Determining multiples of 3 or 5 in python

Question

I'm relatively new to python and thus trying to set myself up running some simple algorithms. Here's the first problem from project euler, although there are other solutions available to the same problem in python, but I've tried a different approach.

In crux the idea is to find the sum of all multiples of 3 or 5 less than 1000. This is my code.

def main():

    num = input('Insert number:')
    output = sumOfMultiples(num)
    print(output)


def sumOfMultiples(param):

    j = 0
    i = 0
    for i in range(i, param):
        if (i % 3 ==0) or (i % 5 == 0) and (i % 15 != 0):
            j = j + i
    return j

if __name__ == '__main__':
    main()

This is the error that I get

Traceback (most recent call last):
  File "/Users/Soumasish/PycharmProjects/MultiplesOf3And5/Main.py", line 21, in <module>
    main()
  File "/Users/Soumasish/PycharmProjects/MultiplesOf3And5/Main.py", line 7, in main
    output = sumOfMultiples(num)
  File "/Users/Soumasish/PycharmProjects/MultiplesOf3And5/Main.py", line 15, in sumOfMultiples
    for i in range(i, param):
TypeError: 'str' object cannot be interpreted as an integer

Process finished with exit code 1

Answer 1

You need to cast your input as an int:

def main():
    num = int(input('Insert number:'))
    output = sumOfMultiples(num)
    print(output)

Furthermore, you can reduce your second method as follows. I checked and it still gives the correct answer:

def sumOfMultiples(param):
    sum = 0
    for i in range(param):
        if (i % 3 ==0) or (i % 5 == 0):
            sum += i
    return sum

Answer 2

This is an issue with types, when you take an input in with:

num = input('Insert number:')

num is a string type. You can see this if you use type(num). Then later you try to take a modulo with num and the modulo is not a defined operation with a string.

If you make the type correct the problem will go away:

num_str = input('Insert number:')
output = sumOfMultiples(int(num_str)) #this makes the input string an int first

Answer 3

Instead of checking each number by dividing it by 5 and 3 we can easily do it in O(1) with formula.

n=int(input())
a=(n-1)/3
b=(n-1)/5
c=(n-1)/15
ans=(3*a*(a+1)/2)+(5*b*(b+1)/2)-(15*c*(c+1)/2)
print(ans)

Answer 4

num = int(input("enter the number : "))
lt = []
for i in range(num):
  if (i%3==0 or i%5==0):
     lt.append(i)
sum_of_lt=sum(lt)
print(sum_of_lt)