I have a function that returns two values, and I would like use a list comprehension to fill two lists. for example:
def f(x):
return 2*x,x*x
x = range(3)
xlist, ylist = [f(value) for value in x]
EDITS from answers below:
xtuple, ytuple = zip(*[f(value) for value in x])
xlist, ylist = map(list,zip(*[f(value) for value in x]))
where the expected return should be:
xlist = [0, 2, 4]
ylist = [0, 1, 4]
my question boils down to:
Currently I get a list of tuples, while this is reasonable I will end up needing two independent lists. currently I could have 1 placeholder (tuple list) variable, and 3 total comprehensions. But I'm wondering if there is a clean way to do it with as single list comprehension.
Worth noting: in the real code my two returns are correlated, so I cannot simply split the function into two.
First of all, you made a small mistake: it should be:
[f(value) for value in x]
# ^ notice the `value`
instead of:
[f(x) for value in x]
Furthermore the point is that:
return 2*x,x
is short for:
return (2*x,x)
so a tuple. Your list comprehension thus generates a list of tuples, not a tuple of lists. The nice thing of zip however is that you can easily use it in reverse with the asterisk:
xlist,ylist = zip(*[f(value) for value in x])
# ^ with asteriskNote that xlist and ylist will be tuples (since zip will be unpacked). If you want them to be lists, you can for instance use:
xlist,ylist = map(list,zip(*[f(value) for value in x]))which results in:
>>> xlist
[0, 2, 4]
>>> ylist
[0, 1, 4]
(note that ranges start counting from 0)
Alternative: Another way to do this is of course:
xlist = [f(value)[0] for value in x]
ylist = [f(value)[1] for value in x]
But this is of course inelegantly and furthermore can be inefficient (given f is computationally expensive).
Let's make this work. The function is fine:
def f(x):
return 2*x, x*x
But you want to define the range as follows, notice the starting and ending values:
x = range(1, 4)
Also, you have to call the function with the value, not with the list as parameter. And the final trick to unzip the result into two lists, is to simply zip(*lst) the result of the list comprehension:
xlist, ylist = zip(*[f(value) for value in x])
Now the result is as expected:
xlist
=> [2, 4, 6]
ylist
=> [1, 4, 9]