lambda in nested loop with condition

Question

I encountered this question in one of my test for applying a new job.

Given this array :

arr = [[-1, 1, 2, -2, 6], [3, 4, -5]]

1. For each element in the list, if negative value is found, we want to exclude it
2. Square the remaining value

I already made an answer using normal looping

for i in arr:
    for j in range(len(i)):
        if i[j]>0:
            asd = i[j]**2
            i[j] = asd
        else:
            i.remove(i[j])
print(arr)

The result should be like this : [[1, 4, 36], [9, 16]]

The problem is, I have to use lambda function to deliver the question.

I tried to use nested loop with condition for lambda but it's very confusing. Any idea how to solve the problem ? Any helps will be very much appreciated.

Answer 1

As you need to use lambda, you can convert what you would get as list comprehension (the most pythonic IMO):

[[x**2 for x in l if x>=0] for l in arr]

into a functional variant:

list(map(lambda l: list(map(lambda x: x**2, filter(lambda x: x>=0, l))), arr))

longer, less efficient, more cryptic, but you do have plenty of lambdas ;)

output: [[1, 4, 36], [9, 16]]

More on this topic: list comprehension vs lambda+filter

Answer 2

How about using a list comprehension?

list_ = [[-1, 1, 2, -2, 6], [3, 4, -5]]

result = [[n*n for n in e if n >= 0] for e in list_]

print(result)

Output:

[[1, 4, 36], [9, 16]]

Answer 3

You can use lambda like this:

arr = [[-1, 1, 2, -2, 6], [3, 4, -5]]

result = lambda nested_list: [[a ** 2 for a in b if a > 0] for b in nested_list]

print(result(arr))

>>> [[1, 4, 36], [9, 16]]

Answer 4

one way of using lambda

>>> arr = [[-1, 1, 2, -2, 6], [3, 4, -5]]
>>> function = lambda x: map(lambda j:j**2, filter(lambda y: y>0, x))
>>> result = list(map(lambda x: list(function(x)), arr))
>>> result
[[1, 4, 36], [9, 16]]

Answer 5

A bit more verbose: the partial is not required but it is a bad practice to assign lambda to variable.

from functools import partial

power_2 = (2).__rpow__
is_positive = (0).__lt__

apply_to_row = partial(lambda row: list(map(power_2, filter(is_positive, row))))
out = list(map(apply_to_row, arr))

or a single line

out = list(map(lambda row: list(map(power_2, filter(is_positive, row))), arr))