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Recursion for Coding Interviews in Python

Count All Occurrences of a Number

Explore how to use recursion to count all occurrences of a specific number in an array. This lesson helps you understand breaking down the problem into smaller parts and recursively summing results, preparing you for coding challenges involving arrays.

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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

Python 3.5
def count(array, key) :
  # Base Case
  if array == []: 
      return 0
  # Recursive case1
  if array[0] == key:
      return 1 + count(array[1:], key)
  # Recursive case2
  else:
      return 0 + count(array[1:], key)
# Driver Code
array = [1, 2, 1, 4, 5, 1]
key  = 1
print(count(array, key))

Explanation

We need to count the number of times key occurs in an array. We do this by checking the first element. If the first element is the specified key, we will add 11 to the result. After checking the first element, remove it, and call the function count again.

It is easier to implement code if we visualize the solution of the larger problem as the sum of the solution to the smaller tasks.

In this case:

count of key in array[0:length−1]={1+count of key in array[1:length−1]     if array[0 ...