Question

for(; max != 0 ; max/=10, pow10*=10)

Pulled this exerpt from the wiki page on “Radix sort.” I’ve not seen a loop that starts with a semicolon like that before. The compiler didn’t catch it so I’m assuming it’s legal. Can anyone explain?

Also, as I’ve only written fairly simple loops I didn’t realize that you could make multiple assignments (right word?) like “max/=10, pow10*=10” at the end of the for() statement … Is there any limit to this? Bad form?

Thanks guys! (and pardon my potentially wrong vocabulary…it’s early, I need more coffee…)

Answer 1

It is perfectly legal. This is a for loop with an empty initialization block.

The most typical for loop looks something like this:

for (int i = 0; i < size; i++) {
  ...
}

This is (almost) equivalent to the following:

int i = 0;
for ( ; i < size; ) {
  ...
  i++;
}

(Except that in the first case the scope of i is limited to within the loop, while in the second case, i is still visible after the loop, as its scope is the enclosing block.)

For loops have three blocks, separated by semicolons: initialization, loop condition and loop increment. Any or all of these can be empty. So this is legal too (resulting in an endless loop):

for(;;)