while (cond) { body } tests cond before running the body, so if cond starts false, the body never runs, zero is a valid iteration count. do { body } while (cond); runs the body first and then tests, so the body always executes at least once, even if cond is false from the start. The practical distinction is when "always do this once before checking" is the natural flow, input validation prompts ("enter a number until they give a valid one") are the textbook do-while use case, because you must read at least one value before you can validate it.
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What's the difference between while and do-while?
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From the lesson
while & do-while Loops
Repeating work while a condition holds, and the one case (do-while) where the body has to run at least once before the test.