Merge Sort C++ Incorrectly Sorting - c++

Question

I'm writing a program that will do 5 different sorting functions and compare times across them. I'm outputting the 1000th element of each array to see if it's sorted correctly, all of my other sorts except merge sort are producing the correct number. Merge sort is close but off, it is within an element or two of getting the correct answer (outputting 25011 rather than 25034 for the 1000th element.) Here is the code for my merge sort:
//Zach Heesaker. CS3. Project 4
#include <iostream>
#include <fstream>
#include <string>
#include <iomanip>
#include <stdio.h>
#include <ctime>
#include <cstdio>
#include <time.h>
#include <stdint.h>
#include<list>
#include<cmath>
using namespace std;
const int n = 10000;
int numswaps, numcompares;
void swap(int& a, int& b);
void print(int arr[], int n, ofstream& outf, double time);
void read(int arr[], int size);
void mergepass(int arr[], int y[], int& n, int& L);
void mergeSort(int arr[], int n);
void merge(int arr[], int y[], int L, int m, int n);
int main()
{
int numsorts = 5;
string whichsort;
ofstream outf("output.ot");
int arr[n + 1];
clock_t start, end;
double time;
outf << "Sort Name: " << setw(5) << " " << "1000th Element: " << setw(1) << " " << "Number of Moves: " << setw(1) << " " << "Time taken: " << endl;
read(arr, n);
numcompares = 0;
numswaps = 0;
start = clock();
mergeSort(arr, n);
end = clock();
time = double(end - start) / double(CLOCKS_PER_SEC);
}
void mergeSort(int arr[], int size)
{
int L = 1;
int y[n + 1];
while (L < n)
{
mergepass(arr, y, size, L);
L = 2 * L;
mergepass(y, arr, size, L);
L = 2 * L;
}
}
void merge(int arr[], int y[], int L, int m, int n)
{
int i, j, k, t;
i = L;
k = L;
j = m + 1;
while ((i <= m) && (j <= n))
{
numcompares++;
if (arr[i] <= arr[j])
{
numswaps++;
y[k++] = arr[i++];
}
else
{
numswaps++;
y[k++] = arr[j++];
}
}
if (i > m)
{
for (t = j; t <= n; t++)
{
numswaps++;
y[k + t - j] = arr[t];
}
}
else
{
for (t = i; t <= m; t++)
{
numswaps++;
y[k + t - i] = arr[t];
}
}
}
void mergepass(int arr[], int y[], int& n, int& L)
{
int i, t;
i = 1;
while (i <= n - 2 * L + 1)
{
merge(arr, y, i, i + L - 1, i + 2 * L - 1);
i = i + 2 * L;
}
if ((i + L - 1) < n)
{
merge(arr, y, i, i + L - 1, n);
}
else
{
for (t = i; t <= n; t++)
{
numswaps++;
y[t] = arr[t];
}
}
}
void swap(int& a, int& b)
{
int temp;
temp = a;
a = b;
b = temp;
numswaps += 3;
}
void print(int arr[], int n, ofstream& outf, double time)
{
outf << left << setw(6) << " " << left << arr[1000] << setw(12) << " " << left << numswaps << setw(10) << " " << left << "\t" << time << endl;
}
void read(int arr[], int size)
{
ifstream ifs("input.txt");
int i = 0;
while (!ifs.eof())
{
ifs >> arr[i];
i++;
}
}
int merge(int arr[], int left, int right)
{
int pivot = arr[right];
int k = (left - 1);
for (int j = left; j <= right - 1; j++)
{
numcompares++;
if (arr[j] < pivot)
{
k++;
swap(arr[k], arr[j]);
}
}
swap(arr[k + 1], arr[right]);
return (k + 1);
}
Any ideas as to what is going wrong in here? Thank you.