Suppose i have the following for loop
L=[]
for e in C:
t=0
for r in D:
if r[0]==e:
t=t+r[2]
L.append((e,t))
To give some more information e is a list, r is a tuple of size 3. I also want each element of L to contain a tuple.
How do i write the following in a list comprehension? I'm unsure as there are variable assignments in the for loop. I'd really appreciate any help! Thanks!!
work it up in reverse.
What is the data to append to L ? a tuple.
What is this tuple made of? e (we have it) and the sum of some terms given a condition.
So without testing I can write:
L = [(e,sum(r[2] for r in D if r[0]==e)) for e in C]
D len(C) times but just once. If we just want to translate OP's loop to a comprehension 1-1, it does not get better than this.
L = [
(
e,
(sum(r[2] for r in D if r[0]==e))
)
for e in C
]
sum…sum(r[2]) this can't be correct
This solution might be more complicated and not a simple list-comprehension but it is also more efficient (O(n) instead of O(n2)). The reason is that we no longer have go through the entire list D for every item e we get from C but only once. As a result, the benefits of this approach scale with increasing size of D or C.
from operator import itemgetter
from itertools import groupby
D = [["A", 0], ["B", 1], ["C", 0], ["D", 2], ["A", 2]]
D.sort(key=itemgetter(0))
glo = {k: sum(f[1] for f in g) for k, g in groupby(D, key=itemgetter(0))}
L = [(e, glo[e]) for e in C]