Skip to content

Instantly share code, notes, and snippets.

@mycodeschool
Last active December 13, 2022 15:02
Embed
What would you like to do?
/* Merge sort in C */
#include<stdio.h>
#include<stdlib.h>
// Function to Merge Arrays L and R into A.
// lefCount = number of elements in L
// rightCount = number of elements in R.
void Merge(int *A,int *L,int leftCount,int *R,int rightCount) {
int i,j,k;
// i - to mark the index of left aubarray (L)
// j - to mark the index of right sub-raay (R)
// k - to mark the index of merged subarray (A)
i = 0; j = 0; k =0;
while(i<leftCount && j< rightCount) {
if(L[i] < R[j]) A[k++] = L[i++];
else A[k++] = R[j++];
}
while(i < leftCount) A[k++] = L[i++];
while(j < rightCount) A[k++] = R[j++];
}
// Recursive function to sort an array of integers.
void MergeSort(int *A,int n) {
int mid,i, *L, *R;
if(n < 2) return; // base condition. If the array has less than two element, do nothing.
mid = n/2; // find the mid index.
// create left and right subarrays
// mid elements (from index 0 till mid-1) should be part of left sub-array
// and (n-mid) elements (from mid to n-1) will be part of right sub-array
L = (int*)malloc(mid*sizeof(int));
R = (int*)malloc((n- mid)*sizeof(int));
for(i = 0;i<mid;i++) L[i] = A[i]; // creating left subarray
for(i = mid;i<n;i++) R[i-mid] = A[i]; // creating right subarray
MergeSort(L,mid); // sorting the left subarray
MergeSort(R,n-mid); // sorting the right subarray
Merge(A,L,mid,R,n-mid); // Merging L and R into A as sorted list.
free(L);
free(R);
}
int main() {
/* Code to test the MergeSort function. */
int A[] = {6,2,3,1,9,10,15,13,12,17}; // creating an array of integers.
int i,numberOfElements;
// finding number of elements in array as size of complete array in bytes divided by size of integer in bytes.
// This won't work if array is passed to the function because array
// is always passed by reference through a pointer. So sizeOf function will give size of pointer and not the array.
// Watch this video to understand this concept - http://www.youtube.com/watch?v=CpjVucvAc3g
numberOfElements = sizeof(A)/sizeof(A[0]);
// Calling merge sort to sort the array.
MergeSort(A,numberOfElements);
//printing all elements in the array once its sorted.
for(i = 0;i < numberOfElements;i++) printf("%d ",A[i]);
return 0;
}
@satoshi-u
Copy link

Why use dynamic memory ?

@RahulSrivatava
Copy link

Thak u it works perfectly fine!!

@kumarmanish52
Copy link

                **merge sort program in C by Studyinight Programmer**

https://codegcloudonline.blogspot.com/2019/12/merge-sort.html
#include<stdio.h>
int merge(int *,int ,int ,int);
void mergesort(int *,int ,int);
main()
{
int n,a[40],i,p,r;
printf("Enter total no of element in an array\n");
scanf("%d",&n);
printf("Enter total %d element in an array\n",n);
for(i=1;i<=n;i++)
scanf("%d",&a[i]);
printf("\nElement Before sorting\n");
for(i=1;i<=n;i++)
printf("%d\t",a[i]);
p=1;
mergesort(a,p,n);
printf("\nElement After sorting\n");
for(i=1;i<=n;i++)
printf("%d\t",a[i]);
}
void mergesort(int *a,int p,int r)
{
int q;
if(p<r)
{
q=(p+r)/2;
mergesort(a,p,q);
mergesort(a,q+1,r);
merge(a,p,q,r);
}
}
int merge(int *a,int p,int q,int r)
{
int n1,n2,l[30],m[30],i,j,k;
n1=q-p+1;
n2=r-q;
for(i=1;i<=n1;i++)
l[i]=a[p+i-1];
for(j=1;j<=n2;j++)
m[j]=a[q+j];
l[n1+1]=1234567897;
m[n2+1]=1234567898;
i=1;j=1;
for(k=p;k<=r;k++)
{
if(l[i]<=m[j])
{
a[k]=l[i];
i=i+1;
}
else
{
a[k]=m[j];
j=j+1;
}
}
return a;
}

@RahulSrivatava
Copy link

RahulSrivatava commented Dec 16, 2019 via email

@tridibsamanta
Copy link

Dynamic memory is used to so that we can de-allocate (free) the left and the right sub-array memory space once the process completes. A good programming practice indeed.

