I need help understanding how the map function in python

Question

def multiply(x):
    return (x*x)
def add(x):
    return (x+x)

funcs = [multiply, add]
for i in range(5):
    value = list(map(lambda x: x(i), funcs))
    print(value)

So I understand that map is used to apply a function/first arg to every item in the list/second arg. What I dont understand is how its being handled on this list of functions.

Output:

[0, 0]

[1, 2]

[4, 4]

[9, 6]

[16, 8]

For every iteration of i, from 0 to 4, you are taking the input i and multiplying it by itself, hence the first element in each list of 0*0, 1*1, 2*2, 3*3, 4*4 and also adding each element to itself, hence hte second element in each list, 0+0, 1+1, 2+2, 3+3, 4+4. The functions are being applied as you entered them in the list: first run multiply function then apply add function — rmilletich, Apr 25 '18 at 19:03
Possible duplicate: [Understanding the map function](https://stackoverflow.com/questions/10973766/understanding-the-map-function) — jpp, Apr 25 '18 at 19:04
BTW: this is not really the usual use of `map`, which might be causing your confusion. Usually you just have a single function (e.g., `multiply`) which you want to apply to a list or iterable (e.g., `range(5)`). — RishiG, Apr 25 '18 at 19:06
it applies two functions to numerals from 1 to 5, just that. — Abr001am, Apr 25 '18 at 19:06
Possible duplicate of [Understanding the map function](https://stackoverflow.com/questions/10973766/understanding-the-map-function) — Aniket Bote, Apr 25 '18 at 19:12

score 1 · Accepted Answer · answered Apr 25 '18 at 19:09

Let's try pulling this out of a loop and see what happens.

Your lambda x: x(i) is calling each function with the argument i and map (roughly) turns this into a list.

list(map(lambda x: x(0), funcs)) -> [0,0]

This is the same as saying: [multiply(0), add(0)].

If we try again with 1:

list(map(lambda x: x(0), funcs)) -> [1,2]

This is the same as saying: [multiply(1), add(1)].

The function that you are applying, the first argument to map, is your lambda. That happens to be a higher order function that returns the result of it's input.

A similar way to rewrite this program would be:

def multiply_and_add(i):
    return [multiply(i), add(i)]

result = map(multiply_and_add, range(5))

for value in result
    print(value)

score 0 · Answer 2 · answered Apr 25 '18 at 19:09

Your map here is used on an array of function. So, for each function it will execute the lambda on it

In the lambda, you have x: x(i) Here, x is the current function (so multiply or add).
And i is your current iteration of the loop (so 0-4).

For example, the 1st time it enters the loop, i == 0. The map function runs multiply 1st, so it returns 0*0, then it runs the add and returns 0+0. So, the output of value is [0,0]

I need help understanding how the map function in python

2 Answers2