While loop coding pattern

Question

Suppose I have this loop:

boolean modified = true;
while(modified == true){
    modified = false;
    for (Object o : objectList){
        // do logic
        if (... condition...){
            modified = true;
        }
    }
}

I don't like that I initially have to declare the modified variable as true. The loop should terminate if in a round no more modifications have occured.

In Java the `break` keyword serves this purpose, although its use is controversial, as it has a definite "procedural" smell to it, and obscures the control flow. What you just wrote is quite common, and its common use makes its purpose clear to the reader. — MikaelF, Oct 13 '19 at 15:20
@JaredFarrish I think what OP was looking for is a way of omitting the declaration of `modified`. — MikaelF, Oct 13 '19 at 15:21
This question is lacking a language tag; solutions may differ according to language (Java vs C++ vs Javascript). — MikaelF, Oct 13 '19 at 15:24
@JaredFarrish I see your point; however, at least in C++ and Java, this [won't](https://stackoverflow.com/questions/18541304/use-variables-declared-inside-do-while-loop-in-the-condition) [work](https://stackoverflow.com/questions/16533859/accessing-a-variable-from-inside-a-do-while-loop). In these languages, `modified` has to be *declared* outside the loop, but *assigned* a value inside the loop. — MikaelF, Oct 13 '19 at 15:30
I edited the question. The modified attribute comes from a nested loop. So breaking is not possible. — Andrew, Oct 13 '19 at 15:38
@Serberker It is in fact possible in Java, using [loop labels](https://stackoverflow.com/questions/3821827/loop-in-java-code-what-is-this-and-why-does-it-compile). As previously stated, this usage is not recommended in Java, as it tends to obscure the control flow. It is however necessary in some algorithms. — MikaelF, Oct 13 '19 at 15:40

Sazzad Hissain Khan · Accepted Answer · 2019-10-13T15:51:38.597

5

If I would have been you, I would write the logic like below.

boolean modified = true;
while (modified) {
    for (Object o : objectList){
        // do logic
        modified = (modified && condition);
    }
}

edited Oct 13 '19 at 15:51

answered Oct 13 '19 at 15:21

Sazzad Hissain Khan

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@Serberker Do while is the best option for your case! – thechaoticpanda Oct 13 '19 at 15:36

MikaelF · Answer 2 · 2019-12-17T19:02:53.190

Here's a construct that will keep modified within the loop's scope, but still requires a declaration:

for (bool modified = false; modified; ) {
    modified = false;
    for (Object o : objectList) {
        modified |= condition; 
        /* If "condition" involves heavy computation, prefer the type of
           condition-checking in @Sazzad's answer, as this will not
           short-circuit to "true" if "modified" is already true. */
    }
}

Alternately, use a while loop with a label (although labels don't really belong to the OO style):

outer:
while(true) {
    for (Object o : objectList) {
        //do stuff
    }
    if(objectList.stream().anyMatch(/* condition predicate */)) {
        break outer;
    }
}

This will work with however many levels of nesting you have.

Here's a third solution using a do-while loop:

do {
    for (Object o : objectList) {
        //do stuff    
    }
} while (objectList.stream.anyMatch(/* predicate */));

Depending on what your condition looks like, you can make the check pretty obvious using a method reference:

objectList.stream().anyMatch(this::condition);

While loop coding pattern

2 Answers2