IDL Programming Tips

This IDL Programming Tips and Tricks database is constantly being updated. I have a large collection of tips I've collected from teaching hundreds of IDL programming classes and from the IDL news group (comp.lang.idl-pvwave). I try to add a half dozen or so new tips per month. Doing so takes quite a bit of time. If you find this information helpful, and you would like to contribute to this free service, please consider making a small PayPal donation to support it. Any amount is appreciated, but Coyote donates all contributions to local charitable organizations, so bigger is always better! See the Coyote's Guide to IDL Programming for details on how you can make your appreciation count.

If you have a question that you would like to see answered here, please fill out this question form. I'll see what I can do to find an answer.

Tip Topics

New Tips
Color Tips
Error Messages
File Reading/Writing Tips
General Programing Tips
Graphics Display Tips (2D)
Graphics Display Tips (3D)
Image Processing Tips
iTool Tips
Math Tips
Map Projection Tips

Miscellaneous Tips
Object Graphics Tips
Object Programming Tips
PostScript Output Tips
Programming Algorithm Tips
String Processing Tips
The IDL Way
Tutorials
Widget Programming Tips
IDL Workbench Tips
Ancient History (circa IDL 5.6)

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New IDL Programming Tips

How can I create a float from two integers, using the second integer as the decimal part of the first?
Is it possible to create a multi-line label widget?
Do you know any interesting facts about exponential operations in IDL?
Is there a way to draw a transparent polygon over my line plot?
Are there other problems with the IDL 7.0.1 Workbench I should know about?

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Color Tips

Can you explain how color works in IDL?
Can you show me how to construct a color table in IDL?
How do I ask for a color by name?
How do I add a color bar to my graphics plot?
How do colors work on a 24-bit display?
XLOADCT doesn't work on my display. Why?
How can I create a 24-bit color image from an 8-bit color image?
I've heard that the program XCOLORS is useful. Can you tell me how it works?
How can I reverse the colors in the color table?
How can I add a color overlay to a gray-scale image so that the image shows through?
TVRD gives me strange results. What is going on?
The colors on my filled contour map projection are completely wrong. What's going on?
How do I find the closest match in a color table to a particular color?
Your nice COLORBAR program won't display my colors. Why not?
How can I annotate COLORBAR with my own labels?
How can I create a color (JPEG, TIFF, PNG, BMP) image from an IDL graphics window?
How can I create a logarithmic color table instead of a linear one?
Can you show me how to display an image with a logarithmic color bar?
When I use your COLORBAR program how come there is a dark line in the color table representation?
How can I convert a 24-bit number to three 8-bit numbers?
Is there some way I can use Brewer Color Tables in IDL?

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Error Messsage Tips

How can I get accurate line error numbers in my CATCH error handlers?
What does the error message Temporary Variables Still Checked Out mean?
I'm trying to subscript an array and I get the message % Too many array elements. What does this mean?
What causes the error message Unable to create pixmap?
Why does IDL think my function is a variable?
How come I can't CATCH type conversion errors in IDL?
What does the error message The Interpreter Stack is not Empty on Exit mean?
Is there a list of error codes somewhere in IDL?
Help!! IDL code that ran perfectly yesterday is not working today! What happened?
Yikes! Running IDL 6.3 on a Macintosh from terminal window crashes IDL.
What does the error message X does not support locale C mean?
What does a java.lang.NullPointerException error mean in the IDL Workbench?
Is there a work-around for the Region_Grow error when the number of regions is large.

