CODEWITHSUNDEEP
python
user4966454
user4966454

Reputation:

Whats the correct list comprehension syntax for this

if I have two lists:

e = [3,1,5,8], w = [1,2,4]

and I want to go in and check which element of e is evenly divisible by every element of w, how do I do this in a list comprehension? so using my two lists, 8 should be returned since 8 % each i in w will give a zero.

in terms of a for loop I'm thinking of something like:

for i in e:
    for j in w:
        if i % j == 0:
            print i

but even that is incorrect!

Upvotes: 3

Views: 184

Answers (3)

jfs
jfs

Reputation: 414139

To check which element of e is evenly divisible by every element of w:

#!/usr/bin/env python
import fractions
import functools

e = [3,1,5,8]; w = [1,2,4]
lcm = functools.reduce(lambda a,b: a*b // fractions.gcd(a, b), w)
result = [i for i in e if i % lcm == 0]
# -> [8]

This is linear algorithm (if numbers are limited) unlike the nested list comprehension that is quadratic algorithm e.g., if e, w were to contain a million numbers each then the linear algorithm would require around a million operations that should be fast even on modest device, at the same time the quadratic algorithm would take ~1e12 (a number with 12 zeros) operations that may take a while.

In general, if you see:

L = []
for i in e:
    for j in w:
        if i % j == 0:
             L.append(i)

then it is equivalent to:

L = [i for i in e for j in w if i % j == 0]

i.e., all you need is to remove colons and add brackets -- the order is exactly the same.

If you need to figure out the correct syntax for a list comprehension then it means that you should probably leave the code as the explicit nested loops.

Upvotes: 1

Peter Korinek TellusTalk
Peter Korinek TellusTalk

Reputation: 257

If a number is evenly divisible by every element in a list of numbers then it also has to be evenly divisible with the product of all the numbers in the list.

e = [3,1,5,8, 16, 32]
w = [1,2,4]


product = reduce(lambda a,b: a*b, w)

answer = [i for i in e if i % product == 0]

# print answer  ->  [8, 16, 32]

Upvotes: 0

Cong Ma
Cong Ma

Reputation: 11302

[i for i in e if all(i % j == 0 for j in w)]

Upvotes: 5

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