Which multiple of 5 is missing?

The answer is 335 .

This solution uses two loops: the first creates a lookup table of the numbers in the file, and the second loop iterates through the multiples of 5 to check if they are in the lookup table. In the below python code, any multiples of 5 that are missing are printed out.

# Python code
def missing_5_multiple():
    with open('cs_missingmultiple.txt', 'r') as f:

        # create a dictionary of numbers in the file
        nums = {}
        for n in f:
            n = int(n)
            if n not in nums:
                nums[n] = 1

        answer = None
        # loop through all multiples of 5 between 5 and 955
        for n in range(5, 955 + 5, 5):
            # if n is missing, print it out
            # Note: if more than one number is missing, all missing ones will be printed
            if n not in nums:
                answer = n
                print answer

        if answer:
            print 'Missing multiple found!"
        else:
            print 'No multiples of 5 missing!'

My solution, using python built-in sets, assumes that only one multiple of five is missing.

with open("D:\Source\multiplesoffive.txt") as f:
    sum_of_multiples = sum({int(i) for i in f.readlines()})

def triangular_number(n):
    return (n * (n + 1))/2

missing_multiple = (triangular_number(955/5) * 5) - sum_of_multiples

# Outputs 335
print missing_multiple