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File Input/Output Tips

I can't make heads or tails of the IDL HDF documentation. Can you help!?
How do I create a color JPEG file from a 2D image?
How do I create MPEG movies in IDL?
How can I read an ASCII data file that has a header and three columns of data?
How can I work with unsigned integers in IDL?
Why can't I read from a file into a subscripted array?
People talk about "little indian" and "big indian" machines. What in the world are they talking about!?
How do I count the number of rows in a text file?
How can I count the number of columns in a text file?
When I read a new variable READF skips to the start of a new line. How can I prevent that?
How do I create a color TIFF image?
How do I create a color JPEG image?
The Findfile function didn't find all the files on my UNIX machine. What is going on?
Is there a size limitation to IDL generated MPEG files?
How do I write my IDL array out as a comma separated value file I can open in my spreadsheet?
How do I print the contents of a graphics window?
I just want to create and print a text file from within IDL. Can this be done?
What is the best way to make a scientific animation in IDL?
Can I write a DICOM file in IDL?
How do I create a PostScript file of object graphic output?
How do I set my MPEG file frame rate to one frame per second?
I'm having trouble storing an associated file variable in a structure. Why?
How do I separate the file name from the file path returned by Dialog_Pickfile?
Why do I have memory leaks from IDL's file reading objects?
Can I make a JPEG file of a line plot without opening a graphics window?
How can I make an IDL plot with a transparent background for inclusion in a PowerPoint slide?
What do you know about working with AVI files in IDL?
I can't open a 2 GByte file in Windows. What's going on?
My repeat format code doesn't repeat properly. What's the matter with it?
No matter what I do I can only get six columns of output in my file. What am I doing wrong?
How can I create PDF files from within IDL?
How can I convert hexadecimal string values into numbers in IDL?
How can I determine if my TIFF file is georeferenced?
How can I suppress the line feed character and print two values on the same line with different Print commands?
How can I create a color (JPEG, TIFF, PNG, BMP) image from an IDL graphics window?
How do I convert my CT data file to Hounsfield Units?
My colleague produces GIF output in IDL, but I can't. What's up with that!?
Is there a way to handle an HDF file open failure more gracefully than “Sorry, Charlie?”
Reading a double from a file produces float accuracy. Why?
How do I write a structure to an HDF5 file?
Is there an easy way to browse and read netCDF files in IDL?
How do I read GRIB data into IDL?

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General Programming Tips

What are the 10 Most Common Programming Errors in IDL?
Can I create an IDL executable file?
How can I compile a program from within another program?
How do I create a stand-alone or run-time application on my Windows computer?
Is there a limit to the number of parameters I can define for a program?
How can I delete all my IDL session variables?
Is it possible to delete all my compiled procedures and functions?
Can someone help me link my FORTRAN programs into IDL?
Do you have any tips for calling C programs from IDL?
In C or C++, there is the notion of include files. Does IDL have such a thing?
Say what!? Avoiding asterisks in array expressions is how much faster?
My office mate's programs never have to be compiled and mine do. What does he know that I don't?
When does it make sense to use the Temporary function?
How can I keep my IDL code backward compatible and still use new features?
Unanswerable Questions: E-Mail from Pablo Neruda.
When is TRUE true and when is it false?
What's this about Dynamically Loadable Modules (DLM)? I can't find any documentation.
TVRD gives me strange results. What is going on?
Whoa! Does is take, like, a ton of memory to subscript arrays?
Oh, oh! What's going on when I do logical operations on two arrays?
Is IDL column-major or row-major?
Endless loops suck! How can I interrupt loops on Windows?
Is there a way to tell if IDL is running in DEMO or RUNTIME mode?
I want to use TeX style string formatting in IDL. Can I?
What is the proper way to name files in IDL?
How can I store a value in a structure when I have a pointer to an array of structures?
OK, my IDL program is slow, but why is it slow?
Is there a memory leak in the IDL Java Export Bridge?

