What is different between while and for loop in c plus plus?

Answer 1

Although both can achieve very similar loops, the one you use for a specific loop usually comes down to whichever seems more intuitive to you. In terms of performance, one method may be slightly more efficient than the other, however there are no general rules that can be applied; you must test each case by comparing the machine code or by physically timing the loops with a high performance event timer (HPET). In my experience, the performance difference is so negligible it is not worth the effort. Background activity has a far greater impact on performance.

Describing the differences between every possible loop you might create would take me a lifetime, but there are a couple of subtle differences that are worth pointing out.

Infinite Loops

for(;;){

} // infinite for loops do not require a condition.

while( true ){

} // infinite while loops MUST have a condition.

To break out of an infinite loop you must test for one or more conditions within the loop itself, and call break to exit the loop when a condition is met. You can also call continue to start the next iteration before reaching the end of the current iteration.

Stepping Loops

When stepping through a series of values, the for() loop is usually the better choice, especially if the step must occur AFTER each iteration. Consider the following similar loops:

for( int x=0; x<10; ++x ){

// if we place continue here, ++x will execute before the next iteration begins.

}

int x=0;

while( x<10 ){

// if we use a continue here, ++x will not execute. We could be here forever!

++x;

}

Another point to bear in mind here is that with the for()loop, x will fall from scope when the loop finishes. If x must remain in scope, initialise x before entering the for() loop. With the while() loop, x is still in scope when the loop finishes so if x must fall from scope, do not use the while() loop.

do...while Loops

The do...while loop is quite different from for()and while() loops, so it is worth mentioning here. Both for() and while() loops test a condition BEFORE the loop begins execution. If the condition is false, the loop may not execute at all. But a do...while loop is guaranteed to enter the loop AT LEAST once, because the condition is tested AFTER the loop begins, at the end of the loop just before it begins to iterate.

int x = 0;

do{

// Whatever you place here will execute at least once.

// If we place continue here, execution jumps to the while(condition) first.

}while(++x != 10)