Example 6: Sum up the digits in an integer 

Given a positive integer n, say 287591, we want to determine the sum of all its digits. The result for 287591 would be 2+8+7+5+9+1 = 32. To solve this problem, we need to determine the individual digits in integer n.

Our program works as follows:

  • Use the remainder operator % to obtain the last digit of the integer and add the digit to the variable sum.
  • Then, use the floor division operator //  to produce the integer without the last digit.
  • Repeat these steps until there are no digits left.
# 02Loops example_06 SumDigits1.py
# add up the digits in an integer using a while loop

n = int(raw_input("Please input an integer:")) 	

sum = 0
while n != 0:               #While there are still digits left in n, the loop continues
    last_digit = n % 10		#Obtain the last digit of n
    sum = sum + last_digit 	#Add the last digit to sum
    n = n // 10 		#Remove the last digit of n with Floor Division operator 



  • Video03NestedLoops2.mp4 uses this problem in a nested loop setting.
  • The above program “destroys” integer n.  If for some reason we need n later, we would need to store it in another variable.
  • If students have already seen for-loops in Python, ask them to try computing the sum of the digits with a for-loop. We will consider the for-loop version of this problem in Example 11.