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Graphics Display Tips (2D)

How do I overplot data on a !P.MULTI plot?
How do I position plots on the display?
How do I position images on the display?
How do I create a graphic with a specified aspect ratio?
How do I turn axis labelling off?
How can I add a second, logarithmic axis to my plot?
Why don't graphics windows resize smoothly on my Windows machine?
How do I leave room for a title when plotting multiple plots?
How can I create a circle in IDL?
How do I draw a rubberband box in a regular graphics window?
How can I print the contents of the current graphics window?
How can I tell if a graphics window is still open and available.
How can I create a plot with irregular tick spacing and values?
How can I create a polar plot with axes through the center?
Can I force IDL to use exponential axis annotation?
There is a hole in my filled contour plot, especially noticable when I make a PostScript file. Why?
My filled contour plot colors look great on the display. But three contours are gray in my PostScript output. Why?
Why doesn't the NLEVELS keyword to the CONTOUR command give me the correct number of contour levels?
I finally got my cheapskate boss to update IDL, only to find my favorite RDPIX program doesn't work. Help!
The colors on my filled contour map projection are completely wrong. What's going on?
What is the best way to make a scientific animation in IDL?
Help! I just want to center my plot on the page with the PRINTER device. Is this possible!?
I'd like to cut and paste a graphics window from IDL to Powerpoint. Can this be done?
I'm trying to do yada yada with log axes and it's not working. What am I doing wrong?
How do I get nice looking fonts in the Z-Buffer?
I found the error bar routines in IDL, but can I plot error bars in the X direction, too?
Is it possible to get the X axis labels on top of the plot instead of at the bottom?
Can I make a JPEG file of a line plot without opening a graphics window?
How do I fill the area under a curve with a color?
Is there any way to change the text orientation of axis labels?
Is there a way to plot the wave length on one axis and the corresponding wave number on another?
I'd like to make a sort of stacked ladder plot. Can this be done in IDL?
There is no room for a title on my plot. What can I do?
What do people use to overplot vectors on their graphics plots?
How can I create a color (JPEG, TIFF, PNG, BMP) image from an IDL graphics window?
I want to plot pressure on the Y axis, but I want the data range reversed. How do I do that?
Have you noticed odd cursor behavior with Mac OS X?
Is there a way to label minor tick marks on a logarithmic axis?
Can you show me how to display a colored background inside my plot axes?
Can you show me how to display an image with a logarithmic color bar?
Is there a routine in IDL for clipping polygons?
Can you show me how to draw a box and whisker plot in IDL?
How come my minor tick marks disappeared?
How can I make a histogram plot that looks like a histogram plot?
How can I display an image with a log-linear aspect ratio in IDL?
Can you show me how to obtain the location and value of a displayed image?
Why does my sine plot look so lousy in IDL?
What is the best way to produce IDL graphics output for Web or Powerpoint display?
Is there a way to draw a transparent polygon over my line plot?

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Graphics Display Tips (3D)

How do I create 3D axes?
How do I add 3D axes to a rendered isosurface?
How do I display a particle trajectory in 3D space?
How do I create a 3D scatter plot?
How can I exaggerate the Z axis of a surface plot?
How can I grid 3D spherical data to view with SLICER?
How can I draw a surface plot with axes through the origin?
How can I shade my surface plot according to its elevation?
How can I create a semi-transparent surface in IDL?
Why can't I use the [XY]Range keywords to display just a portion of a surface?
How can I display my XYZ data as a surface?
Can you show me how to create a 3D representation of a surface from image ROIs?

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Image Processing Tips

How do I build a frequency domain filter for my image?
How do I create a power spectrum of a 2D image?
How can I plot an image profile along any arbitrary line in an image?
How do I interactively adjust the contrast/brightness of an image using direct graphics?
How do I interactively adjust the contrast/brightness of an image using object graphics?
How can I convert an RGB image to a gray-scale image?
I'd like to create an ROI mask for my image. How do I do it?
Can you show me how to draw a boundary around a region of interest?
How do I fit an ellipse to a region of interest?
I've heard you can sharpen an image with a Laplacian operator. Can I use IDL to do that?
Is there an easy way to convert a pixel interleaved image to a band interleaved image?
Is there a function for histogram matching in IDL?
Active Contours and Snakes! Oh, my!
"Pixel-interleaved. Band-sequential..." Just tell me this: Where does the 3 go?
How can I improve the contrast in my 2D image?
How you show me the IDL Way to turn an 8-bit color image into a 24-bit image?
Is there an easy way to display a maximum intensity projection of an image?
How can I display a transparent PNG file in direct graphics?
Can you show me how to do Principal Components Analysis (PCA) in IDL?

