Thread: quicksort program crashes without error

  1. #1
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    quicksort program crashes without error

    I'm trying to create a QuickSort algorithm but while I did have success in writing one that doesn't give me an error, the programm crashes after entering the unsorted numbers.

    I already searched but couldn't find anything, could someone help me?


    Code:
    void swap(int left, int right, int Numbers[]) {
        int temp = Numbers[left];
        Numbers[left] = Numbers[right];
        Numbers[right] = temp;
        return;
    }
     
    void quicksort(int Beginning, int End, int Numbers[]) {
        int pivot = Numbers[((Beginning+1)+End)/2];
        int left = Beginning-1;
        int right = End;
        do {
     
            while (Numbers[left] < pivot) {
                left++;
            }
            while (Numbers[right] < pivot) {
                right--;
            }
     
            if (right > left) {
                swap(left,right,Numbers);
                left++;
                right--;
            }
        } while (left <= right);
        if (Beginning < right) {
            quicksort(Beginning,right,Numbers);
        }
        if (End < left) {
            quicksort(End,left,Numbers);
        }
    }
     
    int main()
    {
        int End,Beginning,Dim,i;
        printf("Number of Elements: ");
        scanf("%d",&Dim);
        int Numbers[Dim];
        for ( i = 0 ; i < Dim ; i++ ) {
            printf("Enter the %d. element: ",i+1);
            scanf("%d",&Numbers[i]);
        }
        Beginning = 0;
        End = Dim-1;
        quicksort(Beginning,End,Numbers);
        printf("\ngsorted:");
        for ( i = 0 ; i < Dim ; i++ ) {
            printf("[%d]: %d",i+1,Numbers[i]);
        }
        return 0;
    }

  2. #2
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    Why do you think this is correct?

    Code:
    int left = Beginning-1;
    Tim S.
    "...a computer is a stupid machine with the ability to do incredibly smart things, while computer programmers are smart people with the ability to do incredibly stupid things. They are,in short, a perfect match.." Bill Bryson

  3. #3
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    Quote Originally Posted by stahta01 View Post
    Why do you think this is correct?

    Code:
    int left = Beginning-1;
    Tim S.
    Well, because I learned that you can declare a variable while setting a value on it and Beginning-1 should be 1 subtracted from the variable Beginning. Is it wrong?

  4. #4
    and the hat of int overfl Salem's Avatar
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    Well if you do beginning-1, when beginning is 0 to begin with, which array element do you think you'll access with an index of -1?
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  5. #5
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    Well seems like I found the first mistake. Thanks for that, I tried another way before (Beginning was set to 1) but then changed my mind to this way and forgot to remove the -1. Well, now I just have to find out why it's stuck in an endless loop

  6. #6
    and the hat of int overfl Salem's Avatar
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    This is where a debugger comes in handy.
    You can set breakpoints, examine variables and step the code one line at a time to really examine what your code is actually doing.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  7. #7
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    Thanks I finally fixed all the errors.

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