Sum of numbers smaller than n that are multiples of 3 or 5

Question

I need to write a program that receives an n number and prints the sum of all numbers smaller than n and that are multiples of 3 or 5.

For this, I must use only the while repeating structure and no data type as set or list, as it is an exercise of an introductory programming course.

Answer 1

Focus on the fact that the expected output is a single number: the sum. A simple way to solve it is to look at each candidate, from the smallest to the largest. If it fits the constraints, add it to a variable that contains the partial result. A pseudo-code that implements this idea:

Seja n um número inteiro
Leia n
Seja soma um inteiro com valor 0
Seja i um inteiro com valor 1

Enquanto i for menor do que n:
    Se i é divisível por 3 ou i é divisível por 5:
        soma <- soma + i
    i <- i + 1

Imprima soma

There is room for optimizations. For example: is it necessary to look at even numbers? And primes? For simplicity’s sake, I left that sort of thing out of the answer, but it’s a good exercise to think about it.