I dont understand this step particularly in mergesort [duplicate] - c

Question

This question already has answers here:
How does the Pseudocode for merge sort work?
(2 answers)
Closed 6 years ago.
In the above code i dont understand this. it calls merge sort again but when do merge come into play as the function repeatedly calls mergesort
but when do merge happens
The code i could not understand is recursive one that how after breaking into halves and repeated calling of mergesort and breaking inrtoa single elment merge comes into play.
REAL QUESTION WAS HOW REAL COMES INTO PLAY
#include <stdio.h>
#include <stdlib.h>
#define MAX_SIZE 50000
void merge (int arr[], int start, int mid, int end){
/* two sorted arrays arr[start ... mid]
and arr[mid+1 ... end] merge them */
int tempvals[MAX_SIZE];
int ptr1, ptr2, ptrnew=0, idx;
ptr1 = start;
ptr2 = mid+1;
/* compare both the sorted arrays element by element
move the element that is smaller to the new aray */
while ( (ptr1 <= mid) && (ptr2 <= end)){
/* compare elements at ptr1 and ptr2 */
if (arr[ptr1] < arr[ptr2]){
tempvals[ptrnew] = arr[ptr1];
ptr1 ++; ptrnew++;
} else {
tempvals[ptrnew] = arr[ptr2];
ptr2 ++; ptrnew++;
}
}
/* at this point --> one of the pointers has reached the end */
while (ptr1 <= mid){
tempvals[ptrnew] = arr[ptr1];
ptr1 ++; ptrnew++;
}
while (ptr2 <= end){
tempvals[ptrnew] = arr[ptr2];
ptr2 ++; ptrnew++;
}
/* transfer back to arr */
for (idx = start, ptrnew = 0; idx <= end; idx++, ptrnew++)
arr[idx] = tempvals[ptrnew];
}
void merge_sort (int arr[], int start, int end){
int mid = (start + end)/2;
/* termination condition */
if( start >= end) return;
/* recurse */
merge_sort(arr,start, mid); /* first half */
merge_sort (arr, mid+1, end); /* second half */
merge (arr, start, mid, end); /* do the merging */
}
void print_array (int arr[], int size){
int i;
for (i=0; i < size; i++)
printf ("%d ",arr[i]);
printf ("\n");
}
int main(){
int arr[MAX_SIZE];
int i;
/* initialize */
for (i=0; i < MAX_SIZE; i++)
arr[i] = rand();
/* sort and print */
merge_sort (arr,0, MAX_SIZE - 1);
}
Blockquote
In the above code i dont understand this
it calls merge sort again but when do merge come into play as the function repeatedly calls mergesort
/* recurse */
merge_sort(arr,start, mid); /* first half */
merge_sort (arr, mid+1, end); /* second half */
merge (arr, start, mid, end); /* do the merging */

Answer 1

It's under the comment recurse, and called out in the line-by-line comments. You recur on merge_sort to sort the left part of the array, the right part. Then you call merge to reunite them, now in order.
You do not call merge_sort forever: note the base case (a.k.a. termination condition) at the top of merge_sort:
if( start >= end) return;
When you get to a list of no more than one element, you simply return -- it's already in order. Then you crawl back up that large call chain, merging single elements, then larger lists, ... until you're done.