Question

What must be done to translate a posttest loop expressed in the form repeat: (...) until (...) into an equivalent posttest loop expressed in the form do: (...) while (...)

Short answer

Invert the condition of the repeat-until loop to convert it into a do-while loop's condition.

Step-by-step solution

Understand the Structure of a Repeat-Until Loop

A repeat-until loop is a posttest loop that executes its block of code at least once before evaluating the condition. The condition in the 'until' clause determines whether the loop continues or breaks.

Analyze the Structure of a Do-While Loop

A do-while loop, like a repeat-until loop, is a posttest loop that executes its code block at least once before evaluating the condition. The difference is that a do-while loop continues while a specific condition is true.

Identify the Loop Condition

In both types of posttest loops, the logic of when to stop the loop is controlled by their conditions. In a repeat-until loop, the loop continues until the given condition is true (meaning it stops when the condition is true), whereas in a do-while loop, the loop continues while the condition is true.

Invert the Condition for the Do-While Loop

To translate a repeat-until loop into a do-while loop, you'll need to invert the stopping condition. If the repeat loop ends when `until condition is true`, then the equivalent do-while loop should continue with `while condition is false`.

Write the Do-While Loop

Convert the block of the repeat-until loop directly into the block of the do-while loop. Then, append the inverted condition after the loop keyword. For example, if the repeat condition is `until x > 10`, the do-while condition will be `while x <= 10`.

Key concepts

Repeat-Until Loop

A "repeat-until" loop is a type of posttest loop, which means it checks its condition after having executed the loop's code block. One of the most significant advantages of using this type of loop is that it guarantees at least one execution of the loop body. This characteristic is useful in scenarios where you want the code to run first and then decide whether to continue.

Here’s how it works:

• The loop starts by executing the statements inside it.
• After the execution, the loop evaluates the condition specified in the "until" statement.
• If the condition is false, the loop continues to execute. If true, the loop terminates.

Think of the "repeat-until" loop like this: run the code "until" a certain situation occurs. This loop type is particularly beneficial when the stopping condition might not be met immediately, thus requiring the loop to run at least once before it can check and potentially stop.

Do-While Loop

A "do-while" loop is another form of posttest loop, sharing its basic nature with the repeat-until loop. Like all posttest loops, a do-while loop executes its body at least once before it checks the condition. However, the direction of the condition differs from that of a repeat-until loop.

The workflow of a do-while loop is as follows:

• The code block in the loop is executed once initially, regardless of the condition.
• After the execution, the loop checks the condition in the "while" clause.
• If the condition evaluates to true, the loop runs again. If false, the loop stops execution.

This means a do-while loop continues executing "while" a specified condition is met. In practical applications, this type of loop is helpful when you're confident that the condition for continuation might only apply after at least one execution cycle. The key difference, therefore, is in the condition used to determine whether to "continue" or "stop" the loop.

Loop Condition Inversion

Converting a repeat-until loop into a do-while loop involves a crucial concept: condition inversion. This process is essential because of the fundamental difference in testing conditions between these loop types.

Here’s a simplified process:

• In a repeat-until loop, the loop stops when the condition becomes true.
• Conversely, a do-while loop continues as long as the condition remains true.

Therefore, to switch a repeat-until loop to an equivalent do-while loop, the condition must be inverted. This logically means changing the stopping condition of the repeat-until into a continuation condition for the do-while.

For example, if the repeat-until condition is **"until x > 10"**, the equivalent do-while condition would be **"while x <= 10"**. By inverting the condition, the loop effectively mirrors the behavior of the original repeat-until loop, maintaining the logic and workflow goals of the original code.