Most efficient way to convert items of a list to int and sum them up

Question

I'm doing something like this to sum up a number of elements of a line:

for line in open(filename, 'r'):
   big_list = line.strip().split(delim)
   a = sum(int(float(item)) for item in big_list[start:end] if item)  
   # do some other stuff

this is done line by line with a big file, where some items may be missing, ie, equal to ''. If I use the statement above to compute a, the script becomes much slower than without it. Is there a way to speed it up?

Answer 1

As Padraic commented, use filter to trim out empty strings, then drop "if item":

>>> import timeit
>>> timeit.timeit("sum(int(float(item)) for item in ['','3.4','','','1.0'] if item)",number=10000)
0.04612559381553183
>>> timeit.timeit("sum(int(float(item)) for item in filter(None, ['','3.4','','','1.0']))",number=10000)
0.04827789913997549
>>> sum(int(float(item)) for item in filter(None, ['','3.4','','','1.0']))
4
>>> 

Counterproductive in this example, but might reduce in your context. Measure to see.

see also this answer

Answer 2

This isn't tested, but intuitively I would expect skipping the intermediary float conversion would be helpful. You want to grab the integer to the left of the decimal, so I would try doing that directly via regular expression:

import re

pattern = re.compile("\d+")

Then replace the float parsing with the regex match:

sum(int(pattern.search(item).group(0)) for item in big_list[start:end] if item)

If you don't need to keep the old decimal strings, you could also get these on the fly as you build big_list . For example, say we have the line "6.0,,1.2,3.0," . We could get matches like this:

delim = ","
pattern = re.compile("(\d+)\.\d+|" + re.escape(delim) + re.escape(delim) + "|$")

The results of this pattern on the line would be: ['6', '', '1', '3', ''] , which could then be sliced and filtered as usual without the need of float parsing:

for line in open(filename, 'r'):
    big_list = pattern.findall(line)
    a = sum(int(item) for item in big_list[start:end] if item)