DO...LOOP

Description

A control structure that is a general-purpose iteration statement used to execute a block of statements while or until a condition is true.

DO... LOOP has four formats:

DO UNTIL Executes a block of statements until the specified condition is true. If the condition is true on the first evaluation, the statement block does not execute.
DO WHILE Executes a block of statements while the specified condition is true. The loop ends when the condition becomes false. If the condition is false on the first evaluation, the statement block does not execute.
LOOP UNTIL Executes a block of statements at least once and continues until the specified condition is true.
LOOP WHILE Executes a block of statements at least once and continues while the specified condition is true. The loop ends when the condition becomes false.

In all four formats of the DO...LOOP control structure, DO marks the beginning of the statement block that you want to repeat. The LOOP statement marks the end.

You can nest DO...LOOP control structures.

Syntax

DO UNTIL condition
   statementblock
LOOP

DO WHILE condition
   statementblock
LOOP

DO
   statementblock
LOOP UNTIL condition

DO
   statementblock
LOOP WHILE condition

Parameter	Description
condition	The condition you are testing
statementblock	The block of statements you want to repeat

Usage

Use DO WHILE or DO UNTIL when you want to execute a block of statements only if a condition is true (for WHILE) or false (for UNTIL). DO WHILE and DO UNTIL test the condition before executing the block of statements.

Use LOOP WHILE or LOOP UNTIL when you want to execute a block of statements at least once. LOOP WHILE and LOOP UNTIL test the condition after the block of statements has been executed.

Examples

Example 1

DO UNTIL The following DO UNTIL repeatedly executes the Beep function until A is greater than 15:

integer A = 1, B = 1

Example 2

DO UNTIL A > 15

		Beep(A)

		A = (A + 1) * B

LOOP

Example 3

DO WHILE The following DO WHILE repeatedly executes the Beep function only while A is less than or equal to 15:

integer A = 1, B = 1

Example 4

DO WHILE A <= 15

		Beep(A)

		A = (A + 1) * B

LOOP

Example 5

LOOP UNTIL The following LOOP UNTIL executes the Beep function and then continues to execute the function until A is greater than 1:

integer A = 1, B = 1

DO

		Beep(A)

		A = (A + 1) * B

LOOP UNTIL A > 15

Example 6

LOOP WHILE The following LOOP WHILE repeatedly executes the Beep function while A is less than or equal to 15:

integer A = 1, B = 1

Example 7

DO

		Beep(A)

		A = (A + 1) * B

LOOP WHILE A <= 15