I use the function map() in python,and convert the results into a list,but the result is an empty list

Question

Why I am geting a wrong list when I use the function map() in python? here is my code,

When I use print() function to print list(r)

print(list(r))

my result is an empty list.

But When I write the code：

rList= list(r)

and then

print(rList)

my result is right.

here is my code:

def f(x):
    return x * x
r = map(f, [1, 2, 3, 4, 5, 6, 7, 8, 9])
rList= list(r)
print( list(r) )
print( rList)

here is my result:

[]
[1,4,9,16,25,36,49,64,81]

and here is expected result

[1,4,9,16,25,36,49,64,81]
[1,4,9,16,25,36,49,64,81]

@coldspeed Perhaps [stackoverflow.com/questions/45017937/python-casting-map-object-to-list-makes-map-object-empty](https://stackoverflow.com/questions/45017937/python-casting-map-object-to-list-makes-map-object-empty)? — RoadRunner, Jan 09 '19 at 11:03
@coldspeed No worries, I'll just make sure to close vote next time. — RoadRunner, Jan 09 '19 at 11:05

score 1 · Accepted Answer · answered Jan 09 '19 at 10:28

1

When you cast r (map object) to list it means that you run f function all over your list, and it will be finished. you can not use the result of map twice. like itertools.groupby.

answered Jan 09 '19 at 10:28

Mehrdad Pedramfar

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Thaks a lot !!!! I understand !!! – noobshane Jan 09 '19 at 10:39
@noobshane your welcome, please consider this: https://stackoverflow.com/help/someone-answers – Mehrdad Pedramfar Jan 09 '19 at 10:40

I use the function map() in python,and convert the results into a list,but the result is an empty list

1 Answers1