A function is a program unit containing one or more IDL statements that returns a value. This unit executes independently of its caller. It has its own local variables and execution environment. Referencing a function causes the program unit to be executed. All functions return a function value which is given as a parameter in the RETURN statement used to exit the function. Function names can be up to 128 characters long.

The general format of a function definition is as follows:

FUNCTION Name, Parameter1, ..., Parametern

    Statement1

    Statement2

    ...

    ...

    RETURN, Expression

END

Function Example

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To define a function called AVERAGE, which returns the average value of an array, use the following statements:

FUNCTION AVERAGE, arr

    RETURN, TOTAL(arr)/N_ELEMENTS(arr)

END

Once the function AVERAGE has been defined, it is executed by entering the function name followed by its arguments enclosed in parentheses. Assuming the variable X contains an array, the statement,

PRINT, AVERAGE(X^2)

squares the array X, passes this result to the AVERAGE function, and prints the result. To return the result in a variable, use a function call as follows:

vAvg = AVERAGE(X^2)

Parameters passed to functions are identified by their position or by a keyword. See Using Keyword Parameters. If a function has no parameters, you must specify empty parentheses in the function call.