help me to fix this merge sort code in c++

Question

what is the problem with this code and how can i fix it ?
*******************************
#include "stdafx.h"
#include <iostream>
using namespace std;

// merge the two halves into a sorted array
void Merge(int *a, int from, int to, int mid) {
int i = from;
int j = mid + 1;
int k = 0;

int temp[to - from + 1];

// merge the two parts into temp[]
while (i <= mid && j <= to) {
if (a[i] < a[j]) {
temp[k] = a[i];
k++; i++;
}
else {
temp[k] = a[j];
k++; j++;
}
}
// insert all the remaining values from i to mid into temp[]
while (i <= mid) {
temp[k] = a[i];
k++; i++;
}
// insert all the remaining values from j to to into temp[]
while (j <= to) {
temp[k] = a[j];
k++; j++;
}
// assign sorted data stored in temp[] to a[]
for (i = from; i <= to; i++) {
a[i] = temp[i - from];
}
}

void MergeSort(int *a, int from, int to) {
int mid;
if (from < to) {
mid = (from + to) / 2;

MergeSort(a, from, mid);
MergeSort(a, mid + 1, to);

Merge(a, from, to, mid);
}
}

int main() {
int n = 10; // number of elements
int arr[10] = { 10, 54, 63, 6, 420, 11, 41, 32, 17, 22 };

MergeSort(arr, 0, n - 1);

// print the array
for (int i = 0; i < n; i++)
cout << arr[i] << " ";

return 0;
}
**********************
the vs2017 say that the error is at line 9
int temp[to - from + 1];
the error is with " to "
expression must have a constant value

Answer 1

Yes, the value inside [here] must be known before the program runs.
You can use a std::vector or change it to:
int* temp = new int[to - from + 1];
But you must delete[] the int* when you don't need it anymore.

Answer 2

#include <iostream> using namespace std; // merge the two halves into a sorted array void Merge(int *a, int from, int to, int mid) { int i = from; int j = mid + 1; int k = 0; int temp[to-from+1]; // merge the two parts into temp[] while (i <= mid && j <= to) { if (a[i] < a[j]) { temp[k] = a[i]; k++; i++; } else { temp[k] = a[j]; k++; j++; } } // insert all the remaining values from i to mid into temp[] while (i <= mid) { temp[k] = a[i]; k++; i++; } // insert all the remaining values from j to to into temp[] while (j <= to) { temp[k] = a[j]; k++; j++; } // assign sorted data stored in temp[] to a[] for (i = from; i <= to; i++) { a[i] = temp[i-from]; } } void MergeSort(int *a, int from, int to) { int mid; if (from < to) { mid=(from+to)/2; MergeSort(a, from, mid); MergeSort(a, mid+