I'm trying to take two lists and use one list to make changes in the second list as the following code shows:
a = [1,2,3,4,5]
b = [0,0,0,0,0]
for aA, bB in zip(a,b):
bB = aA*4
print(b)
I get the result b = [0,0,0,0,0], I want b = [4,8,12,16,20]
I know that you shouldn't modify containers while iterating with a for loop. But is there any work-around in python to be able to get the intended result of modifying a container?
Python unlike many other languages is dynamically typed as in you don't need to initialize prepopulated arrays(lists) before populating them with data. Also a standard convention is list comprehensions which in lamens are in-line loops that return a list.
a = [1,2,3,4,5]
b = [num * 4 for num in a]
This is equivilent to:
a = [1,2,3,4,5]
b = []
for num in a:
b.append(num * 4)
Why not use index?
for i in range(len(a)):
b[i] = a[i]
The standard advice is "don't change a list while you're iterating over it". We say this because if you add or remove an element then the iterator will start doing weird hard-to-predict things. But that's not what's happening here: we're iterating over a and changing b, knowing in advance they're the same size, so there's no undefined or hard-to-understand behavior.
The values you want to put into b only depend on a, so there is no reason to initialize and iterate on b. You could do either:
# build list b as you go
b = []
for aA in a:
b.append(aA*4)
print(b)
or
# use a list comprehension
b = [aA*4 for aA in a]
print(b)
Please try this:
b.append( aA*4)
