Create a function

Functions can be used in all three calculators in ILWIS: MapCalc, in TabCalc, in scripts or in the pocket line calculator. Some 50 internal functions are available (see also MapCalc and TabCalc), but also user-defined functions can be created.

A user-defined function generally contains of a combination of:

existing MapCalc/TabCalc operators and functions, and
user-defined parameters which represent maps and/or columns on which the function will be applied later on.

To create a user-defined function:

in the Main window, open the File menu and choose Create Function, or
in the Operation-tree, expand the Create item and double-click New Function, or
in the Operation-list , double-click the New Function item.

A user-defined function can be created:

to perform calculations in MapCalc and TabCalc which use difficult or long formulas that are tedious to type in on the command line, or
to define calculations for MapCalc and TabCalc which you want to use a number of times on different maps or columns.

Dialog box options:

Function name:

Type a new name for the user-defined function.

Expression:

Type the expression that the function should perform.

An expression is a combination of user-defined parameter names with which you want to calculate and some MapCalc and TabCalc operators and functions.

Parameter names:

must start with a character from A to Z,
may contain characters from A to Z, digits 0 to 9, and underscores,
may be longer than 8 characters.

Description:

Type a description for the user-defined function. The description is visible on the status bar of the Main window when moving the mouse pointer over the function in the Catalog.

The Function editor is opened. For more information on function syntax, see Function editor : functionality.

Example:

To sum for each record in a table (e.g. Rainfall), the values of two columns (e.g. January and February), you can create a function (e.g. sumcols).
The expression for this function can be: a + b
To apply the function, open the table Rainfall and type on the command line of the table window:

newcol = sumcols(January,February)

To use the same function on columns March and April, type on the command line:

newcol2 = sumcols(March,April)

In Map calculation : creating and applying user-defined functions and in Table calculation : creating and applying user-defined functions other 'basic' examples are available.
For more advanced user-defined functions, e.g. user-defined functions with IF, THEN, ELSE loops, FOR loops, WHILE loops and REPEAT/UNTIL loops, see Map and Table calculation : User-defined functions (advanced).

Tips:

User-defined functions can be used on the command line of the Main window for map calculations, on the command line of a table window for table calculations and in scripts for a sequence of map and table calculations.
ILWIS operations can be started from the command line of the Main window by typing an ILWIS expression. For more information, see Appendices : ILWIS expressions.
To perform a function on multiple maps or columns at a go, or to perform a series of ILWIS operations at a go, use a script. For more information, see ILWIS script language (syntax).