Thread: Bidirectional loop? (Increase/decrease depending on values)

  1. #1
    Registered User
    Join Date
    Dec 2010

    Bidirectional loop? (Increase/decrease depending on values)

    I have two values: x_start and x_end. From time to time any value can be the highest.
    How can I make a single for or while-loop to iterate through x in any directions to count either from x_start to x_end or the other way around?

    Of course I can do it with an if statement and two for loops, but since I'm gonna fill the loop with lots of stuff, I'd like to smack it into one loop. Is that possible?

    int step = 2;     // Can vary
    int x_start = 10; // This could be 20
    int x_end = 20;   // And this could be 10
    for (int x = x_start; x<=x_end; x+=step) {
      printf ("x = %d\n", x);

  2. #2
    Registered User
    Join Date
    Nov 2010
    Long Beach, CA
    Yep. Have a second set of variables, called smallest and largest. Use the classic
    int smallest = x_start < x_end ? x_start : x_end;
    // similar for largest
    for (int x = smallest; x <= largest; x += step)

  3. #3
    and the Hat of Guessing tabstop's Avatar
    Join Date
    Nov 2007
    Or, if you have to start at start and stop at end (regardless of whether that's up or down), then you do the same sort of ?: trickery, except making step positive or negative.

  4. #4
    Algorithm Dissector iMalc's Avatar
    Join Date
    Dec 2005
    New Zealand
    Either make step negative when x_end < x_start, or put the body of the loop into its own function, and just go with two loops.
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  5. #5
    Registered User
    Join Date
    Dec 2010
    Thanks a lot, that worked out great. Have coded PHP for 10 years, never had to use that before. But now when coding a game, that was handy

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