How can I use a conditional expression (expression with if and else) in a list comprehension?

Question

I have a list comprehension that produces list of odd numbers of a given range:

[x for x in range(1, 10) if x % 2]

That makes a filter that removes the even numbers. Instead, I’d like to use conditional logic, so that even numbers are treated differently, but still contribute to the list. I tried this code, but it fails:

>>> [x for x in range(1, 10) if x % 2 else x * 100]
  File "<stdin>", line 1
    [x for x in range(1, 10) if x % 2 else x * 100]
                                         ^
SyntaxError: invalid syntax

I know that Python expressions allow a syntax like that:

1 if 0 is 0 else 3

How can I use it inside the list comprehension?

Asked By: ducin

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Source

Answer 1

x if y else z is the syntax for the expression you’re returning for each element. Thus you need:

[ x if x%2 else x*100 for x in range(1, 10) ]

The confusion arises from the fact you’re using a filter in the first example, but not in the second. In the second example you’re only mapping each value to another, using a ternary-operator expression.

With a filter, you need:

[ EXP for x in seq if COND ]

Without a filter you need:

[ EXP for x in seq ]

and in your second example, the expression is a “complex” one, which happens to involve an if-else.

Answered By: shx2

Answer 2

[x if x % 2 else x * 100 for x in range(1, 10) ]

Answered By: lucasg

Answer 3

Just another solution, hope some one may like it :

Using: [False, True][Expression]

>>> map(lambda x: [x*100, x][x % 2 != 0], range(1,10))
[1, 200, 3, 400, 5, 600, 7, 800, 9]
>>>

Answered By: James

Answer 4

You can do that with list comprehension too:

A=[[x*100, x][x % 2 != 0] for x in range(1,11)]
print A

Answered By: Stefan Gruenwald

Answer 5

#how you can squre a list of an array of negative and positive values

my_list=[-2,-3,0,1,5]
squred_values=[]
  

squred_values=[-i**2 if i<0 else i**2 for i in my_list]

#or

for i in my_list:
    if i<0:
        squred_values.append( -i**2)`enter code here`
    else:
        squred_values.append( i**2)

Answered By: Mido

Answer 6

two for in one list comprehension

ex = [['obi', 'is', '#alive'],['oge', 'is', 'beautiful'],
                ['Ade', 'the', '#comedian', 'de', '#rich'],['Jesus', 'wept']]
res = [j if j.startswith("#") else "_"+j for i in ex for j in i]

Answered By: Jagat Deshmukh

How can I use a conditional expression (expression with if and else) in a list comprehension?

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