptiness in this graph
plot,alog10(x),y^2+10,/ynozero   ; /ynozero is alternative for ynozero=1
plot,abs(fft(y,1)^2),/ylog       ; power spectrum on logarithmic scale
plot_io,x,abs(y)+0.1)   ; log-linear plotter, not in the IDL help?
erase                   ; wipe current plot window
wdelete                 ; kill current plot window
while !d.window ne -1 do wdelete,!d.window ; kill all IDL windows ("easy")

Coyote cg window alternative
----------------------------
cgplot,x,y,/window ; resizable window, "save-as-postscript" clicker

plot beautification
-------------------
plot,x,y,psym=1          ; defined for psym=1-7,10; try them out
; something=something: optional "keyword" parameter
; check PLOT (? plot); check GRAPHICS KEYWORDS
plot,x,y,psym=-4         ; plot both curve and diamonds at sample values
plot,x,y,linestyle=1     ; defined for linestyle=0,...,5, try them all
oplot,x,y*2,linestyle=2  ; overplot another graph in the same frame
plots,20,70,psym=2,symsize=1.5      ; mark location with asterisk
plots,[20,70],[-0.5,+0.5]           ; overplot line segment [x1,x2],[y1,y2]
plots,[50,50],[-1E10,1E10],noclip=0 ; overplot line cut at edges (NOT /clip)
plot,x,y,xtitle='x axis',ytitle='y axis',thick=2,xthick=2,ythick=2,$charthick=2,charsize=2 ;$ extends to next line
ytitle=textoidl("sin(x/\alpha) e^{-x/\beta}")
print,ytitle          ; !U=up, !D=down, !N=normal, !7=Greek, !X=entry font
angstrom=textoidl("\AA")
angstrom=string(197B)  ; alternative = "byte constant" character code
; find symbol codes such as this by Googling <table isolatin1>
; but they may not be valid in the PostScript font you choose
; why the &$#@$% doesn't IDL accept full latex strings for all fonts?
set_plot,'ps'
angstrom=cgsymbol("angstrom") ; Coyote cg, probably most robust
set_plot,'x'
xtitle='x  ['+angstrom+']'
print,xtitle
plot,x,y,xtitle=xtitle,ytitle=ytitle,charsize=2
alphaspec=textoidl("\alpha = ")+strtrim(string(alpha),2) ; not so simple...
alphaspec=cgsymbol("alpha",/ps)+' = '+ strtrim(string(alpha),2) ; cg for ps
alphaname=strlowcase(scope_varname(alpha))  ; get variable name as string
alphaspec=greek(alphaname)+' = '+ strtrim(string(alpha),2)
xyouts,80,0.7,alphaspec,charsize=2              ; x,y in data units
xyouts,0.7,0.8,/norm,alphaspec,charsize=2       ; x,y in window size units
plot,x,y,xrange=[-10,+110],yrange=[-1.5,1.5]    ; your choice axis lengths
plot,x,y,xrange=[-10,+110],yrange=[-1.5,1.5],xstyle=1,ystyle=1
; now the axes obey your ranges exactly

plot beautification in a Coyote cg window
-----------------------------------------
cgplot,x,y,xtitle=xtitle,ytitle=ytitle,charsize=2,$; similar to above /window,$       ; resizable window
aspect=2./3,$; fixed aspect ratio psym=-15,$      ; many more choices; see doc_library,'symcat'
/_extra,thick=2,xthick=2,ythick=2,charthick=2   ; any plot keywords
; overplot is now an option of cgplot (add /add), not a separate routine
;   cgplot can also easily color curves, symbols, etc
;   select cgcolor name from palette: color = cgcolor(/selectcolor)
;   but oops: sticky, makes colors add up spoiling your next display

-------------------------------------
set_plot,'ps'                 ; change plot output to postscript format
device,filename='demo1.ps'    ; the plot commands now write to this file
plot,x,y,xtitle=xtitle,ytitle=ytitle,thick=2, xthick=2,ythick=2,$charthick=2,charsize=2 ; redo all the above xyouts,80,0.7,alphaspec,charsize=2 ; idem device,/close ; done, write postscript file set_plot,'x' ; back to output on Unix/linux/MacOS Xwindows screen ; set_plot,'win' ; back to output on a Micro$oft Windows screen
; help,/device        ; /device is the same as device=1 (enable)
$gv demo1.ps ; starting$ on command line escapes to shell
filename='demo1.ps'           ; make it a variable for
spawn,'gv '+filename          ; generic shell escape, also in a program

