CODEWITHSUNDEEP
pythonlistsyntax
Isbister
Isbister

Reputation: 946

Move head and tail 1 step in a List

If I have the big list A = [1,2,3,4,5,6,7,8,9,10] and I have sub lists of 3 elements e.g

B = [1,2,3]

I want to slide 1 step forward according to the A list so B becomes [2,3,4] - is there a smooth way to do this? Or do i just have to pop the first element of B and then append the proper element from A?

Thanks!

EDIT: My answer B = A[i:i+3] Where you can increase 'i' if you want to view forward by 'i' steps.

Upvotes: 1

Views: 403

Answers (4)

Maciej Gol
Maciej Gol

Reputation: 15864

Using itertools:

>>> from itertools import islice, izip
>>> A = [1,2,3,4,5,6,7,8,9,10]
>>> for l in izip(*(islice(A, x, None) for x in xrange(3))):
...     print list(l)
[1, 2, 3]
[2, 3, 4]
[3, 4, 5]
[4, 5, 6]
[5, 6, 7]
[6, 7, 8]
[7, 8, 9]
[8, 9, 10]

islice(A, x, None) creates an iterator over A[x:].

Upvotes: 0

John La Rooy
John La Rooy

Reputation: 304393

>>> A = [1,2,3,4,5,6,7,8,9,10]
>>> for B in (A[i - 3: i] for i in range(3, len(A) + 1)):
...     print B
... 
[1, 2, 3]
[2, 3, 4]
[3, 4, 5]
[4, 5, 6]
[5, 6, 7]
[6, 7, 8]
[7, 8, 9]
[8, 9, 10]

Upvotes: 0

upepo
upepo

Reputation: 41

for i in range(len(a)-2):
    b = a[i:i+3]
    print b 

[1, 2, 3]
[2, 3, 4]
[3, 4, 5]
[4, 5, 6]
[5, 6, 7]
[6, 7, 8]
[7, 8, 9]
[8, 9, 10]

You can make more generally.

Upvotes: 1

roippi
roippi

Reputation: 25974

You can make A a deque:

from collections import deque
A = deque(range(1,11))

And B can be a view of the first 3 elements of A. When you need to "slide", rotate A to the left.

A
Out[71]: deque([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])

from itertools import islice #deques do not support slicing notation
B = list(islice(A,3))

B
Out[74]: [1, 2, 3]

A.rotate(-1)
B = list(islice(A,3))

B
Out[77]: [2, 3, 4]

Upvotes: 3

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