CODEWITHSUNDEEP
python
willyssignal
willyssignal

Reputation: 59

Python adding every other item

If I have a list with say [7,6,5,4,3,2,1] how can I make it add upp every second number, for instance 7 + 5 + 3 + 1?

I've tried adding mylist[0] + mylist[2] etc but it is very tedious.

Upvotes: 3

Views: 11844

Answers (2)

Padraic Cunningham
Padraic Cunningham

Reputation: 180391

If you don't want to create a new list you can use xrange adding every even indexed element:

l = [7,6,5,4,3,2,1]

print(sum(l[i] for i in xrange(0, len(l), 2)))

Or use itertools.islice:

sum(islice(l, 0, None, 2))

Some timings show islice wins on time and space using python2:

In [14]: timeit sum(islice(l, 0, None, 2))
10 loops, best of 3: 123 ms per loop

In [15]: timeit sum(l[i] for i in xrange(0,len(l), 2))
1 loops, best of 3: 363 ms per loop

In [16]: timeit sum(l[::2])
10 loops, best of 3: 148 ms per loop

And they all return the same result:

In [21]: sum(islice(l, 0, None, 2))
Out[21]: 24999995000000

In [22]: sum(l[i] for i in xrange(0,len(l), 2))
Out[22]: 24999995000000

In [23]: sum(l[::2])
Out[23]: 24999995000000

Upvotes: 2

Mathias711
Mathias711

Reputation: 6658

sum(mylist[::2])

the mylist[::2] takes every other item, and sum sums it.

If you want to have the first, third, fifth et cetera item, you can use: 

sum(list[1::2])

This will first omit the first item (with the 1 part), then do the same as in the first command.

Upvotes: 11

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