QUESTION 21 · Loops
An entry-controlled loop tests before the body; an exit-controlled loop tests after it.
QUESTION 22 · Loops
The three loops all support repetition, but differ in where their condition is tested and how their control expressions are arranged.
The test appears before the body. Initialisation is normally written before the loop and the update inside the body. It is useful when a condition, rather than a simple fixed count, is the main reason for repeating. If its first condition is false, its body does not execute.
The header places initialisation, the condition and the update together. Initialisation runs once; then the condition is tested before every body execution. It is convenient for counter-controlled repetition. With continue, the update runs before the next condition test.
The body appears before the condition. Therefore at least one execution occurs. It is useful for a menu that must be displayed before deciding whether to repeat. With continue, control moves to the ending condition test. A semicolon follows that test.
Start i at 5 and use i <= 3 as the condition. A while loop and a for loop skip their bodies. A do-while loop executes its body once before discovering that the condition is false. The choice therefore affects program behaviour, not just appearance.
Use for for a clear counting task, while for a condition-controlled task, and do-while when the first action is compulsory. These are guidelines, not restrictions. Equivalent loops can have the same complexity. Count iterations and body work to determine time; changing the keyword alone does not improve an algorithm.
| Feature | while | for | do-while |
|---|---|---|---|
| Test position | Before body | Before body | After body |
| Minimum executions | 0 | 0 | 1 |
| continue destination | Condition | Update, then condition | Condition |
| Common use | Condition-controlled work | Counting | At least one action |
Related questions in the loops category from the supplied papers. The marks below are the printed allocations, including any combined subquestion.
How is an entry-controlled loop different from an exit-controlled loop? Explain.
Paper P01 · page 1Devise an algorithm for the sum of n numbers using while and for loops.
Paper P02 · page 1