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Count of index pairs with equal elements in an array in C++
We are given with an array of N elements. The goal is to find the index pairs (i,j) which have the same element value such that i!=j. i.e, Arr[i]=Arr[j] and i!=j. This is used to make pairs of gloves of equal size. Out of N gloves only paired gloves are useful to sell.
We will do this by running two loops with 0<=i<n-1 and i<j<n. Compare for each pair of (i,j) if Arr[i]==Arr[j] && Arr[i]>0 && Arr[j]>0, and i!=j. If true, increment the count of such pairs and make these elements as -1 ( Arr[i]=Arr[j]= -1) to remove them from further checking as no gloves can have size -1.
Let’s understand with examples.
Input − Arr[]= { 4,3,2,1,2,4 } N=6
Output − Count of index pairs with equal elements − 2
Explanation −
count=0, Arr[]= [ 4,3,2,1,2,4 ] Arr[0]=Arr[5], 0!=5, count=1 Arr[0]=Arr[5]=-1 → [ -1,3,2,1,2,-1 ] Arr[2]=Arr[4], 2!=4, count=2 Arr[2]=Arr[4]=-1 → [ -1,3,-1,1,-1,-1 ] Now array has no new pairs with equal values, i!=j and > -1. Total pairs=2
Input − Arr[]= { 2,2,2,2,2 } N=5
Output − Count of index pairs with equal elements − 2
Explanation−
count=0, Arr[]= [ 2,2,2,2,2 ] Arr[0]=Arr[1], 0!=1, count=1 Arr[0]=Arr[1]=-1 → [ -1,-1,2,2,2 ] Arr[2]=Arr[3], 2!=3, count=2 Arr[2]=Arr[3]=-1 → [ -1,-1,-1,-1,2 ] Now array has no new pairs with equal values, i!=j and > -1. Total pairs=2
Approach used in the below program is as follows
We take an integer array Arr[] initialized with random numbers for size of gloves > 0.
Take a variable n which stores the length of Arr[].
Function countPairs(int arr[], int n) takes an array, its length as input and returns the pairs which have equal size and different indexes.
Traverse array using two for loops for each element of the pair.
Outer Loop from 0<=i<n-1, inner loop i<j<n
Check if arr[i] and arr[j] are positive. If arr[i]==arr[j] then increment count. (i will never be equal to j according to conditions set in loop, no need to compare ).
Now set arr[i]=arr[j]=-1, to remove them from further comparisons.
At the end of all loops count will have a total number of pairs of gloves.
Return the count as result.
Example
// C++ implementation of above approach
#include <bits/stdc++.h>
using namespace std;
// Function to count equal elements to make pair of gloves
int countPairs(int arr[], int n){
int count = 0;
for(int i=0;i<n-1;i++){
for(int j=i+1;j<n;j++){
if(arr[i]==arr[j] && arr[i]>0 && arr[j]>0){
count++;
arr[i]=arr[j]=-1;
}
}
}
return count;
}
int main(){
int arr[] = { 1,2,4,2,1,2,4 };
int n = sizeof(arr) / sizeof(arr[0]);
cout <<"Pair of gloves ( Equal element pairs ):"<<countPairs(arr, n);
return 0;
}Output
If we run the above code it will generate the following output −
Pair of gloves ( Equal element pairs ):3.
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