Thread: My code works... but I don't understand why. Can someone explain?

  1. #1
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    My code works... but I don't understand why. Can someone explain?

    Hi all. I'm new to C programming and I am having A LOT of trouble understanding nested for loops. Here is a program I wrote.
    Code:
    #include <stdio.h>
    
    int main(void){
        int userNum = 0;
        int i, j;
    
        printf("Enter an integer: ");   scanf("%d", &userNum);
        printf("\n");
    
        for(i = 0; i <= userNum; ++i){
            for(j = 1; j <= i; ++j){
                printf(" ");
            }
            printf("%d", i);
            printf("\n");
        }
        return(0);
    }
    The desired output is:
    0
    _1
    __2
    ___3...... and so on, up to the number that was entered. (the underscores should be spaces instead)

    I understand why it works for the first couple of iterations. But starting on the third line, why does the inside loop iterate twice, producing 2 spaces? Shouldn't 'j' increment and become equal to 'i', therefore only iterating once and printing one space?

    I'm just trying to get a better understanding of all this. Any kind of explanation would be greatly appreciated.
    Attached Files Attached Files
    • File Type: c test.c (270 Bytes, 27 views)
    Last edited by agamemn0n; 09-14-2016 at 09:58 AM.

  2. #2
    Registered User camel-man's Avatar
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    Please post your code in the respective code tags.
    Code:
    int get_random_number(void)
    {
       return 4; //chosen by fair dice roll.
                 //guaranteed to be random
    }

  3. #3
    Programming Wraith GReaper's Avatar
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    That happens because "++j" happens after everything else inside that loop. I don't know how to say it, so I'll show it:

    The following pieces of code are equivalent.
    Code:
    int i;
    
    for (i = 0; i < 10; i++) {
        // Code goes here
    }
    Code:
    int i;
    
    i = 0;
    while (i < 10) {
        // Code goes here
    
        i++;
    }
    Last edited by GReaper; 09-14-2016 at 09:54 AM.
    Devoted my life to programming...

  4. #4
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    Ok, I think I understand.
    So you're saying that '++j' happens AFTER '++i'? I was thinking that everything in the inner loop happened before moving back to the outer loop.

  5. #5
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    That the behavor of for-loops.
    Code:
    for ( initialization ; evaluation ; action )
    {
        loop-code;
    }
    initialization will be executed at the start of the loop.
    evaluation will be checked every time befor the loop-code runs.
    action will be executed every time the loop-code is finish.

    You can rebuild your for-loop with while():

    Code:
    …
        j = 1;           // initialization
        while (j <= i)   // evaluation
        {
            printf(" "); // loop-code (one or more lines)
            ++j;         // action (last line)
        }
    …
    Other have classes, we are class

  6. #6
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    I understand how for loops work on their own. It's when there is a for loop nested inside another for loop that I have an issue.

    In my code above, 'i' starts at zero and 'j' starts at one. During the first iteration the inner loop is ignored because the loop condition is false. During the second iteration (after 'i' has been incremented) the inner loop will also run, printing a space, then incrementing 'j' to 2, then printing the number 1, then incrementing 'i' to 2 also. After this, since 'i' and 'j' are now equal, why would the inner loop run twice, printing 2 spaces? I know that this is super confusing but I'm hoping there's someone that can get what I'm asking and explain it.

  7. #7
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    Quote Originally Posted by agamemn0n View Post
    After this, since 'i' and 'j' are now equal, why would the inner loop run twice, printing 2 spaces?
    In every iteration of the outer loop, the inner loop will start new! This means, the initialization will be executed.
    So, the inner loop starts again and initialize j with 1!
    Other have classes, we are class

  8. #8
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    Ahhh yes! It all makes sense now. Thank you so much friend.

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