I'd like to edit some elements in python's list inside a loop. How can I make a proper loop for this? This code doesn't work:
for i in X:
i=(i-C1)/C2
5 Answers
Use a list comprehension:
X = [(i-C1)/C2 for i in X]
And a minor point, the Python Style Guide recommends using all lower case for your variable names, e.g.:
x = [(i-c1)/c2 for i in x]
Capital letters are usually used to start class names (and all-capitals denotes a constant).
Comments
try this:
X=[(i-C1)/C2 for i in X]
Comments
Note that although the following is possible it may not be recommended:
for i,v in enumerate(X):
X[i]=(v-C1)/C2
3 Comments
Rusty Rob
People don't recommend editing a data structure while iterating through it, although I don't see the problem in this case.
John La Rooy
@robert, you shouldn't add or remove elements while iterating. Replacing items is ok
John La Rooy
This is the way to do it if you need to include other statements in the loop, or perhaps if the formula is more complicated
You can do
X = [(y - C1) / C2 for y in X]
This will not modify your list, but create a new list based on the change you wanted to do and assign it back to X.
Comments
using X[:] is better than X, as it allows splice assignment:
X[:] = [(y - C1) / C2 for y in X]
if you wanted to loop through, I would recommend using enumerate(X).
e.g.
for i,y in enumerate(X):
X[i] = (y - C1) / C2
Here i is assigned the position in the array (i = 0..len(X)-1) and y is the value of X[i].
look up enumerate if you're interested.
Of course you must be careful if you edit a list (or any data structure) while iterating through it in case you change values before they have been iterated. e.g:
x = [1,2,3]
for i,y in enumerate(x):
x[-1] = 99
print(i,y)
>>>
0 1
1 2
2 99
notice you get "2 99" instead of "2 3"
1 Comment
John La Rooy
The slice assignment is a good way to save memory if you use a generator expression instead of a list comprehension
X[:] = ((y - C1) / C2 for y in X)