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iTool Tips

Can I improve the cryptic error handling in iTools?
How can I replace the image data in iImage programmatically?
My iTool application won't run on the Virtual Machine. What trick am I missing?
How do I change iTool properties from the command line?
Does the iTool system leak memory?
Is it possible to create and save an iTool without it appearing on the display?
How can I make an iPlot plot fill the graphics window on start-up?

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Map Projections Tips

How do I contour irregular data on a map projection?
How come !P.Multi doesn't work with map projections?
Aaauuugh! How can I center a map projection over the Pacific ocean?
How do I add an 8-bit image to a map projection?
How do I add a 24-bit image to a map projection?
The colors on my filled contour map projection are completely wrong. What's going on?
My contours extend over the ocean. How can I mask these sea contours out of my map projection?
How can I use my own political boundary database (shapefiles) on map projections?
How do I convert normal longitude values (-180 to 180) to values ranging from 0 to 360?
How can I georeference an image I downloaded from the Internet?
How do I display a satellite image on a map projection?
Are there better options for continental outlines on maps than the built-in database?
Can you show me how to draw a map with a day/night terminator shown?
Can I associate a map coordinate system with an image that is already in a map projection?
Can you show me how to overlay point source data on a map?
Be careful using MAP_PROJ_INIT with GCTP map projections.
How can I match point source data with gridded maps?
The shapefile I made from CONTOUR doesn't quite line up on my map projection. Why not?
Can you show me how to convert UTM coordinates to latitude/longitude?
Can you show me how to extract latitude/longitude values for each pixel in a GeoTIFF image?
“Choose wisely when choosing Mercator (or any other) map projection.”
Can you show me how to overlay geographic information on a GeoTIFF image?
Can you show me how to navigate a GOES image?

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Math Tips

How can I calculate the local standard deviation or variance in an image?
How can I determine if a point is inside a polygon?
What's up with the IDL FFT function? I don't get the same values as my FORTRAN FFT function.
How do I obtain either a binary or hexadecimal representation of a number?
I want a more flexible curve fitting program. Can you help?
How can I calculate the logarithm base 2 of a vector in IDL?
How do you check for NANs in an array?
How do I test for NANs in my programs?
How do I calculate the center of mass of a 2D or 3D array?
What is the most efficient way to rotate a volume?
Help! The sky is falling with the way this number is represented in IDL!
I'd like to fit a polygon about a set of points. How do I do that?
How do I add two vectors that have NANs in them?
Do these floating underflow errors mean anything!?
Is there a fast way to get the sum of the diagonals of an array?
How do I get a Gaussian distributon of numbers with a specified mean and sigma?
Whoa! Did the behavior of ATAN change recently?
I'm having some trouble getting random points on the surface of a sphere. Can you help?
Histogram: On the Razor's Edge. A fable of interest to all computer users.
Are the Sigma and Chi-Square parameters in CURVEFIT screwy? Or am I just losing my mind?
Yikes! My double precision value is not very precise.
How can I interpolate a value on a regular grid?
I'm having trouble calculating derivatives in IDL. Can you help?
Is there a easy way to interpolate bad data points in a vector?
Is there a better way to represent missing data then using large numbers?
How is a floating point number really stored in IDL?
Do you know any interesting facts about exponential operations in IDL?

