Procedural Programming Example

The following function, Factors(), computes the prime factors of an integer x. It illustrates many of the key constructs of procedural programming.

See below for an explanation of each of these constructs, and cross references to where they are.

Numbers identify features below	Function Factors(x) Definition:
1	`VAR result := [1];`
2	`VAR n := 2;`
3	`WHILE n <= x DO`
4	`BEGIN`
2	`VAR r := Floor(x/n);` `IF r*n = x THEN`
5	`(result := Concat(result, [n]);`
6	`x := r)` `ELSE n := n + 1`
4,7	`END; /* End While loop */`
7, 8	`result /* End Definition */`

This definition illustrates these features:

VAR x := e construct defines a local variable x, and sets an initial value e. See Defining a Local Variable.
You can group several expressions (statements) into a definition by separating them using “;” (semicolons). Expressions can be on the same line or successive lines. See Begin-End for Grouping Expressions.
While test Do body construct tests condition Test, and, if True, evaluates Body, and repeats until condition Test is False. See While(Test) Do Body.
Begin e1; e2; … End groups several expressions separated by semicolons “;” — in this case as the body of a While loop. See “Begin-End, (), and “;” for grouping expressions” on page 376.
(e1; e2; …) is another way to group expressions — in this case, as the action to be taken in the Then case. See Begin-End for Grouping Expressions.
x := e lets you assign the value of an expression e to a local variable x or, as in the first case, to a parameter of a function. See Assigning to a local variable.
A comment is enclosed between /* and */ as an alternative to { and }.
A group of expressions returns the value of the last expression — here the function Factors returns the value of result — whether the group is delimited by Begin and End, by parentheses marks ( and ), or, as here, by nothing.