What's the most pythonic way of performing an arithmetic operation on every nth value in a list? For example, if I start with list1:
list1 = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
I would like to add 1 to every second item, which would give:
list2 = [1, 3, 3, 5, 5, 7, 7, 9, 9, 11]
I've tried:
list1[::2]+1
and also:
for x in list1:
x=2
list2 = list1[::x] + 1
You could use slicing
with a list comprehension as follows:
In [26]: list1 = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
In [27]: list1[1::2] = [x+1 for x in list1[1::2]]
In [28]: list1
Out[28]: [1, 3, 3, 5, 5, 7, 7, 9, 9, 11]
numpy
allows you to use +=
operation with slices too:
In [15]: import numpy as np
In [16]: l = np.array([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])
In [17]: l[1::2] += 1
In [18]: l
Out[18]: array([ 1, 3, 3, 5, 5, 7, 7, 9, 9, 11])
Use enumerate
and a list comprehension
>>> list1 = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
>>> [v+1 if i%2!=0 else v for i,v in enumerate(list1)]
[1, 3, 3, 5, 5, 7, 7, 9, 9, 11]
list1 = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
for i in range(1, len(list1), 2):
list1[i] +=1
print(list1)
using i%2 seems not very efficient
Try this:
list1 = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
for i in range(1,len(list1),2):
list1[i] += 1
You can create an iterator representing the delta ( itertools.cycle([0, 1])
and then add its elements to your existing list.
>>> list1 = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
>>> [a + b for a,b in zip(list1, itertools.cycle([0,1]))]
[1, 3, 3, 5, 5, 7, 7, 9, 9, 11]
>>>
a = [i for i in range(1,11)]
#a = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
b = [a[i]+1 if i%2==1 else a[i] for i in range(len(a))]
#b = [1, 3, 3, 5, 5, 7, 7, 9, 9, 11]
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