What Functions and Procedures do

You don't have to have any arguments or keywords. (N_PARAMS function returns the number of parameters, KEYWORD_SET checks if a keyword is defined).

If your function/procedure is called `new' the file should be called new.pro

All the variables you set up within a procedure/function are local (they are defined ''whilst inside the procedure/function''), updates to the parameters, keywords (and the returned result for a function) are passed out to where it was called from

If IDL has a problem and stops within a sub-program, you will find that the variables you can look at or print are those belonging to the sub-program.

You can leave the `scope' of the sub-program and return, or return, value to the main level by typing

IDL> retall 
;OR for a procedure:
IDL> return 
;OR for a function:
IDL> return, avalue

Note: passing elements of vectors to procedures e.g.

 
Function times_two,x
;+
;pro times_two,x , doubles the value of x. JOHN MARSHAM 22/4/05
;-
x=x*2.
end

passing a scalar or a vector:

 
in=[1.,2.,3.]
a=1.
print,times_two(a)
                ;2.00000 - no surprise
print,times_two(in)
                ;2.00000      4.00000      6.00000 - no surprise

BUT passing an elemsnt of a vector or array

in=[1.,2.,3.]
print,times_two(in(0))             
                ; 2.00000  
print,in
                 ;     1.00000      2.00000      3.00000 ;DOES NOT UPDATE IN(0)!

you have to do

in=[1.,2.,3.]
a=in(0)
in(0)=times_two(a)
print,in           ;      2.00000      2.00000      3.00000