How to get the square of list of lists using map?-CodePudding

Consider the following list of lists:

arr = [[1, 2, 3], [3, 2, 1]]

I want to get the square of it, using map with a lambda function. The result should be:

arr = [[1, 4, 9], [9, 4, 1]]

I tried this:

print(list(map(lambda x: [lst**2 for lsts in arr for lst in lsts], arr)))

But I get that as answer:

[[1, 4, 9, 9, 4, 1], [1, 4, 9, 9, 4, 1]]

CodePudding user response：

Try this:

arr = list(map(lambda List: [x**2 for x in List], arr))
# Output : [[1, 4, 9], [9, 4, 1]]

You can also use nested mapping:

arr = list(map(lambda List: list(map(lambda x: x**2, List)), arr))
# or
square = lambda x: x**2
arr = list(map(lambda List: list(map(square, List)), arr))

CodePudding user response：

Something like this:

arr = [[1,2,3],[3,2,1]]
output = list(map(lambda x: [i**2 for i in x], arr))
print(output)

CodePudding user response：

This works combining both lambdas and list-comprehension:

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

print(list(map(lambda x: [i * i for i in x], arr)))

Output:

[[1, 4, 9], [16, 25, 36]]

CodePudding user response：

Consider using numpy:

arr = [[1,2,3],[3,2,1]]

arr_numpy = np.array(arr)

result = arr_numpy ** 2

CodePudding user response：

You mention that you want to use map and lambda but for reference, you can do this just using list comprehensions,

[[y * y for y in x] for x in arr]