Tricky for loop

This is a discussion on Tricky for loop within the C Programming forums, part of the General Programming Boards category; I found a piece of code a while ago with a for loop that I don't understand. The purpose is ...

  1. #1
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    Tricky for loop

    I found a piece of code a while ago with a for loop that I don't understand. The purpose is counting the number of bits.

    Code:
    #include <stdio.h>
    
    int main()
    {
      unsigned int i;
      unsigned int v = 14;
    
      for (i=0; v; v>>=1) {
        i+= v & 1;
      }
    
      printf("# bits %d\n",i);
    
      return 0;
    }

    The question I have is what is happening here?
    Especially the code in the "for" statement, (some shifting):

    Code:
    v>>=1
    and later on (ANDing one bit or whole vector?):
    Code:
    i+= v & 1;

    I have no problem understanding bits and bytes. The result here is of course 3 bits.

  2. #2
    and the hat of int overfl Salem's Avatar
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    Perhaps print the value in v each time around the loop (in binary for extra points), and also print it when the loop exits.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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    I have of course done this. However I still have problem understanding the "v>>=1".

  4. #4
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by hzcodec
    However I still have problem understanding the "v>>=1".
    Read up on bit shifting.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

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    Well ... "v>>1" is shifting one step but what is the "=" sign doing in the for loop?

  6. #6
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by hzcodec
    Well ... "v>>1" is shifting one step but what is the "=" sign doing in the for loop?
    You may be familiar with syntax like:
    Code:
    x += 2;
    as shorthand for:
    Code:
    x = x + 2;
    The same idea applies here.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

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    Oh yes! I'm familiar with the shorthands. And with a little a bit of experimenting with the code I finally got it. Just confusing myself. laserlight got me thinking in the right direction. Thank you for your help.

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