Using recursion to add digits of a number
- 4 minsAdding digits of a number recursively
In this post I am going to write a C program that adds digits of a positive integer recursive approach.
I’ll start with a simple recursive function that adds the digits of a positive interger just once. I’ll expand this code in the next post to show how the result of this addition can be added repeatedly until the function spits out a single-digit number. e.g. In our program, recursive_sum(891) will produce the output 8+9+1 = 18. In the next post, I’ll go one step further to add the digits of the result as well i.e. 1 + 8 = 9.
In this solution, the digits of the input integer x will be added just once. The recurrence relation involves extracting the last digit(done using x mod 10) and adding it to the result of the recursive call to the number without its last digit(done using x / 10). The base case involves checking whether x has been reduced to a single digit. Let me explain the flow of the program below.
The first time recursive_sum() is called, 891 is passed as an argument. It checks against the base case and as it is not true, the else portion runs. This block of code separates the last digit and adds it with the result of recursive_sum() call on the first two digits i.e 89. In code: 1 + recursive_sum(89). The state of the current function call is saved before it jumps to the newer recursive call involving recursive_sum(89). Now, the previous steps repeat until the last remaining digit i.e 8 is separated. Then it checks against the if condition which actually tests whether the passed integer is a single digit number. Since, 8 is a single digit, it run the if code block and returns the digit itself i.e. 8 to previous instance of the recursive call, that is, recursive_sum(8). Now, the previous saved instance of the recursive function, that is, recursive_sum(89) runs, and evaluates 9 + recursive_sum(8) where recursive_sum(8)=>8. Thus, the expression is now 9 + 8. After evaluating recursive_sum(89) => 17, the previous call i.e. recursive_sum(891) is resumed. The call to recursive_sum(891) actually evaluates 1 + recursive_sum(89) which is, 1 + 17, thus giving us the answer => 18.
Srijal Joshi
I think and I know things - Lyrion Tannister