Multiple list comprehension in a single list comprehension
I am trying to change the values of a list using a list comprehension. I can do this using 3 concept lists
clr = [1,2,2,1,3,1,2,3]
clr= ["green" if i== 1 else i for i in clr]
clr = ["yellow" if i==2 else i for i in clr]
clr = ["black" if i == 3 else i for i in clr]
where using the below code a syntax error occurs
clr = ["green" if i== 1 else "yellow" if i==2 else "black" if i == 3 for i in clr]
Is there a better way to do this?
source to share
Yes. You can, for example, define a dictionary :
the_dic = { 1 : 'green', 2 : 'yellow', 3 : 'black' }
and then do the matching like:
clr = [the_dic.get(i,i) for i in clr]
Or using map(..)
(inpython-3.x works like a generator (thus lazy):
clr = map(the_dic.get,clr)
Insert None
if item at is clr
not in the dictionary.
So this will add i
to the list clr
if it is not in the dictionary. This is because we are using the_dic.get(i,i)
. The first i
is the key we are looking at in the dictionary. The second i
is the "fallback" value: the value we return in case the key is not found.
If you want to filter these , you can use:
clr = [the_dic[i] for i in clr if i in the_dic]
source to share
colors = collections.defaultdict(int,{ 1:'green', 2:'yellow', 3:'black' })
clr = [ colors[i] for i in clr ]
You can add if 1 <= i <= 3 condition
to list comprehension if it is important not to put an extraneous 0 in the output. In this case, you can use normal dict instead of defaultdict.
source to share
You can actually do this in a list comprehension:
[("green" if i == 1 else ("yellow" if i == 2 else "black")) for i in clr]
and if you want to skip values that are neither 1, 2, nor 3:
[("green" if i == 1 else ("yellow" if i == 2 else "black")) for i in clr if i in range(1, 4)]
and if you want to leave the values not in [1, 2, 3] unchanged, do the following:
[("green" if i == 1 else ("yellow" if i == 2 else ("black" if i == 3 else i))) for i in clr]
source to share