Recursion in Ruby

The process in which a function calls itself directly or indirectly is called recursion and the corresponding function is called a recursive function. Recursion makes the process easier and it definitely reduces a lot of compiling time. In Ruby, we do have an option of putting all the actions in a loop, so that they could be repeated a given number of times. So why is there a need for Recursion?

Since in Ruby, we introduce real-life variables, Recursion plays a very important role in solving major real-life problems in Ruby. 

Simple code: We can understand the steps while considering a very simple problem of summing the array. Writing an iterative code would mean running a loop through the array and adding each element to a variable and then return the variable.

Example:

Ruby

# Iterative program to execute the summing of a given array of numbers.
def iterativeSum(arrayofNumbers)
  sum = 0
  arrayofNumbers.each do |number|
    sum += number
  end
  sum
end

iterativeSum([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])

Output : 

55

Writing the recursive code:
In this code, the method calls itself. In the recursive program, the solution to the base case is provided and the solution of the bigger problem is expressed in terms of smaller problems.

Example:

Ruby

# Recursive method to calculate the sum of all numbers in a given array.
def RecursiveSum(arrayofNumbers)
  # Base Case: If the array is empty, return 0.
  return 0 if arrayofNumbers.empty?

  # Recursive code: Adding each element to the variable by calling the method.
  sum = arrayofNumbers.pop
  return sum + RecursiveSum(arrayofNumbers)
end

RecursiveSum([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])

Output : 

55

The concept of how actually the Recursive steps are carried on can be understood using a yet another example. In the following example, we would execute a simple program of finding the Factorial of a given number. 
The code for the above mentioned program is as follows: 
Example :

Ruby

# Ruby code for calculating the factorial of a number recursively.

def RecursiveFactorial(number)

# Base Case:

  if (0..1).include?(number)
    return 1
  end

#Recursive call:

    number * RecursiveFactorial(number - 1)
end
# Calling the method:

RecursiveFactorial(6)

Output : 

720

The code works as follows: 

The method for calculating the factorial of 6 is called first which further calls the one to calculate for 5 then for 4 until it reaches a point where RecursiveFactorial(1) is called which returns 1 as the answer. Here the recursion calls work as it multiplies the result of each call with the already existing answer. The same can be shown in following steps:

RecursiveFactorial(6) = 6 * RecursiveFactorial(5) 
= 6 * 5 * RecursiveFactorial(4) 
= 6 * 5 * 4 * RecursiveFactorial(3) 
= 6 * 5 * 4 * 3 * RecursiveFactorial(2) 
= 6 * 5 * 4 * 3 * 2 * RecursiveFactorial(1) 
= 6 * 5 * 4 * 3 * 2 * 1 
= 720 

Another example is a code for calculating the Nth Fibonacci number. Here the base case would be when the N is less than 2, as then the Fibonacci number would be itself. The recursive call would be calling the method for calculating the (n-1)th and (n-2)nd Fibonacci number and adding it to get the result for Nth number. 

Example:

Ruby

# Ruby program for calculating the Nth Fibonacci number.
def Fibonacci(number)

  # Base case :  when N is less than 2.
  if number < 2
    number
  else

    # Recursive call : sum of last two Fibonacci's.
    Fibonacci(number - 1) + Fibonacci(number - 2)
  end
end

Fibonacci(3)

Output :

2

The concept of Recursion solves many problems and makes the process easier as when calculating through the Iterative process. Thus, if a person masters the concept of Recursion, one could solve many problems in lesser time and lesser lines of code. However, Recursion in Ruby sometimes produces “SystemStackError: stack level too deep” when a large number is used as an input(This number varies with the system). This means that we would have to use an iterative approach to a problem involving large inputs.