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Recursion for Coding Interviews in JavaScript

Count all Occurrences of a Number

Explore how to count every occurrence of a specific number within an array using recursion in JavaScript. Understand the step-by-step recursive process that breaks the problem into smaller tasks by checking elements and summing the results. This lesson builds foundational skills for solving array recursion problems and prepares you for interview challenges.

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What does “Count all Occurrences of a Number” mean?

Our task is to find the number of times a key occurs in an array.

For example:

Number of occurrences of a key.
Number of occurrences of a key.

Implementation

Javascript (babel-node)
function count(myArray, key) {
  // Base case
  if (myArray.length == 0) {
    return 0;
  }
  // Recursive case1
  if (myArray[0] == key) {
    return 1 + count(myArray.slice(1), key);
  }
  // Recursive case2
  else {
    return 0 + count(myArray.slice(1), key);
  }
}
// Driver Code
myArray = [1, 2, 1, 4, 5, 1];
key  = 1;
console.log(count(myArray, key));

Explanation

We need to count the number of times key occurs in an array.

We do this by:

  1. Checking the first element.
  2. If the first element is the specified key, we will add 11 to the result.
  3. Else, add 00 to the result.
  4. After checking the first element, remove it, and call the function count again on the rest of the array.

Remember, it is easier to implement recursive code if we visualize the solution of the larger problem as the sum of the solution to the smaller tasks.

Let’s visualize the solution to this problem:

In the next lesson, we will learn how to invert an array using recursion