Functions and a Bit about Procedures or something more useful

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Functions and a Bit about Procedures(or
something more useful)
IDL Tutorial Day 2 Nick Hill hillnich_at_grinnell.ed
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Functions

• They perform calculations and assign the result
  of the calculation to a variable
• IDLgt x function_name(y)
• Distinguishable by the parentheses following
  function name with argument(s) enclosed
• IDL comes with several, several functions, and
  they are used all the time in procedures/programs
• Examples sin(x), cos(x), exp(x), min(x),
  max(x), etc.

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Common Functions

• Math functions youre used to seeing
• IDLgt y cos(x) IDLgt y exp(x) IDLgt y tan(x)
• Sizing arrays for programs
• IDLgt x intarr(15) IDLgt x fltarr(13, 12)
• Converting variables to different types
• IDLgt x fix(15.2) converts the argument 15.2
  to an integer such that x 15
• Information about array indices
• IDLgt x where (y eq min(y)) more about Boolean
  later, but what is going on here?

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Writing Functions

• Sometimes youll want to write a function for a
  series of calculations you may use multiple times
  in a program
• Youll treat it just like a program (save as .pro
  text file), but with slightly different syntax
• FUNCTION name_of_function, input
• commands
• RETURN, output
• END
• To use the function, compile it, and call it as
  you would any function
• IDLgt .compile name_of_function.pro
• IDLgt x name_of_function (input)

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Procedures/Programs

• Procedures can perform calculations, create
  plots, etc, and are at the core of what youll be
  doing
• Arguments are input, but does not have to output
  values. You HAVE TO TELL IT you want output
• IDLgt prog_name, arg1, arg2, output1, output2
• To write a program, open a text file, and use the
  syntax
• PRO prog_name, input, output
• commands
• END
• You must then save the text file with ending
  .pro
• my_program.pro

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Programs contd

• Any program you write must be compiled and syntax
  error free before it can be used
• IDLgt .compile prog_name.pro
• Programs are written in text editors (ie emacs,
  nedit), and must merely have that .pro ending
• IDL has a lot of programs for you to use. Try
  looking for one to do what you want before trying
  to write a new one
• Well talk more about writing programs in the
  next 2 days

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Common Programs

• print it prints variable values to a screen, or
  to a text file if specified
• IDLgt print, y
• plot makes plots given values
• IDLgt plot, x, sin(x)
• set_plot tells IDL where to output plots
• IDLgt set_plot, ps sends plot to a
  postscript file. More
• on this in the exercises
• x2jpeg takes whatever image is being displayed
  in a window (ie a plot) and saves it as a jpeg
  in your current working directory (really useful
  for your log)
• IDLgt x2jpeg, file_name.jpg

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Save Files and Restoration

• You often want to save the results you obtain
  from programs, and this is easily done with a
  save file
• Saves files simply contain variables you want to
  keep and end in .sav
• To create one
• IDLgt save, x, y, z, filewhatever.sav
• To restore the save file, so you can access
  those variables in IDL again
• IDLgt restore, whatever.sav, /ver /ver just
  allows you see what variables are being
  restored

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Batch Files/Macros

• These are text files with a series of commands to
  be executed in a sequential order
• Useful when you have a large sequence of programs
  that need to be run, but require a lot of time to
  run
• Only requires that at end of text file, you place
  end
• Executed in IDL like so
• IDLgt .r batch_file

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Practice