file: idl-simple-manual.txt = introduction to IDL basics last: Apr 6 2022 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 <> The html and pdf versions have active weblinks. This manual was written in the early 1990s for second-year astronomy students at Utrecht University doing the "Stellar Spectra" exercises at <> 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 licenses are excessively expensive, it long 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 <> but SunPy is on its way to replace all of it. I haven't tried the public IDL-replacing Fawlty Language. My habits --------- I run ancient IDL 6.4 (2007) under Ubuntu linux in the emacs IDLWAVE shell. I often use SolarSoft routines from <> I sometimes use Coyote Graphics "cg" routines from <> 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 <> A searcher that can also search IDL Google groups resides at <> 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. Weblinks: Numerous url's for astronomical IDL are collected at <> 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: <> textoidl.pro: get the version under pro/plotting in the Sloan library at <> Astronomy IDL library (not used here; it has been converted to cg): <> SolarSoft = "ssw" = solar physics IDL library: <> IDL startup ----------- IDLWAVE for Emacs Recommended modus of IDL operation, offering many keystroke shortcuts and debugging options: <> The IDLWAVE settings in my own .emacs file are shown at <> 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, +, -, * spectrum_AR10910=1 ; variable names must start with alphabetic character print,100^2 ; did you expect that? print,200^2 ; did you expect that? d=32767 ; "short" integers run from -32768 to + 32767 print,d+1 ; did you expect that? print,d+1. ; IDLWAVE: SHIFT mouse2 = print variable under cursor print,2^15 ; once more 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(a)) ; a in radians 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,acos(cos(!pi/a))*!radeg ; another one 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[0],..,a[99]=0 a=intarr(100) ; define a as integer array a[0],..,a[99]=0 a=fltarr(100) ; define a as floating number array a[0],..,a[99]=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[0],a[19] ; 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[0],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 ; 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 alpha=5 ; let's add annotation 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 cgplot,x,y*2,/overplot,/add,color='darksalmon',thick=5,linestyle=2 ; 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 ; see also doc_library,'cgcolor' or browse program.documentation.html cgtext,80,0.7,alphaspec,charsize=2,/addcmd ; replaces xyouts PostScript figure with traditional IDL ------------------------------------- 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 sequence of plot commands that made your nice on-screen display once 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 ----------------------------------------- <> 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"),$ ytitle='whatever averzones was about' 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[0]+(temp-temprange[0])/(temprange[1]-temprange[0])*$ (taurange[1]-taurange[0]) ; 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: <> 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: <> 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[1] ; ssize[0] = number dimensions ny=5*ssize[2] ; 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) ; color pixels with byte value 111 cyan ; (for my 8-bit colors: device,retain=2,decomposed=0; 23-bit came in IDL 7.1) bytim=bytscl(image) wherecolor=where(bytim eq 111) if (wherecolor[0] ne -1) then begin cc=cgcolor('cyan') bytim[where(bytim eq cc)]=cc-1 ; muck original cc-valued pixels bytim[wherecolor]=cc endif 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[1000] ; 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: <> 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='' for iskip=1,4 do readf,1,dummy ; skip 4-line header readf,1,falc h=reform(falc[0,*]) tau5=reform(falc[1,*]) etcetera as a structure, with read_struct.pro (Google for it; in sdssidl library): 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[0].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,$ missing_value=0,template=falctemplate) ; read into structure 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,.... ;+ ; standard header with information ;- IDL statements IDL statements ; all local parameters are only known within this pro end function functionname,param1,param2,...,keyword1=keyword1,.... ;+ ; standard header with information ;- 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): function addup,arr ;+ ; sums 1D array ARR (but IDL's total is faster and more general) ;- arraysize=SIZE(arr) if (arraysize[0] ne 1) then print,'addup input is not a 1D array' sumarr=0 for i=0,arraysize[1]-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,addup(try) 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 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]) but instead: 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 <> 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. Example: common fourier,nx,ny,nt,cad 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: <> <> 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") - do not 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 parameters instead of numbers to get dynamical adaptivity - use size(array) to get unknown array dimensions in procedures - choose clear variable names (in English please) - add lots of explanatory comments (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