Wrote a python program that added up numbers from 1 to a given number using the Gauss equation. It worked for 100 and 10 but when I do 3 it says the sum is 4 instead of 6. The equation works out in my head, did I mess up the code?
def numberSum(num):
nim = int(num)
num = (nim/2)*(nim+1)
return num
print numberSum(raw_input())
from __future__ import division
def number_sum(num):
"""
Return sum of 1 .. num
"""
return num * (num + 1) // 2
num = int(raw_input(": "))
print(number_sum(num))
Things I changed:
number_sum should not be responsible for type-casting input; if it expects a number, you should give it some kind of number, not a string.
if num is an int, then either num or num + 1 is even, ie dividing num * (num + 1) by 2 will result in an int. This is not necessarily true of num; therefore you should divide the product by 2, not num, if you want an integer result.
in Python 2.6+, from __future__ import division makes / always return a float result and // always return an int result, consistent with Python 3.x
I suppose you are working with python2, cause by default it applies integer division like other languages. Besides, you can notice this feature in this post Integer division in python 2 and python 3.
To solve your problem you can follow the next approach:
def numberSum(num):
nim = int(num)
return ((nim * 1.0) / 2) * (nim + 1)
print numberSum(raw_input())