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Miscellaneous Tips

It can't get command line editing to work in my LINUX installation of IDL. What's going on?
Is there a way to make the SPAWN command non-blocking on Windows NT platforms?
Why are plot character sizes larger on SGI machines?
How can I run IDL programs if I don't have a three-button mouse?
I hear you read poetry in your IDL classes. Do you have any programming poems?
I'm going to buy a new computer. Which one runs IDL fastest?
I've heard the idlwave emacs mode is the greatest thing since sliced bread. What's up with that?
I'm having trouble getting on-line help to work with my Linux version of IDL. Any ideas?
Uh, did RSI forget to include the on-line help file in the IDL 5.5 distribution?
What is the IDL Expert Programmers Association, and how can I join?
I heard Mark Hadfield's Motley Library was now on your site. Where is it?
How can I get automatic updates to New IDL Programming Tips?
Do you answer questions about ENVI?
How do I configure IDL to run correctly on LINUX machines?
How can I run IDL without an X server?
Does anyone really expect me to use the IDLDE in an X-Windows enviroment?
My Macintosh OS X X11 xterm window cannot gain focus. What can I do?
Help! I can't get my double- precision values converted to strings properly.
Help! The Cursor command doesn't work on my Mac!
Yikes! Running IDL 6.3 on a Macintosh from terminal window crashes IDL.
Yo Mama's So...IDL Humor?!
Is JULDAY returning the correct value?
After upgrading to MacOS 10.4.8, IDL crashes with a segmentation fault. Any ideas?
My IDL/MacOS 10.4.7 combination crashes intermittently with a corrupted descriptor error. Any ideas?
Can I download IDL Programming Techniques as a PDF file?
What kind of books do IDL programmers read in their spare time?
Whoa! Simple graphics commands are causing segmentation faults. What happened!?
Is there an IDL programming Style Guide anywhere?
How does IDL stack up against other programming languages with respect to speed of execution?
How can I get mouse cursor events to work on a remote IDL session?
What in the world is the DIST function used for?
Yikes! Here is a fix to a potentially dangerous TIMEGEN bug.
Yikes! Is there a problem with IDL and the October 2007 Mac Leopard release?
Every time I call MESH_DECIMATE, IDL crashes. What's going on?
Is there a command line version of IDL for Windows?
Tired of IDL? Need a good book? Here are Coyote's recommendations.
How come DIALOG_PICKFILE starts looking for files in a strange directory?

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Object Graphics Tips

How do I create a line plot using IDL object graphics?
How do I create a surface plot using IDL object graphics?
How do I create a contour plot using IDL object graphics?
How do I create a image plot with axes using IDL object graphics?
Can I put object and direct graphics into the same display window?
Can you show me how to create an object graphics colorbar?
Can you show me how to select and move graphics objects?
Can you show me how to create an elevation-shaded surface using object graphics?
How do I size symbols appropriately in object graphics?
I'm having trouble getting my object graphics axes positioned. What is going on?
The objects in my IDL run-time application don't seem to work. What's going on?
When I add new data to my graphics object the axes annotations go crazy. What's going on?
Can you show me how to use object instancing to create a animated graphical display?
I am trying to draw a rubberband box in object graphics, but it is painfully slow. Can I speed this up?
How do I reverse an object graphics axis without the text going completely bonkers?
How do I add an image as a texture to a surface?
How do I zoom into an image while preserving the aspect ratio of the image subset using object graphics?
How do I interactively adjust the contrast/brightness of an image using object graphics?
How do I create a PostScript file of object graphic output?
I'm trying to represent a 3D surface in object graphics, but I can't seem to get the surface lighted correctly. Can you help?
Why do my object graphics programs run slower on my LINUX machine than my Windows machine?
Is there a maximum size for the IDLgrBuffer object?
How can I use special characters and Greek symbols in object graphics True-Type fonts?
My object graphics rendering is S-L-O-W, even with hardware rendering selected. What am I doing wrong?
My texture mapped image appears warped on my polygon. Am I doing something wrong?
My object graphics text is fuzzy in PowerPoint slides. Is there a way to fix this?
How can I keep a light source in one model from interfering with another model?
How can I obtain the 3D location of a point on my surface?
Is there an iTools program that is analogous to XYOUTS?

