How do you add a range of numbers in python

Question

I have a list of odd numbers but I still need to add them:

for n in range(100, 200):
   if n % 2 == 1:
       print sum([n])

Note: `range(100, 200)` is not list of odd number, it has all number from `100 to 199`. — Grijesh Chauhan, Sep 22 '13 at 06:42
`sum([n])` makes a list whose only element is the single number `n`, then evaluates the sum of that list, producing `n` again. That's not what you want; you need to keep track of the sum of all the odd numbers you've gone through so far. — user2357112, Sep 22 '13 at 06:43
@user2357112 no it is not works like this read Sberry's answer he explained from fundamental also covered how to use `sum()` function. — Grijesh Chauhan, Sep 22 '13 at 07:22

score 10 · Accepted Answer · answered Sep 22 '13 at 06:10

If you are looking to sum the odd numbers in the range of 100 to 200, then the most straight forward way would be:

sum(range(101, 200, 2))

Start at 101 (odd), go till 199 (odd) and increment by 2 so that each number is odd. For instance,

>>> range(101, 110)
[101, 102, 103, 104, 105, 106, 107, 108, 109]

Then you can just sum them.

If you have a preexisting list of numbers then either of the two following methods should fit your need:

>>> nums = [1, 2, 4, 5, 6, 9, 11, 15, 20, 21]
>>> sum(filter(lambda x: x % 2, nums))
62
>>> sum(num for num in nums if num % 2)
62

And this is probably what you were trying to do:

>>> total = 0
>>> for num in nums:
...     if num % 2:
...          total += num
...
>>> total
62

`sum(filter(lambda x: x % 2, nums))` could be replace by `sum(nums[: : 2])` Correct? Which one you prefer? — Grijesh Chauhan, Sep 22 '13 at 06:37
No, my example was showing numbers that were not in a particular pattern of odd, even, odd, even, odd. So slicing like this will not work. — sberry, Sep 22 '13 at 07:10

score 5 · Answer 2 · answered Sep 22 '13 at 08:50

def sum_all(N):
    return N * (N + 1) // 2

The sum of all even numbers from 1 to N (inclusive) is double the sum of all numbers from 1 to N//2.

def sum_even(N):
    return sum_all(N // 2) * 2

The sum of all odd numbers from 1 to N (inclusive) is the difference of these.

def sum_odd(N):
    return sum_all(N) - sum_even(N)

Finally, the sum of all odd numbers between a and b is the sum of all odd numbers from 1 to b minus the sum of all odd numbers from 1 to a - 1.

def sum_odd_range(a, b):
    return sum_odd(b) - sum_odd(a - 1)

To answer the original question:

print sum_odd_range(100, 199)

Note that unlike solutions using sum(), these are O(1) and will be arbitrarily faster for larger inputs.

2 Answers2