Dont understand this syntax

Question

Given this loop, why is there a semi colon at the end?

for(s = string; *s == ' '; s++)
    ;

thanks

edit * so is it possible to reverse this procedure so it starts at the end of a string and checks for a space and decreases until it finds a charachter?

Answer 1

It causes the loop to do nothing. All that happens is that the pointer is advanced.

Answer 2

The semicolon means that nothing is done inside the loop. In your code the loop just loops until the character in the string is not a space. That is, after that line s points to the first character in the string that is not a space.

Answer 3

The semicolon makes an empty statement in the loop. This loop searches for the first non-blank char in string.

Answer 4

It is an empty statement, which is a no-op in C. It's as if you had said:

for(s = string; *s == ' '; s++)
{
    // do nothing
}

You use this when everything can be done in the for( ... ) construct itself - the C grammar requires that there be a controlled statement, but there is nothing for it to do.

Answer 5

There has to be a statement or block as the "body" of the for loop. This is what is executed each time through the loop (as long as s is still pointing to a space).

A semicolon on its own is the empty statement -- that is, nothing happens in the loop body, just the s++ on the for loop line.

Answer 6

Because in C (and others) for grammar is: for (init; condition; step) body

Where body can be a closure ( a block of code in {}), but body can also be a empty with ;

Answer 7

I'd like to point out that the loop could be written like this to make it more explicit what is going on and increase readability:

 for(s = string; *s == ' ';)
     s++;

Or using a while loop:

 s = string;
 while(*s == ' ')
     s++;

But I think the first approach with the empty body is more "c-idiomatic" (and harder to read).