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Object Programming Tips

Can you recommend a book to learn about object-oriented programming in IDL?
How can I restore an object's methods at the same time I restore a saved object?
Can you show me how to make and return a copy of my object from within a method.
How can I write a general extraction method for any field in an object?
The objects in my IDL run-time application don't seem to work. What's going on?
What's up with IDL objects!? My INIT (or CLEANUP) method isn't running, even though I compiled it.
My objects are slow to initalize. What's going on?
Is defining a gazillion GetProperty/SetProperty keywords getting you down?
Can I tell if an object has a particular method?
I've heard about the Catalyst Library as an object framework for building IDL applications. Where is it?

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PostScript Output Tips

How come my images look like they are using only 16 colors?
How can I make my PostScript output look like my display output?
How does PostScript resolution affect my output?
My background and foreground colors were switched. How come?
Can I choose a background color other than white?
Where do the extra colors in my image come from?
Can someone help me position or size images in PostScript?
How can I use PostScript fonts in my output?
Help! My plot disappeared when I selected landscape mode.
How do I print PostScript files from within IDL?
Can I include IDL PostScript graphics in other documents?
Can I create multiple PostScript plots?
How can I dump the contents of a graphics window to a PostScript file?
Is there a good way to print PostScript files on Windows machines?
Can I get 24-bit PostScript output in IDL?
How can I get an encapsulated PostScript preview image in my Microsoft Word document on my PC?
How do I create a PostScript file of object graphic output?
PostScript output is great, but I can't get IDL's PostScript output into Microsoft Word. Isn't this a huge limitation in IDL?
What is the relationship between point size in PostScript and line thickness in IDL?
How can I place a less-than-or-equal sign (≤) in my PostScript output?

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Programming Algorithm Tips

Can I write a recursive function in IDL?
How do I delete or undefine a variable?
How can I convert WHERE output to image subscripts?
Can you show me how to write a recursive function to return all the directories rooted at a target directory?
Can you show me how to write a recursive function to return all the tags of a structure?
Can you show me a simple way to reverse the order of the even rows in a 2D array?
I'm having trouble concatenating structures into an array. Can you help?
Is there a general way extract a field from a structure when the field name is a variable?
I'm having trouble sorting my data set. Can you help?
How can I find the size of a structure in bytes?
How can I make my computer beep progammatically?
How can I tell if a keyword was used in my program?
How can I access or set IDL main-level variables from within a procedure or function?
How do I get the screen size of a remote display?
How can I find the perimeter or convex hull around a set of points?
Does the Sort command sort correctly?
How can I make multiple line dialog messages?
Is there a way to discover programatically what module I'm in and where that module is located?
How can I create an angstrom symbol (&#197) in IDL?
How can I determine if IDL is running on an 8-bit or 24-bit display?
Is it possible to find the maximum in each row of a 2-D array without using a FOR loop?
How can I repair the damage to my graphics window in IDL?
How do I dynamically delete an element from an array?
How do I get rid of floating point underflow error messages?
How can I locate a file in a different directory structure from my own?
Can I get HELP on just the variables, without all that other nonsense?
How can I obtain a random permutation of an index array?
I would like to know which number in an vector is closest to a specific value. Can IDL do this?
Is there such a thing as a null array in IDL?
How do I create sequentially numbered file names?
PV-WAVE had a nifty HAK (Hit Any Key to Continue) program. Does such a thing exist in IDL?
What is an efficient way to perform string substitutions in IDL?
How do I handle data that is given in units of "hours since 0001-01-01"?
I have a 3D model array, sampled on a 3.75 degree lat/lon grid. How can I resample my array to 2.5 or 5.0 degrees?
What is the fastest way to find the number of points in a sphere?
How do I calculate the MODE of an array in IDL?
How do I find the value of a particular bit in a number?
What is the best way to obtain random index numbers?
Can I use shared memory with the IDL_IDLBridge?
Can you show me a fast way to smooth the spectral bands of a hyperspectral dataset?
How can I convert a day-of-year number to a month and day number?
What is a fast way to match two lists with respect to a given parameter?
How can I make a scalar value of a particular data type at run-time?
How can I determine if two floating-point arrays are equal?
How can I create a blank string of a particular length?
How can I convert date strings to Julian numbers in IDL?
How can I make a checkerboard image in IDL?
How can I convert a 24-bit number to three 8-bit numbers?
How do I create an array to arrays with variable numbers of elements?
The Sky is Falling...Again (or What in the world is going on here!?)
Can you help me grid irregularly spaced data in IDL?
How can I add a new field to an anonymous structure array in IDL?
How can I prevent dividing by zero in my IDL code?
How can I scale image data for display so that zero is in the middle of the data range?
Is there a way to draw a transparent polygon over my line plot?
How can I create a float from two integers, using the second integer as the decimal part of the first?