@tridibsamanta
Copy link

tridibsamanta commented May 7, 2020

void merge(int a[], int low, int mid, int high)
{
int b[10000];
int i = low, j = mid + 1, k = low;

while (i <= mid && j <= high) {
    if (a[i] <= a[j])
        b[k++] = a[i++];
    else
        b[k++] = a[j++];
}
while (i <= mid)
    b[k++] = a[i++];

while (j <= high)
    b[k++] = a[j++];

for(i=low;i<=high;i++)
    a[i]=b[i];
}

}

void mergesort(int a[], int low, int high)
{
if (low < high) {
int m = (high + low)/2;
mergesort(a, low, m);
mergesort(a, m + 1, high);
merge(a, low, m, high);
}
}

why are we not implementing this code?

Following issues with the above mentioned method -

  1. What if the array size if more than 10000 ?
  2. There can be lot of empty cells in array 'b[]', and wastage of memory is costly !
  3. What happens to array 'b[]' when the merge() finishes execution for the last time ? After that we don't need the array 'b[]' again, as the elements are in sorted order in array 'a[]', right ? See, we need to free the memory allocated to array 'b[]', which is not done here. :(

@DonutsDevil
Copy link

Java Implementation of the Merge Sort!

public class MergeSort {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		int arr[] = {2,10,7,5,1,3,0,11};
		mergeSort(arr);
		for(int i : arr) {
			System.out.print(i+" ");
		}

	}
	
	
	
	static void mergeSort(int A[]) {
		
		int n = A.length;
		int mid = n/2;
		
		if(n<2) {
			return ;
		}
		
		int left[] = new int[mid];
		int right[] = new int[n-mid];
		
		for(int i = 0 ; i < mid ; i++) {
			left[i] = A[i];
		}
		
		for(int i = mid ; i < n ; i++) {
			right[i-mid] = A[i];
		}
		mergeSort(left);
		mergeSort(right);
		merge(left,right,A);
	}
	
	
	// this merges the sorted left and right array into one array;
	static void merge(int left[],int right[], int arr[]){
		
	int l = 0;	
	int r = 0;
	int k = 0;
	
	while(l < left.length && r < right.length) {
		
		if(left[l] <= right[r]) {
			arr[k] = left[l];
			l++;
		}else{
			arr[k] = right[r];
			r++;
			}
		
		k++;
		
		}//end of while
	while(l<left.length) {
		arr[k] = left[l];
		k++;
		l++;
		}
	while(r<right.length) {
		arr[k] = right[r];
		k++;
		r++;
		}
	}

}// end of class

@JESSE-max1
Copy link

why are we taking arrays as pointers?

@Mayur8991
Copy link

// c++code!

#include
using namespace std;

void mergearrays(int A, int Left,int leftcount, int* Right, int rightcount){

int i,ii,jj;
i=0;ii=0;jj=0;
while(i< leftcount+rightcount){

    if(ii == leftcount) A[i++] = Right[jj++];
    else if(jj == rightcount) A[i++] = Left[ii++];
    else{
        A[i++] = (Right[jj] > Left[ii]) ? Left[ii++]: Right[jj++];
    }
}

}
void mergeSort(int* A, int n){
int mid = n/2;
if(n<2)return;

int* Left= new int[mid];
int* Right= new int[n-mid];

for(int i=0;i<mid;i++)Left[i] = A[i];
for(int i=mid;i<n;i++)Right[i-mid] = A[i];

mergeSort(Left, mid );
mergeSort(Right, n-mid);
mergearrays(A, Left, mid, Right, n-mid);


delete(Left);
delete(Right);
}

int main() {
cout << "Hello World!\n";
int A[]={1,3,4,24,6,7};

int sizel= sizeof(A)/sizeof(A[0]);
mergeSort(A, sizel);
for( int p=0;p<sizel;p++) cout<< A[p]<< endl;

}

@Swagnika
Copy link

Java Code

public class MergeSort {

public static void main(String args[]) {
	int[] a = {100,9,7,4,3,1,11,89,65,43,05,54,21,0,};
	a=ms(a);
	for(int x : a) {
		System.out.print(x+" ");
	}		
}
private static int[] ms(int[] a) {
	int n = a.length; 
	if(n<2) {
		return a;
	}
	else {
	int mid = n/2;

	int[] l = new int[mid];
	int[] r = new int[n-mid];
	for(int i=0;i<mid;i++) {
		l[i]=a[i];
	}
	for(int i=mid;i<n;i++) {
		r[i-mid]=a[i];
	}
	ms(l);
	ms(r);
	merge(l,r,a);
	return a;
	}
	
