Rexx Programming/How to Rexx/conditional loop
A conditional loop tests for a condition around the loop, and repeatedly executes a block of instructions whilst the condition is true. Types of conditional loops include while loops, until loops and repeat loops.
A while loop is a conditional loop that repeatedly executes a block of instructions whilst the condition is true. The while statement is used as a syntactical cocomponent of do and end constructs to produce a while loop:
l = 0 do while l <= 10
say l l = l + 1
An until loop is a conditionalloop that repeatedly executes a block of instructions until the condition is true. In Rexx, the until statement is used together with do and end constructs to produce an until loop:
l = 1; do until l > 10
say l l = l + 1
A *repeat loop* is a [conditionalloop] that repeatedly executes a [block] of instructions whilst the condition is true. The [do] statement is used together with an [until] syntactical cocomponent to produce a repeat loop:
attempts = 1 do until attempts==10 say "Knock! Knock!" attempts = attempts + 1 end
The block of code within a repeat loop will always run at least once
Note than unlike [while] loops, with a repeat loop, the [condition] is evaluated after the [block] of code within the [loop]. even though it is written at the beginning. This means that the [block] of code within a repeat loop will always run at least once.
The cocomponents do and while are evaluated at the start of the loop
If a [do] and [while] cocomponents are used instead of [do] and [until], the loop will behave as a [while] loop with the condition being evaulated at the beginning of the loop. This may cause the loop to not execute at all, if the expression evaluates to a boolean [false] value: