Apply a function to every item of an arbitrarily nested list

Question

I have a nested list a = [1, 2, [3, 4], 5] and I want to apply a function that will raise every number to the power of 2. The result will be like this:

a = [1, 4, [9, 16], 25]

I tried a = [list(map(lambda x : x * x, x)) for x in a] but it gives this error

'int' object is not iterable

How we can fix this issue? How can I apply a function over a nested list?

Answer 1

You probably need a recursive function that distinguishes between lists and scalars:

def square(item):
    if isinstance(item, list):
        return [square(x) for x in item]
    else:
        return item * item

square(a)
#[1, 4, [9, 16], 25]

Incidentally, this approach works for arbitrary-nested lists.

Here's a more general solution:

def apply(item, fun):
    if isinstance(item, list):
        return [apply(x, fun) for x in item]
    else:
        return fun(item)

apply(a, lambda x: x * x)
#[1, 4, [9, 16], 25]

Answer 2

You are decomposing your list into its elements -some of them are lists which can not be multiplied with itself ([3,4]*[3,4]).

Not sure if that is smart - but you can use a "recursing" lambda:

a =[1, 2, [3, 4], 5]

l = lambda x : x * x if isinstance(x,int) else list(map(l,x))
a = list(map(l, a))

print(a)

Output:

[1, 4, [9, 16], 25]

---

Works also for "deeper" levels:

a =[1, 2, [3, [7,8], 4], 5]

Output:

[1, 4, [9, [49, 64], 16], 25]

but will crash if you mix non-iterables things into the mix