CODEWITHSUNDEEP
pythonlistlist-comprehension
S_Ymln
S_Ymln

Reputation: 421

Multiple list comprehension into single list comprehension

I am trying to change the values of a list using list comprehension I can do that by using 3 list comprehensions 

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 as using the below mentioned code is throwing syntax error

clr = ["green"  if i== 1 else "yellow"  if i==2 else "black"  if i == 3 for i in clr]

Is there any better way to do it??

Upvotes: 3

Views: 415

Answers (3)

willeM_ Van Onsem
willeM_ Van Onsem

Reputation: 477684

Yes. You can for instance define a dictionary:

the_dic = { 1 : 'green', 2 : 'yellow', 3 : 'black' }

and then perform a mapping like:

clr = [the_dic.get(i,i) for i in clr]

Or by using map(..) (in this works as a generator (thus lazily):

clr = map(the_dic.get,clr)

This will insert Nones in case the element in clr is not in the dictionary.

This will thus add i to the clr list, if it is not in the dictionary. This is beause we use the_dic.get(i,i). The first i is the key we lookup in the dictionary. The second i is the "fallback" value: the value we return in case the key is not found.

In case you want to filter these out, you can use:

clr = [the_dic[i] for i in clr if i in the_dic]

Upvotes: 4

AGN Gazer
AGN Gazer

Reputation: 8388

You can actually do this within list comprehension:

[("green"  if i == 1 else ("yellow" if i == 2 else "black")) for i in clr]

and if you want to skip over values that are neither 1, 2, or 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 this:

[("green"  if i == 1 else ("yellow" if i == 2 else ("black" if i == 3 else i))) for i in clr]

Upvotes: 0

verisimilidude
verisimilidude

Reputation: 475

    colors = collections.defaultdict(int,{ 1:'green', 2:'yellow', 3:'black' })
    clr = [ colors[i] for i in clr ]

You could add if 1 <= i <= 3 condition to the list comprehension if it is important not to put the extraneous 0's in the output. In that case you could use an ordinary dict instead of a defaultdict.

Upvotes: 0

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