OOPS! ..|.. IDL!  The ps plot differs much from what you had on
your screen.  The thickness parameters in plot do NOT apply to ps
output.  Also the charsize multiplier in plot does NOT work.  The
vertical annotation spacing differs (even hardware-dependently,
depending on the character pixel matrix).  So, this demo exhibits
severe IDL shortcomings.  First, there is no clicker or single
command to obtain ps output that reproduces exactly what you have on
your screen - you cannot develop a nice on-screen display and then
hit or command "save as ps".  Instead, you have to repeat the whole
again for the ps "device", as shown above.  Second, there are
inconsistencies between such plotting on the screen and in ps, and
some of these are hardware-dependent.  The awkward upshot is that
there is not much point in beautifying the on-screen product.
Instead, you should beautify the ps output, independent of what you
get on the screen.  Because the plot thickness keywords do not work
for ps, one then has to muck around with the various !p.thick system
parameter settings.  These are sticky, so changes must subsequently
be undone not to get problems later (for example in the next
on-screen plot).  Similary, the IDL font codes for Greek characters
differ between the screen and some ps fonts.  Argh...

However, David Fanning's cg routines with their /window option can
serve to develop IDL figures on your screen and obtain ps output
like these (and raster pixmaps) without explicit sequence repeat but
indeed per clicker or a single command.  The sequence repeat still
occurs but is hidden within cg routines that call the coyote-library
"ps_start" and "ps_end" routines internally.  Fanning added
"evalkeyword" and "evalparams" options to provide run-time
evaluation for things that differ between devices, such as thickness
keywords and Greek characters.  See below.  They work nicely for
simple figures, but for elaborate ones you may prefer to go back to
the traditional repeat-sequence approach (I usually do).

Below I first demonstrate the traditional way of making postscript
graphs through repeating the entire plot sequence, then coyote cg
usage to avoid such repetition.

Postscript figure following Alfred de Wijn
-----------------------------------------
http://www.iluvatar.org/~dwijn/idlfigures    set_plot,'ps'                                ; postscript format
!p.font=1                                    ; true-type fonts
!p.thick=2 & !x.thick=2 & !y.thick=2         ; & = multiple commands/line
!p.charthick=2                               ; reset system default
xsize=8.8                                    ; cm; this is A&A column width
ysize=xsize*2/(1+sqrt(5))                    ; aspect golden ratio 1.61803
filename='demo2.eps'
device,filename=filename,xsize=xsize,ysize=ysize,/encapsulated,/portrait,$/tt_font,set_font='Times',font_size=11 ; fit size to publication font ytitle=textoidl("sin(x/\alpha) e^{-x/\beta}") ; repeat for ps font alphaspec=textoidl("\alpha = ")+strtrim(string(alpha),2) ; idem plot,x,y,$
position=[0.2,0.2,0.95,0.95],/normal,$; set margins around plot xticklen=0.03,yticklen=0.03*ysize/xsize,$  ; same-length ticks
xtitle=xtitle,ytitle=ytitle
xyouts,80,0.7,alphaspec                      ; x,y in data units
device,/close
set_plot,'x'          ; back to output on Unix/linux/MacOS Xwindows screen
!p.font=-1            ; back to default (Hershey) fonts for screen display
!p.thick=0 & !x.thick=0 & !y.thick=0 & !p.charthick=0  ; reset defaults
spawn,'cat '+filename+$; replace irritating '| sed "s|Graphics produced by IDL|'+filename+$  ; IDL plot banner
'|" >  idltemp.ps; mv idltemp.ps '+filename      ; with the file name
spawn,'gv '+filename              ; set gv to "watch file" for rewrites
; NB: textoidl doesn't give true-type font but at least it works in ps;
for Greek it has to be run again, now in the ps device environment
; NB: I minimize the bounding box later with epstopdf, pdfcrop, pdf2ps

PostScript figure with Coyote ps_start and ps_end
------------------------------------------------
xsize=8.8 & ysize=xsize*2/(1+sqrt(5))
ps_start,filename='ctdemo2.eps',font=1,tt_font='Times',$/nomatch,xsize=xsize,ysize=ysize,/metric,/encapsulated,charsize=0.9 ; default ps thicknesses are temporarily reset to 2 !p.thick=3 & !x.thick=3 & !y.thick=3 & !p.charthick=3 ; if you prefer 3 ytitle=textoidl("sin(x/\alpha) e^{-x/\beta}") ; textoidl repeat for ps alphaspec=textoidl("\alpha = ")+strtrim(string(alpha),2) ; idem plot,x,y,$
position=[0.2,0.2,0.95,0.95],/normal,$xticklen=0.03,yticklen=0.03*ysize/xsize,$
xtitle=xtitle,ytitle=ytitle
xyouts,80,0.7,alphaspec
ps_end     ; back to screen windows, Hershey fonts, original ! values
spawn,'gv ctdemo2.eps'

