If I have a list:
to_modify = [5,4,3,2,1,0]
And then declare two other lists:
indexes = [0,1,3,5]
replacements = [0,0,0,0]
How can I take to_modify's elements as index to indexes, then set corresponding elements in to_modify to replacements, i.e. after running, indexes should be [0,0,3,0,1,0].
Apparently, I can do this through a for loop:
for ind in to_modify:
indexes[to_modify[ind]] = replacements[ind]
But is there other way to do this?
Could I use operator.itemgetter somehow?
A little slower, but readable I think:
>>> s, l, m
([5, 4, 3, 2, 1, 0], [0, 1, 3, 5], [0, 0, 0, 0])
>>> d = dict(zip(l, m))
>>> d #dict is better then using two list i think
{0: 0, 1: 0, 3: 0, 5: 0}
>>> [d.get(i, j) for i, j in enumerate(s)]
[0, 0, 3, 0, 1, 0]
You can solve it using dictionary
to_modify = [5,4,3,2,1,0]
indexes = [0,1,3,5]
replacements = [0,0,0,0]
dic = {}
for i in range(len(indexes)):
dic[indexes[i]]=replacements[i]
print(dic)
for index, item in enumerate(to_modify):
for i in indexes:
to_modify[i]=dic[i]
print(to_modify)
The output will be
{0: 0, 1: 0, 3: 0, 5: 0}
[0, 0, 3, 0, 1, 0]
numpy has arrays that allow you to use other lists/arrays as indices:
import numpy
S=numpy.array(s)
S[a]=m
for index in a:
This will cause index to take on the values of the elements of a, so using them as indices is not what you want. In Python, we iterate over a container by actually iterating over it.
"But wait", you say, "For each of those elements of a, I need to work with the corresponding element of m. How am I supposed to do that without indices?"
Simple. We transform a and m into a list of pairs (element from a, element from m), and iterate over the pairs. Which is easy to do - just use the built-in library function zip, as follows:
for a_element, m_element in zip(a, m):
s[a_element] = m_element
To make it work the way you were trying to do it, you would have to get a list of indices to iterate over. This is doable: we can use range(len(a)) for example. But don't do that! That's not how we do things in Python. Actually directly iterating over what you want to iterate over is a beautiful, mind-liberating idea.
what about operator.itemgetter
Not really relevant here. The purpose of operator.itemgetter is to turn the act of indexing into something, into a function-like thing (what we call "a callable"), so that it can be used as a callback (for example, a 'key' for sorting or min/max operations). If we used it here, we'd have to re-call it every time through the loop to create a new itemgetter, just so that we could immediately use it once and throw it away. In context, that's just busy-work.
Why not just:
map(s.__setitem__, a, m)
You can use operator.setitem.
from operator import setitem
a = [5, 4, 3, 2, 1, 0]
ell = [0, 1, 3, 5]
m = [0, 0, 0, 0]
for b, c in zip(ell, m):
setitem(a, b, c)
>>> a
[0, 0, 3, 0, 1, 0]
Is it any more readable or efficient than your solution? I am not sure!
Source: Stackoverflow.com