Example of a while loop that can't be written as a for loop

Question

I know a while loop can do anything a for loop can, but can a for loop do anything a while loop can?

Please provide an example.

Answer 1

If you have a fixed bound and step and do not allow modification of the loop variable in the loop's body, then for loops correspond to primitive recursive functions.

From a theoretical viewpoint these are weaker than general while loops, for example you can't compute the Ackermann function only with such for loops.

If you can provide an upper bound for the condition in a while loop to become true you can convert it to a for loop. This shows that in a practical sense there is no difference, as you can easily provide an astronomically high bound, say longer than the life of the universe.

Answer 2

Using C

The basic premise is of the question is that while loop can be rewritten as a for loop. Such as

init;
while (conditional) {
  statement;
  modify;
}

Being rewritten as;

for ( init; conditional; modify ) {
  statements;
}

The question is predicated on the init and modify statements being moved into the for loop, and the for loop not merely being,

init;
for (; conditional; ) {
  modify;
}

But, it's a trick question. That's untrue because of internal flow control which statements; can include. From C Programming: A Modern Approach, 2nd Edition you can see an example on Page 119,

n = 0;
sum = 0;
while ( n < 10 ) {
  scanf("%d", &i);
  if ( i == 0 )
    continue;
  sum += i;
  n++;
}

This can not be rewritten as a for loop like,

sum = 0;
for (n = 0; n < 10; n++ ) {
  scanf("%d", &i);
  if ( i == 0 )
    continue;
  sum += i;
}

Why because "when i is equal to 0, the original loop doesn't increment n but the new loop does.

And that essentially boils down to the catch,

Explicit flow control inside the while loop permits execution that a for loop (with internal init; and modify; statements) can not recreate.

Answer 3

While loop does not have as much flexibility as a for loop has and for loops are more readable than while loops. I would demonstrate my point with an example. A for loop can have form as:

for(int i = 0, j = 0; i <= 10; i++, j++){
// Perform your operations here.
}

A while loop cannot be used like the above for loop and today most modern languages allow a for each loop as well.

In my opinion, I could be biased please forgive for that, one should not use while loop as long as it is possible to write the same code with for loop.

Answer 4

Yes, easily.

while (cond) S;

for(;cond;) S;

Answer 5

In C-like languages, you can declare for loops such as this:

for(; true;)
{
    if(someCondition)
       break;
}

In languages where for is more strict, infinite loops would be a case requiring a while loop.

Answer 6

The while loop and the classical for loop are interchangable:

for (initializer; loop-test; counting-expression) {
    …
}

initializer
while (loop-test) {
    …
    counting-expression
}