PostScript figure from a Coyote cg screen window
-----------------------------------------------
cgplot,x,y,/window,$charsize=2,xtitle=xtitle,position=[0.25, 0.25, 0.9, 0.9],$
evalkeywords=['thick','xthick','ythick','charthick','ytitle'],$thick='(!d.name eq "PS")?5:1',$      ; 5 for ps, 1 for screen
xthick='(!d.name eq "PS")?5:2',$; ps thick because size is large ythick='(!d.name eq "PS")?5:2',$     ; PS must be in capitals
charthick='(!d.name eq "PS")?5:1',$ytitle='textoidl("sin(x/\alpha) e^{-x/\beta}")' ; Greek, redo for ps cgtext,0.7,0.8,/norm,$
'greek(alpha)+" = "',evalparams=[0,0,1],$; Greek, redo for ps charsize=2,/addcmd cgtext,0.77,0.8,/norm,$          ; 0.77 results from manual fitting on ps
strtrim(string(alpha),2),$; normal parameter charsize=2,/addcmd ; click on file > save as postscript > ps output; or instead enter: cgcontrol,create_ps='cgdemo2.eps',/ps_encapsulated,/ps_metric spawn,'gv cgdemo2.eps' add second axis --------------- example of adding a top x-axis with nonlinear scaling with respect to the bottom x-axis (in this case mu = cos(theta) over the solar disk versus r/R_sun = sin(theta) with theta the viewing angle) plot,rvalues,averzones,psym=-4,$
position=[0.2,0.2,0.8,0.8],$; wide margins to accommodate extra axes xrange=[0,1],yrange=yrange,$
xstyle=9,ystyle=1,$; no axis along top xtitle=textoidl("r/R_{sun} = sin \theta"),$
mutickpos=[1.0,0.9,0.8,0.7,0.6,0.5,0.4,0.0]
muticknames=['1.0','0.9','0.8','0.7','0.6','0.5','0.4','0.0']
nmuticks=n_elements(mutickpos)-1
rmuticks=sqrt(1.-mutickpos^2)
axis,xaxis=1,xticks=nmuticks,xtickv=rmuticks,xtickname=muticknames,$xminor=1,xtitle=textoidl("\mu = cos \theta") add zero to a second axis ------------------------- IDL's AXIS routine to generate extra axes has the annoying failure that it may not plot the label zero when an axis starts at zero. Below an example how to correct this, plotting functions "tau(height)" and "temp(height)", the tau axis at left, the temp axis at right: heightrange=[0,2300] taurange=[-3,7] plotaspect=1.62 ; golden ratio plot,height,alog10(tau),$
position=[0.2,0.2,0.8,0.95],/normal,$; set margins around plot xticklen=0.03,yticklen=0.03/plotaspect,$    ; same-length ticks
xtitle='height  [km]',ytitle='log  (optical depth)',$xrange=heightrange,yrange=taurange,xstyle=1,ystyle=9,linestyle=1 temprange=[0,30000] tempscaled=taurange+(temp-temprange)/(temprange-temprange)*$
(taurange-taurange)                  ; rescale temp to log(tau)
oplot,height,tempscaled,thick=3              ; overplot temp(height)
axis,yaxis=1,yrange=temprange,ystyle=1,$; dummy axis to get ticks ytickinterval=1000,ytitle=”,ytickname=replicate(' ',60),$
ytick_get=tempaxticks
tempticknames=string(tempaxticks,format='(i5)')
axis,yaxis=1,yrange=temprange,ystyle=1,$; plot temp axis at right ytickinterval=1000,ytitle='temperature [K]',ytickname=tempticknames multi-panel figures ------------------- IDL offers !p.multi for stacking multiple plots into one display. Quite cumbersome and non-versatile. Alfred de Wijn has a better recipe at: http://www.iluvatar.org/~dwijn/idlfigures I myself never make multi-panel displays with IDL. Instead, I make fully-annotated separate graphs and stack them up in LaTeX, using LaTeX macros to remove superfluous annotation between panels. This way I choose the figure layout only when writing the paper, which makes collaboration in the analysis phase much easier. See: https://robrutten.nl/rrweb/rjr-edu/manuals/student-report/cutmultipanel.tex ARRAY/IMAGE PLOTTING ==================== two-dimensional array plotting ------------------------------ k=indgen(100) ; let's make a nice 100x100 array f=sin(k/5.)/exp(k/50.) ; the same f(x) as y(x) above g=cos(k/5.)*exp(k/50.) ; similar function g(y) for the other coordinate s=f#g ; make an array help,s ; a 2-dim (100,100) float array print,s[0:4,0:9] ; better use square brackets for array elements plot,s[7,*] ; plot 8th column (mind the virtual zero finger) oplot,s[*,95],linestyle=5 ; overplot 96th row, dashed tvscl,s ; view as byte-scaled image ; Compare the image (in the bottom-left plot corner), graph, and printout. ; The first index is the column number, the second index the row number. ; IDL's [column,row] is opposite to matrix algebra. See ? array majority. ; IDL's [column,row] fits the notion of an image f(x,y), that's why. ; The printout has s[0,0] at the top-left corner, but ; the image display has s[0,0] at its lower-left corner ("origin"). print,minmax(s) ; show extrema print,array_indices(s,where(s eq max(s))) ; the two plots sample