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incrementation inside for loop

This is a discussion on incrementation inside for loop within the C Programming forums, part of the General Programming Boards category; Code: int i, j = 0; for (i = 0; i++ < 10; ++i, j++); j += i; printf("%d %d", ...

  1. #1
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    incrementation inside for loop

    Code:
    int i, j = 0;
    for (i = 0; i++ < 10; ++i, j++);
    j += i;
    printf("%d %d", i, j);
    The output is
    Code:
    11 16
    . I've tried modifying the code to
    Code:
    int i, j = 0;
    for (i = 0; i++ < 10; ++i, j++);
    {
          j += i;
          printf("%d %d", i, j);
    }
    and the output is still the same. Does this imply the for loop is executed only once? Can anyone explain to me how the above code works please, thanks!

  2. #2
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    It's the same because you follow both for() headers with ; -which is basically a for loop with a single NULL statement, it does nothing during the loop.

    So in both cases, the j+= i; and printf() statements don't happen until the loop exits.
    drawar likes this.

  3. #3
    Cat
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    Code:
    int i, j = 0;
    for (i = 0; i++ < 10; ++i, j++);
    j += i;
    printf("%d %d", i, j);
    Line 2 executes 5 times, but because of the semicolon, the loop only executes the i++, ++i, and j++ expressions. After 5 iterations, i is 11 and j is 5, and the loop terminates.

    Line 3 then adds i to j giving 11 and 16, and line 4 prints.
    drawar likes this.
    You ever try a pink golf ball, Wally? Why, the wind shear on a pink ball alone can take the head clean off a 90 pound midget at 300 yards.

  4. #4
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    Ah I see, thank you all!
    So in the first iteration, the original value of i(0) is used to do the comparison, after that it's incremented by 1, and since the condition is true it's incremented by 1 again?

    I've also tried changing the position of '++' in the for loop like for(i = 0; ++i < 10; ++i, j++); or for(i = 0; i++ < 10; i++, j++); and still get the same output as before. Why is that so?

  5. #5
    Registered User whiteflags's Avatar
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    Code:
    for(i = 0; ++i < 10; ++i, j++);
    You only need the semicolon if the loop is not supposed to have a body. Even then, you should put the semicolon on the next line, so people notice it.
    drawar likes this.

  6. #6
    Cat
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    Quote Originally Posted by drawar View Post
    Ah I see, thank you all!
    So in the first iteration, the original value of i(0) is used to do the comparison, after that it's incremented by 1, and since the condition is true it's incremented by 1 again?

    I've also tried changing the position of '++' in the for loop like for(i = 0; ++i < 10; ++i, j++); or for(i = 0; i++ < 10; i++, j++); and still get the same output as before. Why is that so?
    There actually is a slight difference internally, but the output is the same.


    for(i = 0; ++i < 10; ++i, j++);
    for(i = 0; i++ < 10; i++, j++);

    The expressions in blue are equivalent - the only time that i++ vs. ++i matters is if the result is being used in another expression, and these are not.

    The expressions in red actually are different, but they produce the same final result because each iteration increments twice:

    With ++i, you make the following comparisons:
    1 < 10 - True
    3 < 10 - True
    5 < 10 - True
    7 < 10 - True
    9 < 10 - True
    11 < 10 - False

    With i++ you make the following:
    0 < 10 - True
    2 < 10 - True
    4 < 10 - True
    6 < 10 - True
    8 < 10 - True
    10 < 10 - False

    In both cases, the comparison happens six times and the inner body of the loop happens five times, which is why i is incremented 11 times. You WOULD notice a difference if the number being compared to was odd, not even, or if you used a <= rather than a <
    Last edited by Cat; 02-24-2013 at 11:17 AM.
    You ever try a pink golf ball, Wally? Why, the wind shear on a pink ball alone can take the head clean off a 90 pound midget at 300 yards.

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