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String Processing Tips

Is there a way to calculate a string's size in IDL?
Can I size my text proportionally in resizable graphics windows?
How can I include a changing numerical value in a string?
What is an efficient way to perform string substitutions in IDL?
Say what!? StrMid is vectorized!? How come no one told me?
Can you show me the IDL way to do non-alphabetical sorting of string arrays?
How can I sort a string array on two fields at once without loops?
Is there a way to remove a prefix from every string in a string array without using a loop?

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The IDL Way

Is there a fast way to get the sum of the diagonals of an array?
"My IDL program speed improved from 3 days to 32 seconds by removing loops!" A National Enquirer exclusive interview.
Are FOR loops the embodiment of pure evil?
Wow! Are FOR loops really as bad as JD says they are?
How can I find the union, intersection, and difference between sets A and B?
"Drizzling", "chunking" and other array decimation tricks using Histogram, naturally.
How can I do maximum value resampling of an image using a sampling box of arbitrary size?
Small examples to illuminate the IDL Way.
How do I construct a vector with values from one vector and a repeat count from another?
Is there a fast way to find the intersection between a vector and an array?
What is the “IDL Way” to average components of a time series?
What is the best way to do odd-line smoothing of an array?
How you show me the IDL Way to turn an 8-bit color image into a 24-bit image?

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Tutorials

Dimensional Juggling Tutorial: Working array manipulation magic with Rebin and Reform, by JD Smith.
Array Concatenation Tutorial: Achieving harmonious spiritual balance with nested brackets, by JD Smith.
FFT Secrets Revealed: Everything you wanted to know about the FFT function, by Peter Brooker.
HISTOGRAM: The Breathless Horror and Disgust, by JD Smith.
Operator Precedence Tutorial: Who's on first?
Learn everything you need to know about pointers in IDL.
Learn to vectorizing your code with these small examples from the pros on the IDL newsgroup.
How do I write a device-independent graphics display program?
Can you show me how to do Principal Components Analysis (PCA) in IDL?
Can you show me how to do Empirical Orthogonal Function (EOF) Analysis in IDL?

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Widget Programming Tips

How do I detect a double click on a button widget?
How can I process keyboard events from within my widget program?
How do I draw a rubberband box in a widget program?
How do I draw a free-hand line in a draw widget window?
How can I tell which button is currently selected in a set of exclusive buttons?
How can I get the new viewport coordinates when the user scrolls my draw widget?
My widget program produces the error: %XMANAGER: Caught unexpected error from
client application. How can I get a more enlightened message?
How can I remove the scroll bars from my draw widget window?
How can I create a show progress indicator for a time-consuming operation?
How do I get the current value of a droplist widget?
Can you show me how to dynamically add menu items to my program?
What's your opinion of the GUI Builder? Should I use it?
Can you explain the difference between modal and blocking widgets?
Can you explain the GROUP_LEADER, EVENT_PRO and JUST_REG keywords.
I want to run multiple widget programs at the same time and I don't know how to set these keywords.
How can I write a password widget that echos the password in asterisks?
How can I change the maximum value of a slider widget after I have realized it?
Alright, how do you do that resizeable graphics window trick?
How do I get checkbox buttons (or radio buttons) instead of push-buttons in my GUI interface?
How do I set the initial checkbox buttons (or radio buttons) when I create them?
How can I get an error traceback in a widget program, and yet keep the program running?
How do I get my widget buttons to all be the same size?
Given a widget ID, how can I find the ID of the widget at the top of that widget's hierarchy?
Is XManager obsolete in IDL 5.6?
How do I write a pop-up dialog widget in IDL?
Can you show me how to implement a cascading series of menu choices as pop-up windows?
I can't work out how to put checkmarks next to menu items. Any ideas?
How come LIST widgets (or DROPLIST widgets) don't have values?
How can I make my widget buttons look depressed?
Help! I can't get my double- precision values converted to strings properly.
How can I take a snapshot of an entire widget application from within the program?
What do I have to do to use the mouse wheel in UNIX widgets?
How can I create a resizeable list widget?
How can I identify base widget IDs from tab widget events?
Can you show me how to make colored widget buttons?
How can I find the size of a string I want to put in a text widget?
Say, uh, are draw widget press and release values totally off the chart?
Toward a theory of widget geometry.
Is it possible to create a multi-line label widget?