max(s) print,s[5:9,94:96] ; check surface,s ; I dislike such plots, hard to read off values shade_surf,s ; idem show3,s ; yet worse xsurface,s ; primitive tool to change viewing point etc isurface,s ; not for me cgsurface,s ; Coyote alternative, much better ; grab and change viewpoint with left mouse ; zoom in and out with right/middle mouse ; various clicker options cgsurface,s,/shaded ; idem contour,s contour,s,nlevels=50 contour,s,nlevels=20,/downhill cgcontour,s,nlevels=20,/window ; Coyote alternative in resizable window cghistoplot,s,nbins=50,/window ; histogram = occurrence distribution hist=histogram(s,nbins=50,omin=omin,omax=omax) ; the same clumsily a la IDL binsize=(omax-omin)/49. normhist=hist/float(max(hist)) xhist=omin+indgen(50)*binsize plot,xhist,normhist,psym=10 image display ------------- ssize=SIZE(s) ; get array type and size nx=5*ssize ; ssize = number dimensions ny=5*ssize ; etcetera for more dimensions s5=rebin(s,nx,ny) ; resample s for larger display tvscl,congrid(s,188,188,/interp) ; arbitrary resizing (slow) wdelete window,xsize=nx,ysize=ny ; window equal to image size tv,s5 ; oops, tv expects value range 0-255 print,min(s5),max(s5) ; show extrema tv,s5<0 ; same selection, tv wraps negative values tv,(s5-min(s5))/(max(s5)-min(s5))*255 ; rescale to range (0-255) tvscl,s5 ; same s5b=bytscl(s5) ; make bytscale image (8 bits = shades 0 - 255) tv,s5b ; same as tvscl,s5 s5pos=fltarr(nx,ny) ; declare same-size array set to zero s5pos=0.*s5 ; the same if you don't have nx, ny indpos=where(s5 gt 0) ; 1D index vector counting along rows s5pos[indpos]=s5[indpos] ; equate to s5 for only these indices tvscl,s5pos ; shows s5 where s5>0, 0 elsewhere tvscl,s5>0 ; the same but quicker tvscl,s5 gt 0 ; I hope you expected that. Honestly? tvscl,s5<(-1) ; parentheses needed tvscl,s5>(-1)<1 ; clip cutoffs at -1 and +1 tv,bytscl(s5,min=-1,max=1) ; idem indcut=where(s5 gt -1 and s5 lt 1) ; try the same this way s5cut=fltarr(nx,ny) ; where gives 1D vector, need array s5cut[indcut]=s5[indcut] ; s5cut equals s5 where > -1 and < 1 tvscl,s5cut ; why different from tvscl,s5>(-1)<1? profiles,s5cut ; slice image, left mouse toggles rows, columns ; stop with right mouse (with cursor on image) loadct ; set colour table; choose e.g. 4 tv,s5b ; hideous; real scientists prefer monochrome xpalette ; tool to adjust color table xloadct ; idem (I like this one better) tvscl,s5b>127 ; display brighter half (not the same as s5>0) erase tvscl,s5[0:nx/2-1,0:ny/2-1] ; bottom-left quarter bytescaled on its own wdelete ; kill window (I use my wdelall.pro) tvbox,size,x,y,color ; SSW box overplot (color=0 black, 255 white) PostScript image following Alfred de Wijn ----------------------------------------- nx=5 ; define s again but let's now have large pixels ny=5 ; square image xaxisarr=indgen(nx)*float(nx)/(nx-1) ; add 1 for pixelated image yaxisarr=indgen(ny)*float(ny)/(ny-1) ; add 1 for pixelated image xaxisarr=(indgen(nx)*float(nx)/(nx-1)-CRPIX1)*CDELT1+XCEN ; solar X axis xaxisarr=(indgen(nx)*float(nx)/(nx-1)-(nx+1)/2.)*CDELT1+XCEN ; solar X axis axrat=yaxisarr[ny-1]/xaxisarr[nx-1] k=indgen(nx) & f=sin(k/5.)/exp(k/50.) & g=cos(k/5.)*exp(k/50.) & s=f#g set_plot,'ps' ; postscript output !p.font=1 tv ; true type fonts !p.thick=2 & !x.thick=2 & !y.thick=2 & !p.charthick=2 ; I like thick filename='demo3.eps' device,filename=filename,xsize=10,ysize=10*axrat,bits_per_pixel=8,$
/encapsulated,/tt_font,set_font='Times',font_size=12,/portrait
tv,bytscl(s),0.15,0.15,xsize=0.8,ysize=0.8,/normal       ; bytescaled data
contour,s,xaxisarr,yaxisarr,/nodata,/noerase,/xstyle,/ystyle,$; add axes position=[0.15,0.15,0.95,0.95],xticklen=-0.02,yticklen=-0.02*axrat,$
xtitle='x  [px]',ytitle='y  [px]'
; The tv and contour position and size values must correspond
; (here square image as 8 cm square with borders 1.5 and 0.5 cm);
; the wider bottom and left margins (1.5 cm) serve for axis labels.
; Bware: position x and y ranges must be equal for square pixels
; The negative tick lengths produce outward ticks.
; Redefine the indgen arrays for axis scaling
device,/close          ; write ps file
set_plot,'x'           ; back to output on Unix/linux/MacOS Xwindow screen
; set_plot,'win'       ; back to output on Micro$oft Windows screen !p.font=-1 ; back to default IDL (Hershey) fonts !p.thick=0 & !x.thick=0 & !y.thick=0 & !p.charthick=0 ; reset spawn,'cat '+filename+$                            ; replace irritating
