I'm trying to write a statement in python that turns this input (for example):
[[3,4],[1],[1,2]]
into this output:
[3,4,-,1,-,1,2]
using only zip and list comprehension
this is my code:
a = [[1,2],[1],[3,4]]
result = [j for i in zip(a,'-'*len(a)) for j in i]
print(result)
but all I get is:
[[1, 2], '-', [1], '-', [3, 4], '-']
and my output should be this:
[1, 2, '-', 1, '-', 3, 4]
what am I missing?
This should work:
a = [[1,2],[1],[3,4]]
result = [a for i in zip(a,'-'*len(a)) for j in i for a in j]
print(result)
Output:
[1, 2, '-', 1, '-', 3, 4, '-']
This also works if you don't want the last '-' to be included:
a = [[1,2],[1],[3,4]]
result = [a for i in zip(a, '-'*len(a)) for j in i for a in j][:-1]
print(result)
Output:
[1, 2, '-', 1, '-', 3, 4]
Adding another for clause and prepending the separator before each inner list except the first:
[k for i in zip([[]]+['-']*len(a), a) for j in i for k in j]
Result (Try it online!):
[1, 2, '-', 1, '-', 3, 4]
The zip(...) gives these three pairs:
([], [1, 2])
('-', [1])
('-', [3, 4])
Then j is the inner lists/strings, and k is the numbers/characters, so we can use k for the result elements of the list comprehension.
You can do something like that:
a = [[3,4],[1],[1,2]]
result = []
for i,l in enumerate(a):
result += l + ["-"] * (i!=len(a)-1)
print(result) # [3, 4, '-', 1, '-', 1, 2]
Here is a solution that reuse part of your code and just use the itertools library to achieve your goal:
# import chain
from itertools import chain
a = [[1,2],[1],[3,4]]
result = list(chain.from_iterable(
[j for i in zip(a,'-'*len(a)) for j in i]))[:-1]
print(result)
and your output:
[1, 2, '-', 1, '-', 3, 4]