}
private static void merge(int[] l, int[] r, int[] a) {		
int nl = l.length;
int nr = r.length;
int i =0,j=0,k=0;

while(i<nl && j<nr) {
	if(l[i]<=r[j]) {
		a[k]=l[i];
		i++;
	}
	else {
		a[k]=r[j];
		j++;
	}
	k++;
}
while(i<nl) {
	a[k]=l[i];
	k++;i++;
}
while(j<nr) {
	a[k]=r[j];
	k++;j++;
}		
}

}

@Jasmine-liang
Copy link

Jasmine-liang commented Feb 15, 2021

Java Code!

public class Test { 
    
    public static void main(String[] args) {
    int[] arr = {6,2,3,1,9,10,15,13,12,17};
    Test.sort(arr);
    for (int i : arr){
        System.out.print(i+ " ");
    }

}
    public static void Merge(int[] left, int[] right, int[] arr){
        int i = 0, j = 0, k = 0;
        while (i < left.length && j < right.length){
            if (left[i] < right[j]){
                arr[k] = left[i];
                i++;
            }
            else {
                arr[k] = right[j];
                j++;
            }
            k++;
        }
        while (i < left.length){
            arr[k] = left[i];
            k++; i++;
        }
        while (j < right.length){
            arr[k] = right[j];
            k++; j++;
        }

    }
    public static void sort(int[] arr){
            int n = arr.length;
            if(n < 2) return;
            int mid = n/2;
            int[] left = Arrays.copyOfRange(arr,0, mid);
            int[] right = Arrays.copyOfRange(arr, mid, n);
            sort(left);
            sort(right);
            Merge(left, right, arr);

    }

}

@ImranWahidCoder
Copy link

Thank you so much, sir.

@d3rrick
Copy link

d3rrick commented Mar 7, 2021

Thanks alot sir, best explanation on internet.

Python implementation, did variables with much wordings for a simpler follow-along,

def mergeSort(array):
	arrayLength = len(array)
	if arrayLength<2:
		return
    #cmpute array mid-point
	mid = arrayLength//2

    # create two lists/array and initialize with default values
	leftArray= list(range(mid))
	rightArray= list(range(arrayLength-mid))
	
    # populate the two arrays with values from original array/list
	for idx in range(mid):
		leftArray[idx] = array[idx]
	for idx in range(mid, arrayLength):
		rightArray[idx-mid] = array[idx]
    
    # call a mergeSort method recursively until the base case of len <2 is reached 
    # means one element is remaining in a given list
	mergeSort(leftArray)
	mergeSort(rightArray)

    #then start merging picking left and right array from call stack
	merge(leftArray,rightArray,array)

    # meged array
	return array
		

def merge(leftArray,rightArray,array):
	leftArrayLength = len(leftArray)
	rightArrayLength = len(rightArray)
	
	leftIdx=rightIdx=arrayIdx=0
	
    # loop until the len of either array
	while(leftIdx < leftArrayLength and rightIdx < rightArrayLength):
		if leftArray[leftIdx] <= rightArray[rightIdx]:
			array[arrayIdx] = leftArray[leftIdx]
			leftIdx +=1
		else:
			array[arrayIdx] = rightArray[rightIdx]
			rightIdx +=1
		arrayIdx +=1
		
    # case when right array is exhausted before the left one
	while leftIdx < leftArrayLength:
		array[arrayIdx] = leftArray[leftIdx]
		leftIdx +=1
		arrayIdx +=1
	
    # case when left array is exhausted before the left one
	while rightIdx < rightArrayLength:
		array[arrayIdx] = rightArray[rightIdx]
		rightIdx +=1
		arrayIdx +=1


# tests
testArray =[6,2,3,1,9,10,15,13,12,17]
expected = [1, 2, 3, 6, 9, 10, 12, 13, 15, 17]

assert mergeSort(testArray) == expected

@row-star-134
Copy link

Merge sort in java

`class mergeSortAlgorithms{

public void merge(int arr[], int start , int mid , int end){

//find the size of right and left halves
int n1 = mid - start +1;
int n2 = end - mid;

//make the temporary array equal to right and left halve
int left[] = new int[n1];
int right[] =  new int[n2];

