One-line list comprehension: if-else variants

问题:

It's more about python list comprehension syntax. I've got a list comprehension that produces list of odd numbers of a given range:

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

This makes a filter - I've got a source list, where I remove even numbers (if x % 2). I'd like to use something like if-then-else here. Following code 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

There is a python expression like if-else:

1 if 0 is 0 else 3

How to use it inside a list comprehension?

回答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.

回答2:

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

回答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]
>>>

回答4:

You can do that with list comprehension too:

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

回答5:

I was able to do this

>>> [x if x % 2 != 0 else x * 100 for x in range(1,10)]
    [1, 200, 3, 400, 5, 600, 7, 800, 9]
>>>