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Repeat Until Loop

Module by: Kenneth Leroy Busbee. E-mail the author

Summary: An introduction to the repeat until control structure.

Introduction to Test After Loops

There are two commonly used test after loops in the iteration (or repetition) category of control structures. They are: do while and repeat until. This module covers the: repeat until.

Understanding Iteration in General – repeat until

The concept of iteration is connected to possibly wanting to repeat an action. Like all control structures we ask a question to control the execution of the loop. The term loop comes from the circular looping motion that occurs when using flowcharting. The basic form of the repeat until loop is as follows:


repeat
  some statements or action
  some statements or action
  some statements or action
  update the flag
until the answer to the question becomes true 

In every language that I know the question (called a test expression) is a Boolean expression. The Boolean data type has two values – true and false. Let's rewrite the structure to consider this:


repeat
  some statements or action
  some statements or action
  some statements or action
  update the flag
until expression becomes true 

Within the repeat until control structure there are three attributes of a properly working loop. They are:

  • Action or actions
  • Update of the flag
  • Test expression

The English phrasing is, "You repeat the action until the expression becomes true". This is looping on the false. When the test expression becomes true, you stop the loop and go on with the next item in the program. Notice, because this is a test after loop the action will always happen at least once. It is called a "test after loop" because the test comes after the action. It is also sometimes called a post-test loop, meaning the test is post (or Latin for after) the action and update.

The repeat until Structure within C++

Well, it just does not exist. Most programming languages have either the do while or the repeat until control structures, but not both.

Definitions

Definition 1: repeat until
A test after iteration control structure that is not available in C++.

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