I'm new to python and I want to use the syntax
a = [3, 6]
for x in a:
x *= 2
so 'a' would be [6, 12]
...but this doesn't seem to work. How should I write the code, as simple as possible, to get the wanted effect?
Add a comment
|
4 Answers
The following code change creates a new int object and rebinds x, and not changes the items of the list.
for x in a:
x *= 2
To change the item of the list you should use a[..] = ...
for i in range(len(a)):
a[i] *= 2
You can also use List comprehension as the answer of @Hyperboreus.
To change the value of the nested list, use nested loop.
for i in range(len(a)):
for j in range(len(a[i]):
a[i][j] *= 2
Alternative that use enumerate.
for i, x in enumerate(a):
a[i] = x * 2
4 Comments
bruno desthuilliers
enumerate(iterable) yields an index, element tuple. Better than range(len(iterable)) when you need both the index and the element.
falsetru
@brunodesthuilliers, Thank you for comment. I added that as an alternative.
John La Rooy
"loop control variable" is an odd thing to call it.
x is the items from the list. If x was mutable, the normal thing is for *= to mutate in place. However int is immutable so it just creates a new int object and rebinds instead.You can use this:
a = [x * 2 for x in a]
And for a nested list:
a = [ [1,2,3], [4,5,6] ]
a = [ [x * 2 for x in x] for x in a]
3 Comments
Eugene Naydenov
I don't want to look picky though the extra whitespaces are not needed in the second example. See python.org/dev/peps/pep-0008/… If it looks messy we can write something from new line or put some nested operations into separate expression/function.
John La Rooy
For this case (multiply by 2), you may find
x + x is a little faster than x * 2
Hyperboreus
@gnibbler Right. On my box I gain around 9% of execute time. But I think this has nothing to do with OP's question.
You can use either list comprehension or map() function.
my_list = [3, 6]
my_list = [x * 2 for x in my_list]
my_list = [3, 6]
my_list = map(lambda x: x * 2, my_list)
2 Comments
Hyperboreus
Which works only in python2.x and OP asked explicitly about 2 and 3.
Eugene Naydenov
OK in Python 3 for the
map() case we can do list(map(func, seq)). This is just an idea of different ways.If you find you need to do a lot of these things, maybe you should use numpy
>>> import numpy as np
>>> a = np.array([3, 6])
>>> a *= 2
>>> a
array([ 6, 12])
2 (or more) dimensional array works the same
>>> a = np.array([[3, 6],[4,5]])
>>> a *= 2
>>> a
array([[ 6, 12],
[ 8, 10]])
But there is an overhead converting between list and numpy.array, so it's only worthwhile (efficiency wise) if you need to do multiple operations.
1 Comment
daniel
Oh, great! This will be very useful. Thank you!