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IDL Workbench Tips

What in the world is the IDL Workbench, and how come I never heard of it?
IDL 7.0 just arrived. How do I set things up in the IDL Workbench?
Is there a minimum hardware requirement for the IDL Workbench?
Say, uh, where did my graphics windows go in the IDL Workbench?
Whoa! Did my file path just change in the IDL Workbench?
When the IDL Workbench starts up, it spends a lot of time analyzing code. Why?
Can you show me how to set-up Subversion version control in IDL 7?
Say, uh, I exited the IDL Workbench, but my IDL program is still running. Is that normal?
How do I select the language the IDL Workbench speaks?
Do you have general instructions for setting up Source Code Management in IDL 7?
How come DIALOG_PICKFILE starts looking for files in a strange directory?
The IDL Workbench won't let me make projects! Is it something I said!?
What does a java.lang.NullPointerException error mean in the IDL Workbench?
How do I insert a programming template into my file in the IDL Workbench?
Can I add other language plug-ins to the IDL Workbench?
Can I add the IDL Workbench to my current Eclipse installation?
Yikes! The IDL 7 Workbench won't quit “analyzing code”. What's going on?
The heartwenching saga of the IDL 7 Workbench Path Problem from Hell.
Do you have a tip that might make me feel warm and fuzzy toward the IDL Workbench?
How come the IDL Workbench doesn't always stop at a breakpoint?
How come the IDL Workbench hangs in simple main-level loops?
Are there other problems with the IDL 7.0.1 Workbench I should know about?

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Ancient History (IDL Tips circa IDL 5.6)

Can a program load color tables identically on 8-bit and 24-bit displays?
How does IDL choose the number of colors in the IDL session?
Can I use a 16-bit color visual class in my UNIX IDL?
Can I make IDL use 256 colors? How about some other number?
Why do my windows disappear when I make an IDL graphics window active?
How come my GIF file white colors are slightly gray?
Is Linux 24-bit color broken in IDL 5.5?
How can I display images with different color tables at the same time?
Help! All my colors have disappeared! What's going on?
My 24-bit image colors look lousy. What's wrong with them?
What are these strange streaks in my color table with the Linux version of IDL 5.5?
What does the error Program unit has too many local variables mean?
I'm running Linux and I see this message: % Unsupported X Windows visual (class: StaticGray, depth: 0). What does it mean?
The Call_External command that worked perfectly in IDL 5.4 causes a Call_External error in IDL 5.5. Why?
Are there problems with the File_Test command on Windows?
I've noticed extraneous text in JPEG and TIFF files created with object graphics programs. What's up with that?
IDL is falling asleep on me. No, really!
Does the SURFACE command have problems with hidden surface removal under certain circumstances?
Is there a faster way to interpolate 1D vectors than INTERPOL?

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Copyright © 1996-2008 David W. Fanning
Last Updated 4 February 2008

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