JavaScript Program Count number of Equal Pairs in a String

Last Updated : 18 Jul, 2024
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In this article, we are going to learn how can we count a number of equal pairs in a string. Counting equal pairs in a string involves finding and counting pairs of consecutive characters that are the same. This task can be useful in various applications, including pattern recognition and data analysis.

Examples:

Input: 'pqr'
Output: 3
Explanation:
3 pairs that are equal are (p, p), (q, q) and (r, r)
Input: 'HelloWorld'
Output: 18

Table of Content

Naive Approach

The straightforward method involves using two nested loops to iterate through the string, identifying all pairs, and maintaining a count of these pairs.

Example:

JavaScript 
// JavaScript program to determine
// the count of equal character pairs
// Function to calculate the 
// count of equal character pairs

function fun(str) {

    // Length of the input string
    let strLength = str.length;
    
    // Variable to store the 
    // count of equal character pairs
    let pairCount = 0;

    // Nested loops to compare 
    // characters for equal pairs
    for (let i = 0; i < strLength; i++) {
        for (let j = 0; j < strLength; j++) {
        
            // If an equal pair is found
            if (str[i] == str[j]) {
                pairCount++;
            }
        }
    }

    return pairCount;
}

// Driver Code
let str = "geeksforgeeks";
console.log(fun(str));

Output
31

Time Complexity: O(n2), n is the length of input string

Space Complexity: O(1)

Efficient Appraoch

  • In this approach, We must efficiently determine the count of distinct pairs of characters in linear time.
  • Notably, pairs like (x, y) and (y, x) are treated as distinct.
  • To accomplish this, we employ a hash table to record the occurrences of each character. If a character appears twice, it corresponds to 4 pairs: (i, i), (j, j), (i, j), and (j, i).
  • By utilizing a hashing mechanism, we keep track of the frequency of each character, and for each character, the count of pairs will be the square of its frequency.
  • The hash table will have a length of 256 since there are 256 distinct characters.

Example: This example shows the use of the above-explined approach.

JavaScript 
// JavaScript program to calculate the
// count of pairs
const MAX = 256

// Function to calculate the count
// of identical pairs
function fun(str) {

    // Hash table to store
    //  character counts
    let charCount = new Array(MAX).fill(0)

    // Iterate through the string and tally
    // the occurrences of each character
    for (let i = 0; i < str.length; i++)
        charCount[str.charCodeAt(i) - 97] += 1

    // Variable to hold the final count of pairs
    let pairCount = 0

    // Iterate through the characters and check
    // for occurrences
    for (let i = 0; i < MAX; i++)
        pairCount += charCount[i] * charCount[i]

    return pairCount
}

// Driver code

let str = "abccba"
console.log(fun(str))

Output
3

Time Complexity: O(n), n is the length of input string

Space Complexity: O(1)

Using Combinatorial Counting

Another efficient approach to count the number of equal pairs in a string is by using combinatorial counting principles. This approach leverages the fact that the number of ways to choose 2 items from n items (where order does not matter) is given by the combination formula C(n, 2) = n * (n - 1) / 2.

Example:

JavaScript 
function countEqualPairs(s) {
    let freq = {};
    for (let char of s) {
        if (freq[char]) {
            freq[char]++;
        } else {
            freq[char] = 1;
        }
    }
    let totalPairs = 0;
    for (let count of Object.values(freq)) {
        if (count > 1) {
            totalPairs += (count * (count - 1)) / 2;
        }
    }
    
    return totalPairs;
}
const inputStr = 'HelloWorld';
const output = countEqualPairs(inputStr);
console.log(output);

Output
4

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