CODEWITHSUNDEEP
c++mergesort
Justin
Justin

Reputation: 111

What am I doing wrong in this C++ mergesort?

The program compiles and runs properly. A list of integers is read from an input file, but the output displays those numbers without any changes. I expect them to be sorted from least to greatest. For reference, I am trying to implement a version similar to the example on wikipedia.

// arrA contains items to sort; arrB is an array to work in
void mergesort(int *arrA, int *arrB, int first, int last) {

    // a 1 element array is already sorted
    // make increasingly longer sorted lists
    for (int width = 1; width < last; width = 2 * width) {
        // arrA is made up of 1 or more sorted lists of size width        
        for (int i = 0; i < last; i += 2 * width) {
            // merge two sorted lists
            // or copy arrA to arrB if arrA is full
            merge(arrA, i, min(i+width, last), min (i + 2 * width,
                    last), arrB);
        } // end for
        // now arrB is full of sorted lists of size 2* width
        // copy arrB into arrA for next iteration
        copy(arrB, arrA, last);
    } // end for    
} // end mergesort


void merge(int *arrA, int iLeft, int iRight, int iEnd, int *arrB) {
    int i0 = iLeft, i1 = iRight;

    // while either list contains integers
    for (int j = iLeft; j < iEnd; j++) {
        // if 1st integer in left list is <= 1st integer of right list
        if (i0 < iRight && (i1 >= iEnd || arrA[i0] <= arrA[i1])) {
            arrB[j] = arrA[i0];
            i0 += 1;
        } // end if
        else { // right head > left head
            arrB[j] = arrA[i0];
            i0 += 1;
        } // end else        
    } // end for
} // end merge


void copy(int *origin, int *destination, int size) {
    for (int i = 0; i < size; i++) {
        destination[i] = origin[i];
    } // end for 
} // end copy

int main() {
   int size = 0, first = 0, *arrA, *arrB;

   // input data
   read(&arrA, &arrB, &size);

   // sorting
   mergesort(arrA, arrB, first, size);

   // output
   write(arrA, first, size);

   // cleanup
   delete [] arrA;
   delete [] arrB;
}

input

33 9 -2

output

33 9 -2

Upvotes: 0

Views: 118

Answers (1)

Mats Petersson
Mats Petersson

Reputation: 129314

I haven't looked very deeply at your code, but this if-statement seems a bit off to me:

    if (i0 < iRight && (i1 >= iEnd || arrA[i0] <= arrA[i1])) {
        arrB[j] = arrA[i0];
        i0 += 1;
    } // end if
    else { // right head > left head
        arrB[j] = arrA[i0];
        i0 += 1;
    } // end else

Surely, the whole point of a pair of if/else clauses is that you do different things in the if vs. the else part. As far as I can tell, it's identical here.

Upvotes: 5

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