The following c program is a quicksort recursive program. Although I have written this code according to the Cormen explanation, yet it is incorrectly sorting the input.For example it is sorting the input 3,8,1 to 3,1,8. Thanks a lot in advance for finding the mistake
#include<stdio.h>
void printa(int a[],int size)
{
int i;
printf("\n");
for(i=0;i<size;i++)
{
printf("%d\n",a[i]);
}
}
void swap(int *a,int *b)
{
int temp;
temp=*a;
*a=*b;
*b=temp;
}
int partition(int a[],int p,int r)
{
int i,j,x;
i=p-1;
x=a[r];
for(j=p;j<r;j++)
{
if(a[j]<=x)
{
i=i+1;
swap(&a[i],&a[j]);
}
swap(&a[i+1],&a[r]);
}
return i+1;
}
void quicksort(int a[],int p,int r)
{
if(p<r)
{
int q;
q=partition(a,p,r);
quicksort(a,p,q);
quicksort(a,q+1,r);
}
}
main()
{
int a[50],i,size;
printf("enter the size of the array\n");
scanf("%d",&size);
printf("enter the elements of the array\n");
for(i=0;i<size;i++)
{
scanf("%d",&a[i]);
}
quicksort(a,0,size-1);
printa(a,size);
}
Related
I want to sort the given array using quicksort.
So I wrote this code in c for quicksort algorithm but its not giving me correct answer.Its giving me the same output as the input.
eg: If i give input as
3
2
1
then it is giving
Output:
3
2
1
Please help and tell me where is the mistake.
#include<stdio.h>
void quicksort(int a[],int p,int r);
int partition(int a[],int p,int r);
void swap(int a,int b);
int main()
{
int n,i,p,r;
printf("ENter no of elements");
scanf("%d",&n);
int a[n];
printf("ENter the elements");
for(i=1;i<=n;i++)
{
scanf("%d",&a[i]);
}
p=1;
r=n;
quicksort(a,1,n);
printf("\n");
for(i=1;i<=n;i++)
{
printf("%d\n",a[i]);
}
return 0;
}
void quicksort(int a[],int p,int r)
{
int q;
if(p<r)
{
q = partition(a,p,r);
quicksort(a,p,q-1);
quicksort(a,q+1,r);
}
}
int partition(int a[],int p,int r)
{
int x,i,j;
x=a[r];
i=p-1;
for(j=p;j<=r-1;j++)
{
if(a[j]<=x)
{
i=i+1;
swap(a[i],a[j]);
}
}
swap(a[i+1],a[r]);
return (i+1);
}
void swap(int a,int b)
{
int t;
t=a;
a=b;
b=t;
}
void swap(int a,int b)
This line is taking the two parameters as copies of the values. The swapping is done on he copies.
void swap(int* a, int* b)
This is the C-style of passing parameters by reference. You need to pass the references then:
swap(&a[i], &a[j]);
I have implemented each line of merge sort algorithm-wise and can't find error.
al and ar are left and right sub arrays. Arrays are passed along with size.
#include<stdio.h>
#include<conio.h>
void mergesort(int a[] ,int);
void merge(int al[],int,int ar[],int,int a[]);
int main()
{
int i,n;
printf("Enter the no of elements to be sorted\n");
scanf("%d",&n);
int a[n];
for(i=0;i<n;i++)
{
scanf("%d",&a[i]);
}
mergesort(a,n);
printf("\nThe elements after sorting are:");
for(i=0;i<n;i++)
{
printf("%d ",a[i]);
}
getch();
return 0;
}
void mergesort(int a[],int size)
{
int i,n=size,mid;
if(n<2)
return;
mid=n/2;
int left[mid],right[n-mid];
for(i=0;i<mid;i++)
left[i]=a[i];
for(i=mid;i<n;i++)
right[i]=a[i];
mergesort(left,mid);
mergesort(right,n-mid);
merge(left,mid,right,n-mid,a);
}
void merge(int al[],int sl,int ar[],int sr,int a[])
{
int i=0,j=0,k=0;
while(i<sl && j<sr)
{
if(al[i]<=ar[j])
{
a[k]=al[i];
i++;
}
else
{
a[k]=ar[j];
j++;
}
k++;
}
while(i<sl)
{
a[k]=al[i];
i++;
k++;
}
while(j<sr)
{
a[k]=ar[j];
j++;
k++;
}
}
input:
no of elements:4
5 6 3 1
output:
5 16 16 16
Look at this part of your code:
int left[mid],right[n-mid];
for(i=0;i<mid;i++)
left[i]=a[i];
for(i=mid;i<n;i++)
right[i]=a[i];
You are accessing the right array with indexes beyond the array bounds. That should be something like this instead:
right[i - mid]=a[i];
I have correct code for merge sort written in C. Please tally with it.
