19.6.6 Omitted for-Expressions

A fully-fleshed for statement contains all these parts,

for (start; continue-test; advance)
  body

but you can omit any of the three expressions inside the parentheses. The parentheses and the two semicolons are required syntactically, but the expressions between them may be missing. A missing expression means this loop doesn’t use that particular feature of the for statement.

Instead of using start, you can do the loop preparation before the for statement: the effect is the same. So we could have written the beginning of the previous example this way:

int i = 0;
for (; i < n; ++i)

instead of this way:

int i;
for (i = 0; i < n; ++i)

Omitting continue-test means the loop runs forever (or until something else causes exit from it). Statements inside the loop can test conditions for termination and use ‘break;’ to exit. This is more flexible since you can put those tests anywhere in the loop, not solely at the beginning.

Putting an expression in advance is almost equivalent to writing it at the end of the loop body; it does almost the same thing. The only difference is for the continue statement (see continue Statement). So we could have written this:

for (i = 0; i < n;)
  {
    …
    ++i;
  }

instead of this:

for (i = 0; i < n; ++i)
  {
    …
  }

The choice is mainly a matter of what is more readable for programmers. However, there is also a syntactic difference: advance is an expression, not a statement. It can’t include loops, blocks, declarations, etc.