Python - keeping counter inside list comprehension

Question

Is it possible to write a list comprehension for the following loop?

m = []
counter = 0
for i, x in enumerate(l):
    if x.field == 'something':
        counter += 1
        m.append(counter / i)

I do not know how to increment the counter inside the list comprehension.

Note that this can result in division by zero if the first (i.e. "zeroth") element satisfies the condition. Also, what exactly are you trying to achieve? — tobias_k, Jan 05 '15 at 11:36
You are right. I wrote that code on the fly and I forget to increment by 1 i. I want to compute the [average precision](https://en.wikipedia.org/wiki/Information_retrieval#Average_precision) — user2297037, Jan 05 '15 at 11:43

unutbu · Accepted Answer · 2015-01-05T12:11:01.487

11

You could use an itertools.count:

import itertools as IT
counter = IT.count(1)
[next(counter)/i for i, x in enumerate(l) if x.field == 'something']

To avoid the possible ZeroDivisionError pointed out by tobias_k, you could make enumerate start counting from 1 by using enumerate(l, start=1):

[next(counter)/i for i, x in enumerate(l, start=1) 
 if x.field == 'something']

edited Jan 05 '15 at 12:11

answered Jan 05 '15 at 11:32

unutbu

1 Answers1