Question

Implement in C++ the Merge sorting algorithms we saw in class. Test above algorithms by varying the size of the array from 10 to 10000, and compute the elapsed time for the sorting algorithm according to the size of the array.

Answer 1

#include <iostream>
#include <fstream>
#include <chrono>
#include <unistd.h>
using std::ifstream;
using std::ofstream;
using std::cout;
using std::endl;

void fillArray(int * array, int size){
    int x;

    for(int i = 0; i <= size; i++){
        x = rand() % (10000 - 0) + 1;
        array[i] = x;
    }
}

void merge(int *array, int low, int high, int mid) {
    int i, j, k, temp[high-low+1];
    i = low;
    k = 0;
    j = mid + 1;

    // Merge the two parts into temp[].
    while (i <= mid && j <= high) {
        if (array[i] < array[j]) {
            temp[k] = array[i];
            k++;
            i++;
        }
        else {
            temp[k] = array[j];
            k++;
            j++;
        }
    }

    // Insert all the remaining values from i to mid into temp[].
    while (i <= mid) {
        temp[k] = array[i];
        k++;
        i++;
    }

    // Insert all the remaining values from j to high into temp[].
    while (j <= high) {
        temp[k] = array[j];
        k++;
        j++;
    }

    // Assign sorted data stored in temp[] to a[].
    for (i = low; i <= high; i++) {
        array[i] = temp[i-low];
    }
}

// A function to split array into two parts.
void mergeSort(int *array, int low, int high)
{
    int mid;
    if (low < high)
    {
        mid=(low+high)/2;
        // split the array into two halves
        mergeSort(array, low, mid);
        mergeSort(array, mid+1, high);

        // merge together the two sorted arrays
        merge(array, low, high, mid);
    }
}

int main() {

    /* initialize random seed: */
    srand (time(NULL));

    int n = 100000;

    for(int i = 0; i < 10; i++) {

        n += 10000;

        //initialize array of specified size of ints
        int * arrayPointer;

        arrayPointer = new int [n];

        //fill array from data.txt file
        fillArray(arrayPointer, n);

        double start = clock();

        mergeSort(arrayPointer, 0, n - 1);

        double stop = clock();

        double elapsed = ((stop - start) / CLOCKS_PER_SEC);

        cout << "Array: " << (i + 1) << endl;

        cout << "Size: " << n << endl;

        cout << "Elapsed sorting time: " << elapsed << " seconds" << endl;

        /*for(int j = 0; j < n; j++){
            cout << arrayPointer[j] << ' ';
        }

        cout << endl;*/

        delete arrayPointer;
    }

    return 0;
}