Programming in IDL 1

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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