Merge sort in C giving wrong results - c

Question

I wrote a merge sort program but I got wrong results.
I've seen other programs like this, but they don't help me solve my problem. I think the problem is in the merge function.
#include <stdio.h>
#include "stdafx.h"
#define Size 5
//this is the array
int arr[Size] = { 5, 4, 3, 2, 1 };
int sr[10];
void mergesort(int a[], int start, int end, int size);
void merge(int a[], int start, int end, int size);
int main(void) {
mergesort(arr, 0, 4, 5);
for (int i = 0; i < Size; i++) {
printf_s("%i", sr[i]);
}
printf_s("\n");
return 0;
}
void mergesort(int a[], int start, int end, int size) {
if (size < 2)
return;
int s = size / 2;
mergesort(a, start, (start + end) / 2, s);
mergesort(a, (start + end) / 2, end, s);
merge( a, start, end, s);
}
void merge(int a[], int start, int end, int size) {
int left = start;
int right = ((start + end) / 2) + 1;
for (int i = 0; i < size; i++) {
if (left < (start + end)/2) {
if (right >= end) {
sr[i] = arr[left];
left++;
} else
if (arr[left] < arr[right]) {
sr[i] = arr[left];
left++;
} else {
sr[i] = arr[right];
right++;
}
} else {
sr[i] = arr[right];
right++;
}
}
}

Answer 1

(1)
printf_s("%i",sr[i]); should be printf_s("%i ", arr[i]);
(2)
mergesort(a, start, (start + end) / 2, s);//E.g index:{0,1,2,3,4}, start:0, (start + end) / 2 : 2, s: 2, but 0(start),1,2(new end), this length is 3, not 2
mergesort(a, (start + end) / 2, end, s);//Duplicate start position and length should be size - s. E.g size:5, s:2, rest size is 3, not 2.
merge( a, start, end, s);//s should be size
should be like
mergesort(a, start, start + s - 1, s);
mergesort(a, start + s, end, size - s);
merge(a, start, end, size);
(3)
Change according to (2)
Change int right = ((start + end) / 2) +1; to int right = start + size / 2;.
(4)
Add int sr[size]; //Avoid using global variables. It is better to use malloc. E.g int *sr = malloc(size*sizeof(int));...free(sr);
(5)
if (left < (start+end)/2)
{
if (right >= end)
should be
if (left < start + size / 2)
{
if (right > end){//Should be >, not >=
(6) Write back to arr form sr is necessary
Whole code:
#include <stdio.h>
#include <stdlib.h>
void mergesort(int a[], int start, int end, int size);
void merge(int a[], int start, int end, int size);
int main(void){
int arr[] = {5,4,3,2,1};
int size = sizeof(arr)/sizeof(*arr);
mergesort(arr, 0, size - 1, size);
for (int i = 0; i < size; i++){
printf_s("%i ", arr[i]);
}
printf_s("\n");
return 0;
}
void mergesort(int a[], int start, int end, int size){
if (size < 2)
return;
int s = size / 2;
mergesort(a, start, start + s - 1, s);
mergesort(a, start + s, end, size - s);
merge(a, start, end, size);
}
void merge(int a[], int start, int end, int size){
int left = start;
int right = start + size / 2;
int right_start = right;
int *sr = (int*)malloc(size*sizeof(*sr));//Cast(int*) is not necessary in C.
for (int i = 0; i < size; i++){
if (left < right_start){
if (right > end){
sr[i] = a[left++];
} else if (a[left] < a[right]) {
sr[i] = a[left++];
} else {
sr[i] = a[right++];
}
} else {
sr[i] = a[right++];
}
}
for(int i = 0; i < size; ++i)//write back.
a[start + i] = sr[i];
free(sr);
}

Answer 2

Your code is invalid for multiple reasons:
mergesort splits the range into 2 halves of size size / 2, which is incorrect if size is not even.
the arguments to mergesort are incorrect, only the pointer and the size are needed.
the merge function gets values from the global array arr instead of the argument array and stores values into the global temporary array sr, but does not copy it back into the a array.
Here is a corrected and simplified version:
#include <stdio.h>
void mergesort(int a[], int size);
int main(void) {
int arr[] = { 5, 4, 3, 2, 1 };
int size = sizeof(arr) / sizeof(arr[0]);
mergesort(arr, size);
for (int i = 0; i < size; i++) {
printf_s("%i ", arr[i]);
}
printf_s("\n");
return 0;
}
void merge(int a[], int mid, int size) {
int sr[mid]; // temporary array for the left part
if (a[mid - 1] <= a[mid]) { // quick check for sorted case
return;
}
for (int i = 0; i < mid; i++) { // save left part
sr[i] = a[i];
}
// merge into array `a`.
for (int i = 0, left = 0, right = mid; left < mid; i++) {
if (right == size || sr[left] <= a[right]) {
a[i] = sr[left++];
} else {
a[i] = a[right++];
}
}
}
void mergesort(int a[], int size) {
if (size >= 2) {
int mid = (size + 1) / 2; // make left part no smaller than right part
mergesort(a, mid);
mergesort(a + mid, size - mid);
merge(a, mid, size);
}
}