for loop missing initialization

Question

I've seen

for(;;)

and

for ( ; *s != '\0'; s++)

Why is it blank like that. Thanks.

Answer 1

The for statement works like:

for (initialization; test-condition; update)

And any or all of those three can be omitted (left blank). So:

• for (;;) is an infinite loop¹ equivalent to while (true) because there is no test condition. In fact, for (int i=0; ;i++) would also be an infinite loop¹.

• for ( ; *s != '\0'; s++) is a loop with no initialization. s will point to the beginning of (probably) a string and is incremented until it reaches the null character '\0' denoting end-of-string. This essentially means loop through all characters of the string s

¹ The loop will still be interrupted if there's a break statement in the loop body, or a call to exit(), etc...

Answer 2

It is "blank like that" because the author of the code left it blank. The author did not want/need to do anything in the corresponding section of for statement, so it was left blank.

for (;;) is a statement that iterates indefinitely (unless it is interrupted from inside cycle body).

for ( ; *s != '\0'; s++) is a statement that does not need an initialization section, since everything necessary (like the initial value of s) was already initialized before that for statement.