How expensive is modulus in a loop?

This is a discussion on How expensive is modulus in a loop? within the C Programming forums, part of the General Programming Boards category; Code: putchar("rwx"[i % 3]); You might recognise that from this FAQ. It is part of a bit of code to ...

  1. #1
    ... kermit's Avatar
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    How expensive is modulus in a loop?

    Code:
    putchar("rwx"[i % 3]);
    You might recognise that from this FAQ. It is part of a bit of code to list the permission settings on a given file under a *nix type system.

    Now I implemented some code that was not as pretty - it basically was a string of 9 if statements (for the permission bits) which assigned the appropriate char to an array of char. Then I could print the string out when needed.

    As I was mentally comparing these two ways of printing the permission bits of a file, I was wondering how expensive it would be to use modulus like that in a loop. Would it ever make a noticeable difference, or is it something not worth worrying about?

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    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by kermit View Post
    As I was mentally comparing these two ways of printing the permission bits of a file, I was wondering how expensive it would be to use modulus like that in a loop. Would it ever make a noticeable difference, or is it something not worth worrying about?
    For the loop you're talking about, I wouldn't worry. In general though, a modulus is one of the most expensive integer operations. The answer to your question depends on the complexity of the rest of the code in the loop.
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

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