'| sed "s|Graphics produced by IDL|'+filename+$; IDL plot banner '|" > idltemp.ps; mv idltemp.ps '+filename ; with the file name spawn,'gv '+filename ; set gv to "watch file" for rewrites ; NB: Mac users see smoothed pixels in Preview; first use epstopdf PostScript image with Coyote ps_start and ps_end ------------------------------------------------ xsize=8.8 & ysize=xsize*2/(1+sqrt(5)) ps_start,filename='ctdemo3.eps',font=1,tt_font='Times',$
/nomatch,xsize=xsize,ysize=ysize,/metric,/encapsulated,charsize=0.5
!p.thick=3 & !x.thick=3 & !y.thick=3 & !p.charthick=3  ; cg default=2
cgimage,bytscl(s),/keep_aspect,position=[0.15,0.15,0.95,0.95],$/axes,axkeywords={font:1,ticklen:-0.02,xtitle:'x [px]',ytitle:'y [px]'} ps_end ; this also resets the ! thicknesses back to what they were spawn,'gv ctdemo3.eps' ; Other axis scales: define axkeywords xrange and yrange PostScript image from a Coyote cg screen window ----------------------------------------------- cgimage,bytscl(s),/interpolate,/keep_aspect,charsize=2,$
/window,position=[0.15,0.15,0.95,0.95],$/axes,axkeywords={font:1,ticklen:-0.02,xtitle:'x [px]',ytitle:'y [px]'} ; get ps by clicking on 'save window > as ps file' under 'file', or use cgcontrol,create_ps='cgdemo3.eps',/ps_encapsulated,/ps_metric spawn,'gv cgdemo3.eps' ; NB: the cgimage screen image is smoothed by /interpolate, ; whereas the ps output remains pixelated. Use rebin (as above for ; s5) to smooth the latter too. I might do that for a math ; function but I wouldn't for actual data. ; NB: similarly, the addition of an endpoint to the axes befits ; a pixelated image but not a math function. INPUT/OUPUT ============ read/write formatted files -------------------------- openw,1,'myfile.ext' ; open file myfile.ext on "logical unit" 1 for writing printf,1,s ; write free-format file close,1 ; free "lun" 1 openr,1,'myfile.ext' ; now open that file for reading as unit 1 ss=fltarr(100,100) ; define variable type and size readf,1,ss ; read free-format file from unit 1 into array ss help,/files ; show which files are open as "unit" close,/all ; free all units, closing the files read/write binary files ----------------------- writeu,readu ; unformatted binary read/write, faster openr,1,/xdr,'myfile.ext' ; portable binary format, hardware independent random access into a file through assoc --------------------------------------- ; to sample files that exceed the available memory ; very useful for terabyte-challenged laptop owners! get_lun, unit ; the official way to open a file openr,unit,'big-3D-data_cube' ; file is intarr(nx,ny,nt) p = assoc(unit, intarr(nx,ny)) ; define image addressing image=p ; this gets image[*,*,1000] free_lun,unit ; closes the file too FITS files (much used in astronomy; run ssw) -------------------------------------------- writefits,'filename.fits',array [,header] ; adds header if you don't array=readfits('filename.fits' [,header]) ; no lun specification needed mreadfits,file(list),index,data,[....] ; ssw, fits with extensions mwritefits,index,data,[outfile=outfile,..] ; ssw, fits with extensions mwrfits,something,filename,/create ; multi-purpose fits write something=mrdfits(filename) ; multi-purpose fits read header=headfits('filename.fits') ; read header only nx=fxpar(header,'naxis1') ; when header = string array sxaddpar,outheader,'naxis1',nx_new,'new NX' ; (re)set string parameter nx=header.'naxis1' ; when header = structure openr,1,'filename.fits',/swap_if_little_endian ; fits files are big_endian p = assoc(1,intarr(nx,ny),2880) ; N x 2880 = skip fits header data_swap=swap_endian(data) ; swap endian of variable, array, structure mkhdr,header_out,outtype,[nx,ny,nt] ; make simple file header modfits,file,data,header ; replace data or header filelist=file_search(path+filenamepart) ; string with * wild fileonly=file_basename(file) ; remove path in file string filename=repstr(fileonly,'.fits',”) ; filename without extension saving IDL command sequences ---------------------------- journal,'filename' ; copies all typed commands to a journal file save,filename='name.sav' ; saves a full session (not in Student Edition) save,filename='name.sav',var1,var2,... ; save only selected variables restore,'name.sav' ; restart that session (you or your colleague) read ASCII tables ----------------- using as example file falc.dat (solar atmosphere model) at: https://robrutten.nl/rrweb/rjr-edu/exercises/ssb/falc.dat with readcol.pro (Google for it; in SSW/idlastro astrolib library) readcol,'falc.dat',h,tau5,colm,temp,vturb,nhyd,nprot,nel,ptot,$
pgas_ptot,dens,skipline=4
NB: add eg: ,format='I,I,A,F' for initial integer + string columns

