Loop Structures

Introduction to Loop Structures

There are 3 types of loop structures that are used within pseudocode, which are pre-condition, post-condition and count controlled loops.

Pre-Condition (WHILE)

This type of loop is likely the one that most are familiar with, where the loop is controlled by a condition set before the loop. It follows the structure of WHILE, ENDWHILE.

WHILE <condition>
    <code to be executed>
ENDWHILE

It may not execute at all, depending on the set condition. E.g:

WHILE num1 < 5
    num1 <- num1 + 1
ENDWHILE

The expression will continue evaluating until the condition becomes FALSE, at which point the loop terminates.

Post-Condition (REPEAT)

This type of loop depends on a condition set after the code within it has executed once, as the condition is evaluated after the code. It follows the structure of REPEAT, UNTIL.

REPEAT 
    <code to be executed>
UNTIL <condition>

E.g:

REPEAT
    num1 <- num1 + 1
UNTIL num1 = 10

Count-Controlled (FOR)

This type of loop depends on a counter set within its initial statement. It follows the structure of FOR, TO, NEXT.

FOR <integer variable> TO <desired limit>
    <code to be executed>
NEXT <integer variable that was used>

The value of the variable used as a counter can be assigned within the FOR statement.

FOR x <- 1 TO 10
    OUTPUT x
NEXT x

The default increment value for the count variable is 1, however this can be changed with the addition of an extra keyword, STEP. A negative increment can be used, and as soon as the variable reaches a value equal to or above the limit, the loop terminates.

FOR <variable> TO <limit> STEP <increment>
    <code to be executed>
NEXT <variable>