confused about 'for'

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  1. #1
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    confused about 'for'

    Confused about output of following 2 sets of code. Postfix and Prefix notation.

    the way I understood book:

    Using Xcode 3.3 , gcc 4.2


    postfix notation passes on value to i before it increments.

    prefix notation increments before it passes value to i.
    -----------------------------------------------------------
    Postfix notation code
    -----------------------------------------------------------
    for (i = 0; i < 4; i++)

    printf( "first for: i=%d\n", i );

    printf( "After first for loop, i=%d.\n\n", i );
    ------------------------------------------------------------
    Postfix notation code output
    ------------------------------------------------------------
    first for: i=0
    first for: i=1
    first for: i=2
    first for: i=3
    After first for loop, i=4.
    -------------------------------------------------------------
    Prefix notation code
    -------------------------------------------------------------
    for (i = 0; i < 4; ++i)

    printf( "first for: i=%d\n", i );

    printf( "After first for loop, i=%d.\n\n", i );
    -------------------------------------------------------------
    Prefix notation code output
    -------------------------------------------------------------
    first for: i=0
    first for: i=1
    first for: i=2
    first for: i=3
    After first for loop, i=4.

    Why do both sets of code output the same?

    rj
    Last edited by rjreynolds; 07-06-2010 at 08:59 PM. Reason: ommisions

  2. #2
    ... kermit's Avatar
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    Here is another way of thinking about how your for loop works:

    Code:
        i = 0;
        while (i < 5) {
            printf("%d\n", i);
            i++;
        }
    Can you see why it would make no difference at all in your code as to whether you use pre or post increment?

    Anyway, if you want to compare the results you were getting with something more like you wanted, look at this code:

    Code:
    #include <stdio.h>
    
    int main(void)
    {
        int i;
        int x = 0;
        int y = 0;
    
        puts("");
    
        for (i = 0; i < 5; i++) {
            printf("%d\n", i);
        }
    
        puts("");
    
        for (i = 0; i < 5; ++i) {
            printf("%d\n", i);
        }
    
    
        puts("");
    
        for (i = 0; i < 5; i++) {
            printf("x = %d\ty = %d\n", x++, ++y);
        }
    
        puts("");
    
        return 0;
    }
    Last edited by kermit; 07-06-2010 at 09:08 PM.

  3. #3
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    Starting to clear up. After looking at your example and further reading, I a m starting to understand. It will require further study on my part. I want to make sure I have these concepts well understood before I move on. thanks again for your input....

    rj

  4. #4
    ... kermit's Avatar
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    You are definitely on the right track - it is just that how you increment it (either pre or post) only matters in certain contexts.

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