Why is the output coming out like this?

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Why is the output coming out like this?

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#1 by (10 votes)
#2 by (2 votes)

4

I'm trying to make a program in which I use a list and that program returns 2 lists one with the other pairs with the odd ones.

lista = [0,1,2,3,4,5,6,7,8,9,10]
def par(numero):
    if numero % 2 == 0:
        return numero
def impar(numero):
    if numero % 2 != 0:
        return numero

par = list(map(par,lista))
impar = list(map(impar,lista))
print(par)
print(impar)

The output that it gives me is: [0, None, 2, None, 4, None, 6, None, 8, None, 10] [None, 1, None, 3, None, 5, None, 7, None, 9, None] is there any way to do the program without these Nones occurring? PS I have to do it with the higher order functions then use the map

python python-3.x

asked by anonymous 11.12.2016 / 23:12

2 answers

Turn MD5 MessageDigest into string Tie duplicating whole table

score 10 · Answer 1

The ratio of None is because the map function performs its functions for each element of the list, so a return on those functions is always necessary. Since you do not return anything in else (which does not exist in your functions), Python assumes None in these cases.

If you are required to use map , a possible solution is to simply remove the None after calling map , perhaps using a understanding :

lista = [0,1,2,3,4,5,6,7,8,9,10]
def par(numero):
    if numero % 2 == 0:
        return numero
def impar(numero):
    if numero % 2 != 0:
        return numero

par = [i for i in list(map(par,lista)) if i is not None]
impar = [i for i in list(map(impar,lista)) if i is not None]

print(par)
print(impar)

Note that by doing par = ... you change your par previously defined so that it can no longer be used. It might be nice to use another variable name. :)

If on the other hand you do not need to use map , do it directly:

lista = [0,1,2,3,4,5,6,7,8,9,10]

par = [i for i in lista if i % 2 == 0]
impar = [i for i in lista if i % 2 != 0]

print(par)
print(impar)

There is yet another option. If you are using NumPy , you can do this:

import numpy as np
lista = np.array([0,1,2,3,4,5,6,7,8,9,10])

paridx = np.logical_not((lista % 2).astype(bool))
imparidx = (lista % 2).astype(bool)

par = lista[paridx]
impar = lista[imparidx]

print(par)
print(impar)

Explaining:

The command lista % 2 returns a list with the remainders of the division of each element by 2, which for its list it will return [0 1 0 1 0 1 0 1 0 1 0] .

The (lista % 2).astype(bool) command simply converts these 0s and 1s to logical values, making this list become [False True False True False True False True False True False] . Notice how basically it indicates False where the remainder of the division by 2 is zero, and True where the rest of the division by 2 is 1. That is, basically it indicates true where the number in that position (index) is odd.

This is why the final command is (lista % 2).astype(bool) to indicate the indexes of odd numbers and np.logical_not((lista % 2).astype(bool)) (the logical negation of those values) to indicate the indices of even numbers.

Finally, these indexes are used directly to "filter" in the original list the items where the index indicates true ( True ) - a very useful and useful feature when manipulating data: par = lista[paridx] and impar = lista[imparidx]

score 2 · Answer 2

@Luiz Vieira's answer is perfect. Just one more variant: use filter (lets pass only the values approved by a filter function).

Example:

 lista=range(11)
 par = filter(lambda x: x % 2 == 0 , lista)