#include<stdio.h>
void mergesort(int a[],int i,int j);
void merge(int a[],int i1,int j1,int i2,int j2);
int main()
{
int a[30],n,i;
printf("Enter no of elements:");
scanf("%d",&n);
printf("Enter array elements:");
for(i=0;i<n;i++)
scanf("%d",&a[i]);
mergesort(a,0,n-1);
printf("\nSorted array is :");
for(i=0;i<n;i++)
printf("%d ",a[i]);
return 0;
}
void mergesort(int a[],int i,int j)
{
int mid;
if(i<j)
{
mid=(i+j)/2;
mergesort(a,i,mid);
mergesort(a,mid+1,j);
merge(a,i,mid,mid+1,j);
}
}
void merge(int a[],int i1,int j1,int i2,int j2)
{
int temp[50];
int i,j,k;
i=i1;
j=i2;
k=0;
while(i<=j1 && j <=j2)
{
if(a[i]<a[j])
temp[k++]=a[i++];
else
temp[k++]=a[j++];
}
while(i<=j1)
temp[k++]=a[i++];
while(j<=j2)
temp[k++]=a[j++];
for(i=i1,j=0;i<=j2;i++,j++)
a[i]=temp[j];
}
I am trying to merge to arrays without sorting (add one then another) using pointer method but its just printing the first array and then garbage values. What i am trying to do is just combine 2 arrays in one big array. No sorting required(at least for now).
void getarray(int*,int);
int merge(int*,int*,int,int,int*);
main()
{
int a[10],b[10],c[20];
int i,j,n,m,size;
clrscr();
printf("Enter no. of elements in FIRST array: ");
scanf("%d",&n);
getarray(a,n);
printf("Enter no. of elements in SECOND array: ");
scanf("%d",&m);
getarray(b,m);
merge(a,b,n,m,c);
printf("\nMerged Array: \n");
for(i=0;i<n+m;i++)
{
printf("\t%d\t",c[i]);
}
getch();
return 0;
}
void getarray(int *x, int y)
{
int i;
for(i=0;i<y;i++)
{
scanf("%d",x+i);
}
}
int merge(int *a, int *b,int n,int m,int *c)
{
int i,j;
for(i=0;i<n;i++)
{
*(c+i) = *(a+i);
}
for(j=i;j<i+m;j++)
{
*(c+j) = *(b+j);
}
}
Alternatively you can use (assuming c is large enough):
void merge(int *a, int *b,int n,int m,int *c) {
memcpy(c, a, sizeof(int)*n);
memcpy(c+n, b, sizeof(int)*m);
}
You would need to include string.h.
int merge(int *a, int *b,int n,int m,int *c)
{
int i,j;
for(i=0;i<n;i++)
{
*(c+i) = *(a+i);
}
for(j=0;j<m;j++)
{
*(c+n+j) = *(b+j);
}
}
I'm trying to create a simple(?) selection sort program in C that selects the largest integer of an integer array and places it in the location a[n-1], places the second largest number in a[n-2], etc until the smallest number is placed in a[0]. I've run through the below code on paper and it seems like it should work, but when I compile it I'm getting faulty results. Am I missing something obvious?
/* The program implements selection sort*/
#include <stdio.h>
#include "simpio.h"
#define n 5
void GetArray(int a[]);
void SelectionSort(int a[]);
int FindMax(int a[], int high);
void swap(int a[], int p1, int p2);
void PrintArray(int a[]);
main()
{
int a[n];
GetArray(a);
SelectionSort(a);
PrintArray(a);
getchar();
}
void GetArray(int a[])
{
int i;
for(i=0;i<n;i++)
{
printf("Enter integer# %d", i+1);
a[i]=GetInteger();
}
}
void SelectionSort(int a[])
{
int i, max;
for(i=0;i<n;i++)
{
max=FindMax(a,i);
swap(a,max,(n-1-i));
}
}
int FindMax(int a[], int high)
{
int i, index;
index=high;
for(i=high;i<n;i++)
{
if(a[i]>a[index])
index=i;
}
return index;
}
void swap(int a[], int p1, int p2)
{
int temp;
temp=a[p2];
a[p2]=a[p1];
a[p1]=temp;
}
void PrintArray(int a[])
{
int i;
for(i=0;i<n;i++)
printf("a[%d]=%d\n", i, a[i]);
}
Change these method to:
void SelectionSort(int a[])
{
int i, max;
for(i=0;i<n;i++)
{
max=FindMax(a,n-i-1);
swap(a,max,n-i-1);
}
}
int FindMax(int a[], int high)
{
int i, index;
index=high;
for(i=0;i<high;i++)
{
if(a[i]>a[index])
index=i;
}
return index;
}
I actually tested my answer and it works.
