I am trying to run the following code for quicksort but the output is always a garbage value.What should be the modification in the code? - c++

Question

this is the following code
#include<iostream>
using namespace std;
int findPivot(int a[],int startIndex,int endIndex)
{
int pivot=a[endIndex];
int pivotIndex=startIndex;
for(int i=0;i<endIndex-1;i++)
{
if(a[i]<pivot)
{
int temp=a[i];
a[i]=a[pivotIndex];
a[pivotIndex]=a[i];
pivotIndex++;
}
}
int temp=pivot;//swapping pivot element into its position.
pivot=a[pivotIndex];
a[pivotIndex]=temp;
return pivotIndex;
}
void quickSort(int a[],int startingIndex,int endingIndex)
{
int number;
if(startingIndex < endingIndex)
{
int returnValueOfPivot= findPivot(a,startingIndex,endingIndex);
//cout<<returnValueOfPivot<<endl;
quickSort(a,startingIndex,returnValueOfPivot-1);//sorting for left
quickSort(a,returnValueOfPivot+1,endingIndex);//sorting for right
}
}
int main()
{
int number;
cout<<"Enter the total number of elements"<<endl;
cin>>number;
cout<<"Enter the values"<<endl;
int a[number-1];
for(int i=0;i<number;i++)
{
cin>>a[i];
}
quickSort(a,0,number-1);
for(int i=0;i<number;i++)
{
cout<<a[i]<<",";
}
return 1;
}

Answer 1

There are three major problems in your code :
int a[number-1];
You are allocating 1 less space for your array. Note that, array index starts from 0. So array of 5 numbers will be like
array[5] : array[0],array[1],array[2],array[3],array[4]
Swapping array values :
int temp=pivot;//swapping pivot element into its position.
pivot=a[pivotIndex];
a[pivotIndex]=temp;
Here, you swapped pivot value with a[pivotIndex] not a[endIndex]!!
So the correct swap would have been :
int temp=a[endIndex];//swapping pivot element into its position.
a[endIndex]=a[pivotIndex];
a[pivotIndex]=temp;
for(int i=0;i<endIndex-1;i++) is incorrect loop
correct loop would be :
for(int i=startIndex;i<=endIndex-1;i++)
You need to start from the start index and end till the end index. You are currently going from 0 to end - 1. [Think of the right side array loop, it won't start with 0]
Make these changes and your code will work.