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Recursion for Coding Interviews in JavaScript

Count Vowels in a String

Explore methods to count vowels in a string by mastering both iterative loops and recursive function calls in JavaScript. This lesson guides you through implementing and understanding these two approaches, helping you solidify your recursion skills as compared to iteration.

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Problem Statement

Here we’ve to find the number of vowels in a given string. We know that the English alphabet contains 55 vowels: a,e,i,o,ua, e, i, o, u. Let’s solve this problem by using both iterative and recursive methods.

Iterative Method

To solve this problem, we have to traverse the entire string. Therefore, we can initiate a simple for loop.

Let’s have a look at the code:

Javascript (babel-node)
function isVowel(character) { // function to check whether input character is a vowel
  var character = character.toLowerCase(); // convert character to lower case so upper cases can also be handled
  var vowels = "aeiou"; // string containing all vowels
  if (vowels.indexOf(character) != -1) { // check if given character is in vowels
    return true;
  }
  else {
      return false;
  }
}
function countVowels(string) { // function that returns the count of vowels
	var count = 0;
	for (var i = 0; i < string.length; i++) {
		if (isVowel(string[i])) { // check if character is vowel 
			count += 1;
    }
  }
  return count; 
}
// Driver code 
var targetVariable = "Educative";
console.log(countVowels(targetVariable));

In the code snippet above, the main function countVowels() takes a string as input. It traverses over the entire length of the string. The condition:

for (var i = 0; i < string.length; i++)

makes sure that the for loop breaks if i becomes equal to the length of string, calculated by using string.length. Next, we use a condition:

isVowel(string[i])

to determine whether that particular character is a vowel or not. If this condition is satisfied we increment count by 11.

isVowel() is a function that returns true if the character being passed is a vowel and false otherwise.

The illustration below will help you understand the code snippet above:

Recursive Method

Let’s take a look at the recursive counterpart. We reduce the length of the string until the length of the string is equal to 11. Then we check whether the last character in the string is a vowel or not. If it is a vowel, the number of vowels is increased.

Javascript (babel-node)
function isVowel(character) { // function to check whether input character is a vowel
  var character = character.toLowerCase(); // convert character to lower case so upper cases can also be handled
  var vowels = "aeiou"; // string containing all vowels
  if (vowels.indexOf(character) != -1) { // check if given character is in vowels
    return true;
  }
  else {
      return false;
  }
}
function countVowels(string, stringLength) { //function that returns the count of vowels
	// Base case
  if (stringLength == 1) {
	  return Number(isVowel(string[0])); 
  }
  // Recursive case
  return countVowels(string, stringLength - 1) + isVowel(string[stringLength - 1]); 
}
// Driver code 
var string = "Educative";
console.log(countVowels(string, string.length));

The isVowel() function checks whether the character being input to it is a vowel or not. This function is similar to the iterative version. The major change that occurs is the conversion of the for loop to the recursive call.

Here, we reduce the length of the input string and pass it to another instance of the same function. The leftover letter is checked, for whether, it is a vowel or not and the result is appended at the end after the function returns.

Let’s dry run our code:

In the next lesson, we have a challenge for you to solve for yourself.