primitive, as above:
openr,1,'falc.dat'
falc=fltarr(11,80)   ; 11 columns, 80 lines, no string entries
dummy=”
h=reform(falc[0,*])
tau5=reform(falc[1,*])
etcetera

falcfile='falc.dat'
falcstruct={height:0.0,tau5:0.0,mass:0.0,temp:0.0,v_mic:0.0,$n_h:0.0,n_p:0.0,n_e:0.0,p_tot:0.0,p_ratio:0.0,dens:0.0} read_struct,falcfile,falcstruct,falc,nlines=84,skiplines=4 help,/structure,falc plot,falc.height,falc.temp<10000,/ynozero print,falc.height ; print the first value (top of FALC) h=falc.height ; select variable NB: read_struct.pro does not work for columns with irregular strings as a structure with IDL's own read_ascii.pro and ascii_template.pro: falctemplate=ascii_template('falc.dat') ; opens GUI, work through save,falctemplate,filename='falctemplate.sav' ; save for next time restore,'falctemplate.sav' ; use next time table=read_ascii('falc.dat',data_start=1,num_records=80,$
help,table,/struct
h=table.field01[*]                             ; get first column

write ASCII tables
------------------
writecol,'filename.dat',vect1,vect2,vect3,fmt='(3f15.3)'
; in my misc.lib or google for the pro; up to 14 (19) vectors
; alternative: SSW forprint.pro

PROGRAM STRUCTURE
=================

Start a new file filename.pro; edit it (Windows: IDL desktop; Unix:
external editor or idlde.  Emacs with IDLWAVE gives great pro layout
and offers many shortcuts (Google idlwave).

In linux the file name must be lowercase.  Its structure:

pro procedurename,param1,param2,...,keyword1=keyword1,....
;+
;-
IDL statements
IDL statements     ; all local parameters are only known within this pro
end

function functionname,param1,param2,...,keyword1=keyword1,....
;+
;-
IDL statements
IDL statements
something=...                ; value to the function
return,something             ; output of the function
end

; -------- start of main-level program (if any) ----------

;; pro routinename,param1,param2,..,keyword1=keyword1,.. ; in when perfect
IDL statement
IDL statement

procedurename,a,b,keyword=c
x=functionname(a,b,keyword=c)

stop     ; for intermediate command-line inspections, continue with .con

IDL statement
IDL statement
end

The last "main-level part" is a sequence of IDL statements after the
last procedure or function that does not start with PRO or FUNCTION.
It must end with END.  You compile this program with ".com filename"
and run it with ".r filename" or ".rnew filename" which cleans out
earlier variables and recompiles too.  The latter recompiles the
subroutines within the file also.

