Useful code which uses reduce()? [closed]

Answer 1

Find the intersection of N given lists:

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

result = reduce(set.intersection, map(set, input_list))

returns:

result = set([3, 4, 5])

via: Python - Intersection of two lists

Answer 2

reduce can be used to get the list with the maximum nth element

reduce(lambda x,y: x if x[2] > y[2] else y,[[1,2,3,4],[5,2,5,7],[1,6,0,2]])

would return [5, 2, 5, 7] as it is the list with max 3rd element +

Answer 3

I have an old Python implementation of pipegrep that uses reduce and the glob module to build a list of files to process:

files = []
files.extend(reduce(lambda x, y: x + y, map(glob.glob, args)))

I found it handy at the time, but it's really not necessary, as something similar is just as good, and probably more readable

files = []
for f in args:
    files.extend(glob.glob(f))

Answer 4

reduce() can be used to find Least common multiple for 3 or more numbers:

#!/usr/bin/env python
from fractions import gcd
from functools import reduce

def lcm(*args):
    return reduce(lambda a,b: a * b // gcd(a, b), args)

Example:

>>> lcm(100, 23, 98)
112700
>>> lcm(*range(1, 20))
232792560

Answer 5

def dump(fname,iterable):
  with open(fname,'w') as f:
    reduce(lambda x, y: f.write(unicode(y,'utf-8')), iterable)

Answer 6

The other uses I've found for it besides + and * were with and and or, but now we have any and all to replace those cases.

foldl and foldr do come up in Scheme a lot...

Here's some cute usages:

Flatten a list

Goal: turn [[1, 2, 3], [4, 5], [6, 7, 8]] into [1, 2, 3, 4, 5, 6, 7, 8].

reduce(list.__add__, [[1, 2, 3], [4, 5], [6, 7, 8]], [])

List of digits to a number

Goal: turn [1, 2, 3, 4, 5, 6, 7, 8] into 12345678.

Ugly, slow way:

int("".join(map(str, [1,2,3,4,5,6,7,8])))

Pretty reduce way:

reduce(lambda a,d: 10*a+d, [1,2,3,4,5,6,7,8], 0)