Selection sort is process of comparing minimum element from the list and placing from the least index.
Now consider below code snippet.
public void selectionSort(int[] elements) {
for(int i=0;i<elements.length;i++) {
int minPosition = i;
for(int j=i+1;j<elements.length;j++) {
if(elements[minPosition]>elements[j])
minPosition = j;
}
int temp = elements[i];
elements[i] = elements[minPosition];
elements[minPosition] = temp;
}
}
Thanks for reading, let me know feedback to improve from myside
Shouldn't:
max=FindMax(a,i);
swap(a,max,(n-1-i));
Be:
max=FindMax(a,i);
swap(a,max,i);
otherwise, next time through the loop, you'll find the same max value in the top position in the array.
A very basic implementation of selection sort
#include<stdio.h>
main()
{
int i,j,n=7,a[]={1,2,5,3,8,9,5},key;
for(j=1;j<n;j++)
{
key=a[j]; //a[j] is the new element to be added to the sorted
//sequence
i=j-1;
while(i>=0 && key<a[i]) //traverse through the sorted sequence
{a[i+1]=a[i];i--;} //until the place of key is found
a[i+1]=key;
}
for (j=0;j<n;j++)
printf("%d",a[j]);
}
#include<stdio.h>
#include<conio.h>
int removex(int arr[],int small,int n)
{
int i=0;
for(;i<n;i++)
if(arr[i]==small) //searching that no to delete
break;
for(;i<n-1;i++)
arr[i]=arr[i+1]; //delete by overloading no
return n-1;
}
void selectSort(int arr[],int sort[],int n)
{
int j=0,k=0,small;
while(n!=0)
{
small=arr[0];
for(j=0;j<n;j++)
if(arr[j]<small)
small=arr[j]; //finding smallest no
sort[k++]=small;
n=removex(arr,small,n); //removing that from list as we included that no into sorted list
}
}
void main()
{
int arr[10],arr2[10],i,n;
clrscr();
printf("Enter how many elements");
scanf("%d",&n);
for(i=0;i<n;i++)
scanf("%d",&arr[i]);
selectSort(arr,arr2,n);
printf("sorted list is\n");
for(i=0;i<n;i++)
printf("%d\n",arr2[i]);
getch();
}
This is based on the algorithm given in the Cormen's book. What am I doing wrong?
#include <stdio.h>
#include <conio.h>
void mergesort(int a[],int,int);
void merge(int a[],int,int,int);
int main()
{
int i,num,a[50];
printf("Enter the number of elements : ");
scanf("%d",&num);
for(i=1;i<=num;i++)
{
printf("\n%d) ",i);
scanf("%d",&a[i]);
}
mergesort(a,1,num);
for(i=1;i<=num;i++)
{
printf("SoRTED ArrAy \n");
printf("\n%d) ",a[i]);
}
getch();
return 0;
}
void mergesort(int a[],int i, int k)
{
int j;
j=(i+k)/2;
while(i<k)
{
mergesort(a,i,j);
mergesort(a,j+1,k);
merge(a,i,j,k);
}
}
void merge(int a[],int p,int q,int r)
{
int i,j,k,n1,n2;
n1=q-p+1;
n2=r-q;
int l[n1],s[n2];
for(i=1;i<=n1;i++)
{
l[i]=a[p+i-1];
}
for(j=1;i<=n2;j++)
{
s[j]=a[q+j];
}
l[n1+1]=1000;
s[n2+1]=1000;
i=1;
j=1;
for(k=p;k<=r;k++)
{
if(l[k]<s[k])
{
a[k]=l[i];
i++;
}
else
{
a[k]=s[j];
j++;
}
}
}
The main issue is that i and k are never updated inside your while loop in the mergesort function, leading to an infinite loop. It's likely that you don't even need the while loop.
void mergesort(int a[],int i, int k)
{
int j;
j=(i+k)/2;
while(i<k)
{
mergesort(a,i,j);
mergesort(a,j+1,k);
merge(a,i,j,k);
}
}
Nothing inside the while changes the value of i or k. So if i<k, the loop will repeat forever.