JavaScript Program to Check if String Follows Order of Characters Defined by a Pattern or Not

Last Updated : 27 May, 2024

In this JavaScript article, we will see the solution to the problem of checking if a string follows the order of characters defined by a pattern or not. We have given a string and pattern as the input and our task is to develop a program that will check whether the input string follows the same pattern according to the pattern given.

Example:

Screenshot-2023-09-10-at-17-51-12-Browser

To solve this problem, we have three different approaches which are mentioned below:

Table of Content

Using JavaScript Loops
Using JavaScript Map
Using Builtin Functions 'every', 'reduce', and 'split'
Using Regular Expression:

Using JavaScript Loops

In this apporach, we are using the looping in JavaScript, where we are comparing the order of the chatacter occurences between the input pattern and the input string.
We are iterating thorugh them, and also having a track of max and min indexes of he chatacter occurences.
If the patterm's characters order is matched in the input string's order then the true result is returned else the false result will be returned.

Examples:

JavaScript

function checkPattern(str, pattern) {

    // lcmi -> Last char max index
    // ccmi -> current char max index
    // ccmin -> current char min index
    let lcmi = -1, ccmi = -1, ccmin = -1;
    let matchValue = true;
    lcmi = lastOcc(str, pattern[0]);
    for (i = 1; i < pattern.length; i++) {
        ccmi = lastOcc(str, pattern[i]);
        ccmin = firstOcc(str, pattern[i]);
        if (lcmi < ccmi && lcmi < ccmin) {
            matchValue = true;
        }
        else {
            matchValue = false;
        }
        lcmi = ccmi;
        if (matchValue == false)
            break;
    }
    return matchValue;
}
function lastOcc(str, chr) {
    let currentIdx = 99999999, maxIdx = -1;
    for (j = 0; j < str.length; j++) {
        if (chr == str[j]) {
            currentIdx = j;
            if (currentIdx > maxIdx)
                maxIdx = currentIdx;
        }
    }
    return maxIdx;
}
function firstOcc(str, chr) {
    let currentIdx = 99999999, minIndex = 999999999;
    for (k = 0; k < str.length; k++) {
        if (chr == str[k]) {
            currentIdx = k;
            if (currentIdx < minIndex)
                minIndex = currentIdx;
        }
    }
    return minIndex;
}
console.log(checkPattern("engineers rock", "er"));
console.log(checkPattern("engineers rock", "egr"));
console.log(checkPattern("engineers rock", "gsr"));

Output

true
false
false

Using JavaScript Map

In this apporch, we are using the Maps in JavaScript where we have used the two map objects (map1 and map2).
This objects establish the connection between the chacraters in the input pattern and words in the input string, as well as the reverse mapping.
This assures that the mappings are proper and returns true if the character and words are aligned correctly else false will be returned.

Examples:

JavaScript

function checkPattern(str, pattern) {
    let words = str.split(' ')
    if (pattern.length !== words.length) return false
    let map1 = new Map();
    let map2 = new Map();
    for (let i = 0; i < pattern.length; i++) {
        let key = pattern[i];
        let value = words[i];
        if (map1.has(key) || map2.has(value)) {
            if (map1.get(key) !== value) return false;
            if (map2.get(value) !== key) return false;
        } else {
            map1.set(key, value);
            map2.set(value, key);
        }
    }
    return true;
};

console.log(checkPattern("engineers rock", "er"));
console.log(checkPattern("engineers rock", "egr"));
console.log(checkPattern("engineers rock", "gsr"));

Output

true
false
false

Using Builtin Functions 'every', 'reduce', and 'split'

In this apporach we are using inbuilt fucntions rather than using complex data structures.
We firslty split the input string into words and then by using reduce funtion we create a mapping between pattern chatacters and the corresposnding words.
Lastly, we check if all the mapped values are aligned properly witht he words in the input string using the every function.
If they are aligned properly then true result will be returned else false result will be returned.

Example:

JavaScript

function checkPattern(str, pattern) {
    let words = str.split(" ");
    if (words.length !== pattern.length) {
        return false;
    }

    let patternToWord = pattern.split("").reduce((map, character, index) => {
        if (!map.has(character)) {
            map.set(character, words[index]);
        } else if (map.get(character) !== words[index]) {
            return false;
        }
        return map;
    }, new Map());
    let values = [...patternToWord.values()];
    let ans = values.every((value, index) => value === words[index]);

    return ans;
}

console.log(checkPattern("engineers rock", "er"));
console.log(checkPattern("engineers rock", "egr"));
console.log(checkPattern("engineers rock", "gsr"));

Output

true
false
false

Using Regular Expression:

Using Regular Expression: Create a RegExp pattern by joining characters from the input pattern with '.*', representing any character zero or more times. Test if the string matches the pattern anchored at both ends.

Example: In this example The function followsPattern utilizes a regular expression to match if the string str follows the character order defined by pattern, returning true for a match and false otherwise

JavaScript

function followsPattern(str, pattern) {
    const regex = new RegExp('^' + pattern.split('').join('.*') + '$');
    return regex.test(str);
}

const str1 = "abc";
const pattern1 = "abc";
console.log(followsPattern(str1, pattern1)); 

const str2 = "xyz";
const pattern2 = "xzy";
console.log(followsPattern(str2, pattern2));