Merge sort example in C

main.c

#include<iostream>
#include "mergesort.c"
using namespace std;

int main(int argc, char** argv) {
    int num;
    cout << "How many numbers do you want to sort: ";
    cin >> num;
    int a[num];
    for (int i = 0; i < num; i++) {
        cout << (i + 1) << ": ";
        cin >> a[i];
    }

    // Start merge sort
    mergeSort(a, num);

    // Print the sorted array
    cout << endl;
    for (int i = 0; i < num; i++)
        cout << a[i] << " ";

    cout << endl;

    return 0;
}

merge-sort-with-inversion-count.c

void merge(int *arr, int size1, int size2, int *inversions) {
    int temp[size1+size2];
    int ptr1=0, ptr2=0;

    while (ptr1+ptr2 < size1+size2) {
        if (ptr1 < size1 && arr[ptr1] <= arr[size1+ptr2] || ptr1 < size1 && ptr2 >= size2)
            temp[ptr1+ptr2] = arr[ptr1++];

        if (ptr2 < size2 && arr[size1+ptr2] < arr[ptr1] || ptr2 < size2 && ptr1 >= size1) {
            temp[ptr1+ptr2] = arr[size1+ptr2++];
            *inversions += size1-ptr1;
        }
    }

    for (int i=0; i < size1+size2; i++)
        arr[i] = temp[i];
}

void mergeSort(int *arr, int size, int* inversions) {
    if (size == 1)
        return;

    int size1 = size/2, size2 = size-size1;
    mergeSort(arr, size1, inversions);
    mergeSort(arr+size1, size2, inversions);
    merge(arr, size1, size2, inversions);
}

mergesort.c

void merge(int *arr, int size1, int size2) {
    int temp[size1+size2];
    int ptr1=0, ptr2=0;

    while (ptr1+ptr2 < size1+size2) {
        if (ptr1 < size1 && arr[ptr1] <= arr[size1+ptr2] || ptr1 < size1 && ptr2 >= size2)
            temp[ptr1+ptr2] = arr[ptr1++];

        if (ptr2 < size2 && arr[size1+ptr2] < arr[ptr1] || ptr2 < size2 && ptr1 >= size1)
            temp[ptr1+ptr2] = arr[size1+ptr2++];
    }

    for (int i=0; i < size1+size2; i++)
        arr[i] = temp[i];
}

void mergeSort(int *arr, int size) {
    if (size == 1)
        return;  

    int size1 = size/2, size2 = size-size1;
    mergeSort(arr, size1);
    mergeSort(arr+size1, size2);
    merge(arr, size1, size2);
}

clear, thank you

Really nice code but it is missing detailed explanations, if anyone want to read step by step explanations then it could be found here http://www.hellgeeks.com/merge-sort/

This is wrong:

temp[ptr1+ptr2] = arr[ptr1++];

An equals sign is not a sequence point, so you have no way of telling if ptr1 is incremented before or after accessing temp[ptr1+ptr2].

An equal sign need not provide a sequence point. in arr[ptr1++] the value for the index is ptr1 before the side effect of the postfix operator is applied to the pointer. C11 Standard (draft n1570) §6.5.2.4 Postfix increment and decrement operators (2)