//fill the right and left halves
for(int i=0;i<n1;i++){
  left[i] = arr[i+start];
}
for(int j=0;j<n2;j++){
    right[j] = arr[mid+1+j];
}
//intialize variable
int i = 0 ,j=0, k = start;

//sort the array
while(i<n1 && j <n2){

  if(left[i] < right[j]){
    arr[k] = left[i];
    i++;
  }
  else{
    arr[k] = right[j];
    j++;
  }
  k++;
}
//copying the reamaining value of right and left halves
while(i<n1){
  arr[k] =left[i];
  i++;
  k++;
}
while(j<n2){
  arr[k] = right[j];
  j++;
  k++;
}

}
public void mergeSort(int arr[], int start ,int end){
//check if start greater than end
if(start < end){
int mid = (start+end)/2;

  //divide into two halves recursively
  mergeSort(arr,start,mid);
  mergeSort(arr,mid+1,end);


  //merge into two halves
  merge(arr,start,mid,end);
}

}
public static void main(String[] args) {
//random array
int arr[] = {25,20,4,24};

//object of same class
mergeSortAlgorithms array = new mergeSortAlgorithms();
//call the mergesort function
array.mergeSort(arr,0,arr.length-1);
//display sorted array
for (int i=0;i<arr.length ;i++ ) {
    System.out.println(arr[i]);
}

}
}

`

To compile and run online same program : click on this link :
https://debuggingsolution.blogspot.com/2021/09/mergesort.html

To see code :
https://debuggingsolution.blogspot.com/2021/09/mergesort.html

@asesami
Copy link

asesami commented Nov 30, 2021

i did the array split in 1 loop

for(i = 0;i<mid;i++){
	 L[i] = A[i];
	 R[i] = A[mid+i];
}
if(n%2)R[i] = A[mid+i];

@darshandodamani
Copy link

I had implemented the Merge sort class for the divide and conquer method. The main method is written different.

public class divideandconquer {
//Implement the sorting algorithm Merge Sort, which you will have to use in the
//algorithm for solving Closest-Points. (25%)
//**************
//Not implemented the sort() function. Instead of that
//Merge sort class
class Merge{
private static int k;
static void mergeSort(ArrayList A, int start, int end) {
//check condition if start greater than end
if(start < end) {
//using a formula
int mid = (start + (end-1)) / 2;
//Now we divide into two valves for multiple times
mergeSort(A, start, mid);
mergeSort(A, mid+1, end);
//merge into two valves
merge(A, start, mid, end);
}
}
public static void merge(ArrayList A, int startpoint, int midpoint, int endpoint ) {
//Calculate the size of left and right halves
int lefthalve = midpoint - startpoint + 1 ;
int righthvalve = endpoint - midpoint ;
//create a temporary sub-arrays and assigned to calculated left halves
int [] left = new int[lefthalve];
//create a temporary sub-arrays and assigned to calculated right halves
int [] right = new int [righthvalve];
//We fill our sorted right sub-arrays into temporaries
for (int leftIndex = 0; leftIndex < lefthalve; ++leftIndex) {
left[leftIndex] = A.get(startpoint+leftIndex);
}
//We fill our sorted left sub-arrays into temporaries
for (int rightIndex = 0; rightIndex < righthvalve; ++rightIndex) {
right[rightIndex] = A.get(midpoint + 1 + rightIndex);
}
//Initialize the variables, iterators containing current index of temp sub-arrays
int leftIndex = 0; int rightIndex = 0; int k=startpoint;
// copying from leftArray and rightArray back into array
//for (int i = start; i < end + 1; i++) {
while(leftIndex < lefthalve && rightIndex < righthvalve) {
if(left[leftIndex]<right[rightIndex]) {
A.set(k, left[leftIndex]);
leftIndex++;
}
else {
A.set(k, right[rightIndex]);
rightIndex++;
}
k++;
}
// if all the elements have been copied from rightArray, copy the rest of leftArray
while (leftIndex < lefthalve) {
A.set(k, left[leftIndex]);
leftIndex++;
k++;
}
// if all the elements have been copied from leftArray, copy the rest of rightArray
while (rightIndex < righthvalve) {
A.set(k, right[rightIndex]);
rightIndex++;
k++;
}
}
//applying getters and setters of defined k value
public static int getK() {
return k;
}
public static void setK(int k) {
Merge.k = k;
}
}
}

@AKASH-ALAM
Copy link

thanks

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment