Basic Pascal Tutorial/Chapter 3/REPEAT..UNTIL: what changed
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| 1 | 1 | REPEAT...UNTIL | |
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| 3 | 3 | The repeat .. until construct is termed a *post*-test loop, because the controlling condition is tested *after* each iteration of the loop. | |
| 4 | 4 | ||
| 5 | 5 | It has the following syntax: | |
| 6 | 6 | ||
| 7 | 7 | ``` pascal | |
| 8 | 8 | repeat | |
| 9 | 9 | statement1; | |
| 10 | 10 | // statement2; | |
| 11 | 11 | // further statements... | |
| 12 | 12 | until BooleanExpression; | |
| 13 | 13 | ``` | |
| 14 | 14 | ||
| 15 | - | ||
| 15 | + | A `repeat` loop encloses its executed statements, which means they do not need to be further enclosed in a begin ... end block. Note that a repeat loop continues until its controlling Boolean expression is `True`; whereas the `while` loop continues until its Boolean expression is `False`. | |
| 16 | 16 | ||
| 17 | 17 | For instance, the following `repeat` loop executes at least once: | |
| 18 | 18 | ||
| 19 | 19 | ``` pascal | |
| 20 | 20 | repeat | |
| 21 | 21 | WriteLn(Node.Text); | |
| 22 | 22 | Node := GetNextNode; | |
| 23 | 23 | until not Assigned(Node); | |
| 24 | 24 | ``` | |
| 25 | 25 | ||
| 26 | 26 | It assumes that Node is not Nil at the outset. If this assumption is incorrect, the code will fail, and the program may crash. | |
| 27 | 27 | ||
| 28 | 28 | A `while` loop is more defensive, since the needed check is performed before any loop statements are executed: | |
| 29 | 29 | ||
| 30 | 30 | ``` pascal | |
| 31 | 31 | while Assigned(Node) do | |
| 32 | 32 | begin | |
| 33 | 33 | WriteLn(Node.Text); | |
| 34 | 34 | Node := GetNextNode; | |
| 35 | 35 | end; | |
| 36 | 36 | ``` | |
| 37 | 37 | ||
| 38 | 38 | Use a `repeat` loop when the looping statement(s) must execute at least once, whatever the initial value of the controlling Boolean condition. | |
| 39 | 39 | ||
| 40 | 40 | It is not difficult to inadvertently write a Boolean expression controlling a repeat loop that never becomes True. This gives rise to an infinite or endless loop, and causes the program to hang. | |
| 41 | 41 | ||
| 42 | 42 | One programming style deliberately sets up an infinite loop, and inserts a `Break` or `Exit` instruction controlled by some condition evaluated in the middle of the loop to break out of the otherwise infinite loop: | |
| 43 | 43 | ||
| 44 | 44 | ``` pascal | |
| 45 | 45 | repeat | |
| 46 | 46 | statement1; | |
| 47 | 47 | if Condition then | |
| 48 | 48 | Break; | |
| 49 | 49 | statement_that_might_affect_the_Condition; | |
| 50 | 50 | until False; | |
| 51 | 51 | ``` | |
| 52 | 52 | ||
| 53 | 53 | Successful use of this style depends on Condition dependably becoming True, without exception, at some point. Otherwise you have built in an inescapable infinite loop. | |
| 54 | 54 | ||
| 55 | 55 | --- | |
| 56 | 56 | *From the Free Pascal wiki, page "Basic Pascal Tutorial/Chapter 3/REPEAT..UNTIL", as it stood on 2022-08-20 after a change by Kai Burghardt, out of the wiki's dump of 2024-05-26. Copyright stays with the author who wrote it, according to the old wiki's [[Licensing]] page. If you have concerns about this material, [message the admin](/wiki-contact).* |