I am getting 'list' object is not callable error for this code. I need to add zero before each element

Question

a = [1,2,3,4]

newarray = list(map(a,[0,*a]))

print(newarray)

output:

'list' object is not callable error

expected: Add zero to each element in array

Answer 1

Just use a list comprehension:

out = [[0,x] for x in a]

output: [[0, 1], [0, 2], [0, 3], [0, 4]]

Alternatively, itertools.repeat and zip:

from itertools import repeat
out = list(zip(repeat(0), a))
# or keep a generator
# g = zip(repeat(0), a)

output: [(0, 1), (0, 2), (0, 3), (0, 4)]

strings

as your comment is not fully clear ("prepending the string 0"), in case you really want strings, you can use:

out = [f'0{x}' for x in a]

or

out = list(map('{:02d}'.format, a))

output: ['01', '02', '03', '04']

Answer 2

Builtin function map expects two arguments: a function and a list.

You wrote your arguments in the wrong order: you passed list a first instead of second.

And the thing you tried to pass as a function is not really a function.

Here is a possible fix by defining a function with def:

def the_function_for_map(x):
    return [0, x]

newarray = list(map(the_function_for_map, a))

Here is a possible fix by defining a function with lambda:

newarray = list(map(lambda x: [0, x], a))

And finally, here is a possible fix using a list comprehension instead of map:

newarray = [[0, x] for x in a]

In your particular case, you can also use zip with a list full of zeroes:

newarray = list(zip([0]*len(a), a))

Or zip_longest with an empty list and a default value:

from itertools import zip_longest

newarray = list(zip_longest([], a, fillvalue=0))