JavaScript Program Count number of Equal Pairs in a String

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

Approach 1: 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

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

Space Complexity: O(1)

Approach2: 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 = "pqr"
console.log(fun(str))