I understand how the simple list comprehension works eg.:
[x*2 for x in range(5)] # returns [0,2,4,6,8]
and also I understand how the nested list comprehesion works:
w_list = ["i_have_a_doubt", "with_the","nested_lists_comprehensions"]
# returns the list of strings without underscore and capitalized
print [replaced.title() for replaced in [el.replace("_"," ")for el in w_list]]
so, when I tried do this
l1 = [100,200,300]
l2 = [0,1,2]
[x + y for x in l2 for y in l1 ]
I expected this:
[100,201,302]
but I got this:
[100,200,300,101,201,301,102,202,302]
so I got a better way solve the problem, which gave me what I want
[x + y for x,y in zip(l1,l2)]
but I didn't understood the return of 9 elements on the first code
The reason it has 9 numbers is because python treats
[x + y for x in l2 for y in l1 ]
similarly to
for x in l2: for y in l1: x + y
ie, it is a nested loop
List comprehensions are equivalent to for-loops. Therefore, [x + y for x in l2 for y in l1 ]
would become:
new_list = [] for x in l2: for y in l1: new_list.append(x + y)
Whereas zip
returns tuples containing one element from each list. Therefore [x + y for x,y in zip(l1,l2)]
is equivalent to:
new_list = [] assert len(l1) == len(l2) for index in xrange(len(l1)): new_list.append(l1[index] + l2[index])
The above answers will suffice for your question but I wanted to provide you with a list comprehension solution for reference (seeing as that was your initial code and what you're trying to understand).
Assuming the length of both lists are the same, you could do:
[l1[i] + l2[i] for i in range(0, len(l1))]
[x + y for x in l2 for y in l1 ]
is equivalent to:
lis = [] for x in l: for y in l1: lis.append(x+y)
So for every element of l
you're iterating l2
again and again, as l
has 3 elements and l1
has elements so total loops equal 9( len(l)*len(l1)
).
this sequence
res = [x + y for x in l2 for y in l1 ]
is equivalent to
res =[] for x in l2: for y in l1: res.append(x+y)
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