Count all Occurrences of a Number

In this lesson, we will learn how to count all occurrences of a key in a given array.

We'll cover the following

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

Our task here 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

Press + to interact
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 can do this by checking the first element. If the first element is the specified key we will be adding 11 to the result, else, we will be adding 00. After checking the first element pass, remove it, and call the function count again.

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

In this case:

countofkeyinarray[0:length−1]={1+countofkeyinarray[1:length−1]ifarray[0]==key,0+countofkeyinarray[1:length−1]ifarray[0]!=keycount \: of \: key \: in \: array[0:length-1] = \begin{cases} 1 + count \: of \: key \: in \: array[1:length-1] \:\:\:\:\: if \:array[0] == key, \\ 0 + count \: of \: key \: in \: array[1:length-1]\:\:\:\: if \:array[0] != key \end{cases}

Let’s have a look at the sequence of function calls:

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