Skipping elements in a List Python

Question

I'm new to programming and I'm trying to do the codingbat.com problems to start. I came across this problem:

Given an array calculate the sum except when there is a 13 in the array. If there is a 13 in the array, skip the 13 and the number immediately following it. For example [1,2,13,5,1] should yield 4 (since the 13 and the 5s are skipped).

This is what I have so far. My problem is that I don't know what to do when there are multiple 13s...And I would like to learn coding efficiently. Can you guys help? (I'm using python 3.2) Thanks!

def pos(nums):
    for i in nums:
        if i == 13:
            return nums.index(13)
    return False

def sum13(lis):
    if pos(lis)!= False:
        return sum(lis[:pos(lis)])+sum(lis[pos(lis)+1:])
    else:
        return sum(lis)

Answer 1

One tricky thing to notice is something like this: [1, 13, 13, 2, 3]

You need to skip 2 too

def getSum(l):
    sum = 0
    skip = False
    for i in l:
         if i == 13:
             skip = True
             continue
         if skip:
             skip = False
             continue
         sum += i
    return sum

Explanation:

You go through the items in the list one by one

Each time you

• First check if it's 13, if it is, then you mark skip as True, so that you can also skip next item.
• Second, you check if skip is True, if it is, which means it's a item right after 13, so you need to skip this one too, and you also need to set skip back to False so that you don't skip next item.
• Finally, if it's not either case above, you add the value up to sum

Answer 2

You can use the zip function to loop the values in pairs:

def special_sum(numbers):
    s = 0
    for (prev, current) in zip([None] + numbers[:-1], numbers):
        if prev != 13 and current != 13:
            s += current
    return s

or you can do a oneliner:

def special_sum(numbers):
    return sum(current for (prev, current) in zip([None] + numbers[:-1], numbers)
               if prev != 13 and current != 13)

You can also use iterators:

from itertools import izip, chain
def special_sum(numbers):
    return sum(current for (prev, current) in izip(chain([None], numbers), numbers)
               if prev != 13 and current != 13)

(the first list in the izip is longer than the second, zip and izip ignore the extra values).