Programming in IDL 1

Programming Assignment 1:
Adapted from Rutton: http://www.astro.uu.nl/~rutten/rrweb/rjr-material/idl/manuals/idl-briefmanual.txt
IDL
IDL is used often in astronomical image processing.
It is an interactive computer language with the following advantages:
• programming language, not a package – you make up your own stuff
• interactive: test out new tricks by trying them statement by statement
The major disadvantage is that IDL licenses are very expensive. The student version is cheaper
but limited to array size 65536 (256x256 pixel images).
Manuals
The online help is reasonably good.
IDL Tutorial: http://nstx.pppl.gov/nstx/Software/IDL/idl_intro.html
Another one: http://www.msi.umn.edu/software/idl/tutorial/
Tips and Tricks: http://www.dfanning.com/documents/tips.html
The IDL astronomy library: http://idlastro.gsfc.nasa.gov/homepage.html
David Fanning's Coyote's Guide: http://www.dfanning.com/
Since IDL is a proprietary program, you will need to log into the astrolab machines to use it:
ssh -X -l username astrolabXX.astro.washington.edu (or -Y on a mac)
where XX = 1-29
IDL as interpreter
IDL works directly on the command line when you hit return. This makes it easy to try new
statements and statement sequences. The up cursor arrow brings back earlier commands.
Try out the following IDL statements on the command line.
On-line help. Before starting idl, type in the terminal:
% idlhelp
Start idl:
% idl
IDL 1
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Try the following statements. The left column gives the statement you should type into IDL
through the command line. The right column gives you notes in italics and questions in regular
text. Record your input and answers to the questions in an Emacs file.
IDL Statements Notes and Questions
> print, 3*5 1. What happens if you don't use a comma?
> print, 3*5 ; Hello there! 2. What does the ";" do? (try it without the ; )
> a=3*5 This equal sign is an assignment. Read this as "a gets
the value of 3 times 5."
> help,a
> help,A
> d=32767
> print,d+1
3. What does help do? What's the difference between
using a and A? “INT” means integer.
Integers run from -32768 to + 32768.
4. What happened?
> print,d+1. 5. What is the difference between using d+1 and d+1.?
> x = d+1.
> help, x
> print,3/5
> print,3/5.
> width = 20
> height = 5*9
> print, width * height
> a=[1,2,3,4,5,6]
> a=[a,7]
Float: Floating-point, with six significant places.
Double: Floating-point with approximately sixteen
decimal digits.
String: A sequence of 0 to 32,767 characters.
One floating number makes the result a float.
6. Explain what happened in these three lines.
IDL 1
IDL variables can be (multi-dimensional) arrays. You
can lengthen array A by adding value at the end.
7. Lengthen array "a" by adding the number 8. Provide
your command.
> print, a, 1E6*a Single precision: 6 significant digits, print, a, 1D6*a Double precision: 16 significant digits
> print, a, 1/a
> print, a, 1./a
> print, a, a^2
> print, a, alog10(10^a)
> a=1.*a Converts into floats
8. What's the difference between the output of these two
lines? Why?
alog10 is what you normally mean when you write "log"
> print, a, alog10(10^a) 9. Explain the output of this statement.
> print, a, alog(exp(a))
> print, a, acos(cos(a))
> print, a, a mod 2 12. What does “mod” do?
10. Explain the output of these two lines (hint: what is
“alog,” “exp,” etc? Use idlhelp).
11. Do the same as the above, but replace “cos” and
“acos” with sine and arcsine. Provide your command.
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=sqrt(a) Type of b is defined through its assignment.
> help Help without variable shows all variables.
IDL 1
> print,'b=',b 13. What did the quotes do? Write another command
using quotes and a variable. Provide your command and
its output.
> a=intarr(100) Simple arrays: define A as integer array
A(0),...,A(99)=0
> a=a+1 14. What does this do?
> for i=0,99 do a(i)=i This is a one line for loop. BEWARE! IDL starts
counting at 0!
15. What did this for loop do?
> a=fltarr(100)
> a=dblarr(100)
> a=indgen(100)
> print,a(0),a(99)
> print,a(10:19) 18. What does the : do?
16. What do these commands do? What are the
differences between them?
17. What is the value of the 29th element in the array a?
> b=sqrt(float(a)) 19. What does the float do? Why do you need it there?
> for i=0,99 do if (b(i) GT
5) then a(i) = a(i) + b(i)
> exit Exit IDL.
Due: October 12 by 5pm
This is a for loop with an if statement inside it.
20. Explain what this command is doing in a sentence.
21. What does GT mean? What is the opposite of GT?
22. How is = different from EQ?
23. What does the 'i' mean in b(i)?
24. Make an array named c of 100 floating points in
order from 0 to 99 (hint: look for indgen in idlhelp).
Provide your command.
25. Make the most complicated one line for loop with an
if statement you can! Extra points go to those who use
mod, log and trig functions. Provide your command.
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