After the program completion all main-level variables remain
available for inspection and tests on the command line.  Use this
main level for trying out and adding new things.  Insert temporary
stops to check on local variables or diagnose an error.  When your
development is done, then convert the program into a procedure or
function by inserting its name as "pro routinename" or "function
routinename" above the start of the statements, as illustrated above.
This new routine may go to a separate routinename.pro file or may
remain in the present filename.pro file.  You can add a main part
calling it underneath for modification testing.  If you do this
rightaway then on-the-fly testing while developing a subroutine
is very easy when using emacs IDLWAVE.

It is confusing that IDL procedures/functions have extension .pro
but that IDL main programs have these also.  And perhaps your IDL
batchfiles too.  I use .idl for the latter and instruct
emacs to give these IDLWAVE appearance with .emacs entry: (setq
auto-mode-alist (cons '("\\.idl\\'" . idlwave-mode) auto-mode-alist))

It is confusing that somename() is not always interpreted by IDL as
a function but sometimes as a variable, because in older days
(before edition 5.0) IDL used parentheses instead of square brackets
for array indices.  You can ascertain function interpretation and
recompile with: forward_function somename (proname without quotes).

Using procedures and functions
------------------------------
IDL> .run programname            ; compilation (only main program is run)
IDL> .r programname              ; idem; IDL accepts unique abbreviations
IDL> .rnew programname           ; first discard all existing variables
IDL> .r -t programnname          ; show content in manpage format
IDL> .com procedurename.pro      ; compile a procedure or function
IDL> procedurename,param1,...    ; run a compiled procedure
IDL> a=functionname(param1,...)  ; evaluate a compiled function
IDL> reset_session               ; wipe everything, also commons, & restart

IDLWAVE: remain in the emacs window with your program and use its tons
of fast keybindings including (with C = CONTROL):
C-c C-d C-c   ; compile and run program (set auto separate shell opening)
C-c C-d C-p   ; print value of variable under cursor in 2nd window
SHIFT-mouse2  ; idem
C-c ?         ; show help for procedure or keyword under cursor
C ALT q       ; re-indent the routine the cursor is in
C-c C-d C-x   ; jummp to next syntax error

function example (in a separate file addup.pro):
;+
; sums 1D array ARR (but IDL's total is faster and more general)
;-
arraysize=SIZE(arr)
if (arraysize ne 1) then print,'addup input is not a 1D array'
sumarr=0
for i=0,arraysize-1 do sumarr=sumarr+arr[i]
return,sumarr
end
IDL> .com addup               ; recompile after every program change
IDL> try=findgen(100)         ; try = floats 0.,......,99.
IDL> print,total(try)         ; check with IDL array summation

"Disappearing variables": after an error in a procedure or function
your session stops within that procedure/function.  HELP displays
the local variables valid there.  That serves to check out these,
e.g. by printing or plotting or manipulating them.  RETURN gets you
back one level higher.  RETALL gets you back to the top level where
the variables of your main program or session reside.  Recompiling a
routine (.com procedurename) also returns to the top. IDLWAVE offers
slick checkpoint jumping.

If you restart after a stop in a subordinate routine you are likely
to get error messages as:
"Attempt to subscript XXX with <YYY (ZZZ)> is out of range"
"Variable is undefined: XXXX
which means that you forgot to type return or retall and are still stuck
within the subroutine.

STOP in a procedure/function/main stops it right there to let you
inspect the local variables at that place in the statement sequence.
Continue with .continue (or .con).

.skip N on he command line: skip N lines and continue.  Default N=1.
.out on the command line: completes the subroutine but stops after
exiting back to the higher level.

Keyword inheritance: if your program uses e.g. plot, you don't have
to supply all the plot keywords as parameters.  Add a keyword
_extra=plotkeywords to your routine definition and use the same in
its call of plot.  Now you can add any plot keyword to the call of
your program.  See ? inheritance.  Unfortunately, you can specify
only one such inheritance per routine call, but you may have layered
inheritances (one routine calling another, each with its own
_extra=whatever).

conditional statements
----------------------
if (i gt 16) then begin   ; such sequences can also be run interactively
IDL statement           ; on the command line by first typing
IDL statement           ; IDL> .run
endif else begin          ; then enter the sequence, and conclude with
IDL statement           ; IDL> end
IDL statement
endelse

if (y eq 3) then x=2 else x=1  ; relational operators: EQ NE LE LT GE GT

for j=0,9 do number[j]=sin(region[j]*!pi)    ; ! gets system variable

for j=0,20,2 do begin                    ; third number = step 2
number[j]=sin(region[j]*!pi)
region[j]=0
endfor

while (a and (cnt ne 0)) do begin        ; logical operators: AND OR XOR
print,'Still going at count: ',cnt
cnt=cnt-1
endwhile

