Do-until and input loops

Build the idea

Start with the concept and its key vocabulary.

Key idea

Interpret OCR do/until and translate its post-test behaviour into Python.

OCR do/until runs the body first and checks its condition afterwards, so it always executes at least once. Python has no direct do-until statement. A faithful translation uses while True, performs the body, and breaks when the ERL until condition becomes true.

do-until loop

A post-test loop that repeats until its condition becomes true.

Post-test

Checking a stopping condition after the loop body has executed.

Sentinel value

A special input value used to signal that repetition should stop.

break

A Python statement that immediately leaves the nearest loop.

Keep these in mind

The until condition is a stopping condition, not a continuing condition.
The body always runs before the first check.
Python while True with a conditional break can preserve the same order.
Input that may change the stopping condition belongs inside the repeated body.

Build the idea

Start with the concept and its key vocabulary.

Key idea

Interpret OCR do/until and translate its post-test behaviour into Python.

do-until loop

A post-test loop that repeats until its condition becomes true.

Post-test

Checking a stopping condition after the loop body has executed.

Sentinel value

A special input value used to signal that repetition should stop.

break

A Python statement that immediately leaves the nearest loop.

Keep these in mind

The until condition is a stopping condition, not a continuing condition.
The body always runs before the first check.
Python while True with a conditional break can preserve the same order.
Input that may change the stopping condition belongs inside the repeated body.