Main program, Procedures and Functions

Program Examples

To illustrate some of the differences between main programs, procedures, and functions, here are three corresponding versions of the code for determining the length of the first dimension of a given variable (scalar or array) using the built-inSIZE function. Each version is assumed to be stored in a file named get_dim1.pro in your IDL path.

1. [Refer to the definition of the SIZE function in the IDL help files. Note that the form of its output -- the intermediate variable s in the examples -- depends on the number of dimensions in the input variable. Its first element contains the number of dimensions (zero for a scalar); its second element is the length of the first dimension. Note that any text after a semi-colon (;) is optional and is ignored by the compiler.]

2. Main Program

   
            ; MAIN PROGRAM: get_dim1.pro 
            s = size(a_in)
            if (s[0] eq 0) then d1 = 0 else d1 = s[1]
            end
   

3. Procedure

   
            PRO get_dim1,a_in,d1
            s = size(a_in)
            if (s[0] eq 0) then d1 = 0 else d1 = s[1]
            return
            end
               
   

4. Function

   
            FUNCTION get_dim1,a_in
            s = size(a_in)
            if (s[0] eq 0) then d1 = 0 else d1 = s[1]
            return,d1
            end
   

To execute these three routines during an interactive session and to print to the terminal command window the resulting value for the first dimension, the commands would be as follows. In all cases, the input variable a_in must have already been defined during the session. Note that you do not have to give a .run command to compile a procedure or function as long as its file name matches its logical name (except for the .pro extension).

1. Main Program

   
             .run get_dim1
             print,d1
   

2. Procedure

   
             get_dim1,a_in,d1
             print,d1
   

3. Function

   
             print,get_dim1(a_in)
   

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