c++ can't get selection sort to work properly

Question

I am trying to understand sorting algorithms, so based on googled examples/explanations I wrote the below code. Code works 80% of the time. Every once in a while it doesn't sort properly and I can not see why.

#include <iostream>
#include <ctime>
#include <cstdlib>

using namespace std;

void setArray( int *, const int & );
void selectionSorting( int *, const int & );

int main()
{
    int numOfElem;
    cout << "Num of array elements: ";
    cin >> numOfElem;
    cout << '\n';

    int array[numOfElem];
    setArray(array, numOfElem);

    selectionSorting(array, numOfElem);

    cout << '\n';
    return 0;
}

void setArray( int *array, const int & numOfElem ){
    srand(time(0));
    cout << "Original array: " << '\n';
    for (int i=0; i<numOfElem; i++){
        array[i] = rand()%30;
        cout << array[i] << ' ';
    }
    cout << '\n';
}

void selectionSorting( int *array, const int &numOfElem ){
    int eff_size, swap;
    int maxpos = 0;
    for (eff_size = numOfElem; eff_size>1; eff_size--){
        // loop searching for a position of largest number in the array
        for (int i=0; i<eff_size; i++){
            maxpos = array[i] > array[maxpos] ? i : maxpos;
        }
        swap = array[maxpos];
        array[maxpos] = array[eff_size-1];
        array[eff_size-1] = swap;
    }
    cout << "Selection Sorting: " << '\n';
    for (int i=0; i<numOfElem; i++){
        cout << array[i] << ' ';
    }
}

Example output:

Num of array elements: 5

Original array:
7 17 1 12 25
Selection Sorting:
1 7 17 25 12

I can't see any pattern to the sorting failing - it fails in different places, weather there are repeated numbers, regardless of how many numbers I provide etc...

Answer 1

On each iteration of the outer loop(over eff_size) you should re-set maxpos to 0. Otherwise you have the chance that maxpos goes out of the effective portion being sorted(this happens if the maximum element is last in the effective portion i.e. if maxpos==effsize).