Adding up contents of list - project euler 1

Question

I'm trying to do Project Euler Problem 1 in Python (http://projecteuler.net./problem=1) and I'm using a while loop to loop to 1000:

from collections import Counter

x = 0
target = 1000
correctMultiples = list()
while x < target:
    x += 1
    if x % 3 == 0 or x % 5 == 0:
        correctMultiples.append(x)

print(str(correctMultiples) + ' are multiples of 3 or 5')

print('The sum of the multiples of 3 or 5 under 1000 is, ' + str(sum(correctMultiples))) # For some reason, 1000 over, answer is 233168 NOT 234168

This works but the answer I'm getting is 1000 over. I get 234168 instead of 233168.

I've tried checking for duplicates: (following How to find duplicate elements in array using for loop in Python?)

duplicates = Counter(correctMultiples)
print([i for i in duplicates if duplicates[i] > 1])

but I don't think there can be duplicates can they? becuase I'm using if x % 3 or ...

I know this isn't the most efficient method, but still... why doesn't it work? Can anyone help me find why the answer is 1000 over? Thanks

worth noting that if you want to get rid of duplicates you can simply `list(set(myList))` — NDevox, Apr 25 '14 at 13:24

Emil Vikström · Accepted Answer · 2014-04-25T13:27:11.253

2

You include 1000 in your loop, while the question ask for numbers below 1000.

Here you increment after doing the boundary check, so when x==999 you will still run the loop:

while x < target:
    x += 1

This would be so much easier with a for loop:

for x in range(1000):

range does not include the last element.

edited Apr 25 '14 at 13:27

answered Apr 25 '14 at 13:21

Emil Vikström

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Sorry, I'm new to Python - when I say <, does it include the number? – ᔕᖺᘎᕊ Apr 25 '14 at 13:22
4

No, but you increment `x` *after* you do that check. – Emil Vikström Apr 25 '14 at 13:23

score 0 · Answer 2 · answered Apr 25 '14 at 13:25

You are including the number 1000 in your loop, because in the last iteration, x is 999 and gets increased once more.

A better solution would be the construct

for x in range(1,1000):
    ...

which includes all x from 1 to 999 and doesn't compare to 1000 on each iteration.

Adding up contents of list - project euler 1

2 Answers2