if (n eq 0) goto, JUMP
IDL statement
IDL statement
JUMP:
IDL statement
; but since good programmers never use goto, a better solution is:
if (n neq 0) then begin
IDL statement
IDL statement
endif
; or the use of break
for itrans=0,ntrans-1 do begin
IDL statements
if (transition[itrans].i eq i and transition[itrans].j eq j) then break
endfor

if (keyword_set(fontsize) eq 0) then fontsize=9   ; set keyword default
; but keyword_set=0 when supplied keyword=0, giving non-zero default
; therefore better use: if (n_elements(fontsize) eq 0) then fontsize=9

loop speedup
------------
- use implicit loops instead of explicit loops wherever possible,
so not:
for i=0,100 do intensity[i]=planck(wavelength,temp[i])
but:
intensity=planck(wavelength,temp)
by making sure that your function (planck.pro here) can handle
arrays (temperature here, idem for wavelength, but you cannot call both
as unsubscripted arrays).  With my laptop the second version is
typically 20x faster.

- replace an asterisk as first array index on the left-hand side of an
assignment statement by zero, so not:
for i=0,n-1 do array[*,i]=shift(array[*,i],delta[i])
for i=0,n-1 do array[0,i]=shift(array[*,i],delta[i])
which looks like an IDL mistake but actually speeds it up, in my
case typically 3x.  See
http://www.idlcoyote.com/code_tips/asterisk.html
passing parameters
------------------
- main programs
when running a sequence of programs, each with
.r programname
on the command line, the subsequently called programs know the
variables of the earlier called programs. The most primitive way
to pass parameters.

- @batchfile.  A file with a sequence of single-line IDL
commands can be run as @batchfilename on the command line or
from a program (only spaces are then allowed before the @
symbol, on a new line).  The file may not contain begin-end
blocks unless concatenated by \$ signs.  If an @file is run on
the command line it may contain ".r programname" lines.  This
way you can make an @script concatenating multiple main progams.
(I give these files extension .idl instead of .pro, and instruct
IDLWAVE via .emacs to treat these as IDL pro files.)

- procedure/function parameters
The parameter names in the call may of course differ from the
corresponding parameter names in the procedure/function body.
However, if the procedure/function changes the parameters, the
changed versions are passed back to the calling program at the
procedure/function completion.  If values are entered in the
call they do not change.  See IDL help ? passing parameters.

- commons
The traditional FORTRAN manner of passing blocks of parameters.
Put it in all pro's that need the parameters, and in the main
part if need be.  Initiate the parameter values in the main
part, or in the first pro that is called.  The traditional
problem is that the same parameter name may already be used in
another program (by another programmer).  Also, common blocks
cannot be shared between multiple IDL instances.

- structures
The newer way.  Much used in SolarSoft data reduction software.
They collect big parameters sets under a single name or anonymously
to be passed as parameter.  Google "IDL structures".
Example:
a=1.5
b='Never a dull moment with Kees D'
c=1
d=[4.,5.,7.]
s={a:a,b:b,c:c,d:d}   ; definition without name: anonymous structure
print, s.a
print, s.b+'. from whom I took this example'

- pointers
serve for variables that persist outside a routine, for
example pointing at a given location (address) within a
structure.  See:
http://www.idlcoyote.com/misc_tips/pointers.html    http://www.idlcoyote.com/misc_tips/precedence.html        c32=(*hatom.Cij_ptr)[*,2,1]   ; select a vector using a pointer

widgets
-------
Interactive gui's to use mouse actions.  Not treated here but nice examples
(from Oslo) are shown in my movex.pro.

programming hints
-----------------
- never ever forget that IDL array indices start at 0 ("fingers 0-9"
- never forget that you may need to type "retall" at some error
- try, experiment, check on the command line, than insert into program
- split programs in separate procedures and functions, test separately
- use size(array) to get unknown array dimensions in procedures
- choose clear variable names (in English please)
- add detailed explanation at procedure/subroutine start between
;+ and ;- lines for doc_library (as astronlib and SolarSoft do;
Emacs IDLWAVE inserts a template at C-c C-h)
- answer a procedure call without parameters or a function() call with:
if (n_params() lt N) then begin    ; N = nr required parameters
print,'procedurename, yyy, zzz'
print,'   yyy = ...'
return       ; return,-1 for a function called as x=function()
endif
- indent begin ... end structures (two spaces is my habit)
- journal,'filename' records all your command-line entries, useful for
subsequent conversion of the successful trials into programs
- use "save" to copy your work to a colleague

var sc_project=12399232;
var sc_invisible=1;
var sc_security="1abd7b2e"; 