Which loop is guaranteed to execute at least once?

Question

BlackTom AI · Accepted Answer

The do-while loop performs its condition check after executing the loop body, so it will always run the body at least once.

Relevant concepts:
- Pre-test loops (for, while, foreach) evaluate a condition before executing the loop body; if the condition is false initially (or the collection is empty for foreach), the body may never run.
- Post-test loop (do-while) executes the body first, then evaluates the condition to decide whether to repeat—this guarantees one execution.

Step-by-step reasoning for each option:
- for loop: Execution order is initialization → condition check → body → update → condition check → ... If the initial condition is false, the body is skipped entirely. Example: for(int i=0; i<0; i++) { /* never runs */ }.
- while loop: Execution order is condition check → body → update → condition check → ... If the initial condition is false, the body never runs. Example: while(false) { /* never runs */ }.
- foreach loop: Internally attempts to obtain the first element from the collection; if collection is empty, there is no first element and the body is not executed. Example: foreach(var x in new int[0]) { /* never runs */ }.
- do-while loop: Execution order is body → condition check → repeat if true. Because the body executes before the condition is tested, the body runs at least once regardless of the condition’s initial value. Example: do { /* runs once */ } while(false);

Why the correct answer is correct:
- The do-while is the only loop among the options that is a post-test loop. That single structural difference (testing after execution) ensures at least one execution of the loop body.

Why the other options are incorrect:
- foreach, for, and while are pre-test constructs and can skip the body entirely if their initial condition or collection state prevents entry.

Conclusion: The correct answer is do-while, matching the provided answer.

Which loop is guaranteed to execute at least once?单项选择题

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