How to Loop Through a Python List in Pairs, Sliding Windows, and Batches?

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Method 1: Iterating over Consecutive (Sliding) Windows

Python sliding window

Given are:

  • Python list lst
  • Window size n

Problem Formulation: How to loop through the list in consecutive element-windows of size n, so that in each iteration, you can access the n next elements in the list?

# INPUT: 
lst = ['a', 'b', 'c', 'd', 'e', 'f', 'g']
n = 3 # window

# OUTPUT: 
 Window:  ['a', 'b', 'c']
 Window:  ['b', 'c', 'd']
 Window:  ['c', 'd', 'e']
 Window:  ['d', 'e', 'f']
 Window:  ['e', 'f', 'g']

? Solution: To iterate over a Python list lst in windows of size n, iterate over all list indices i from 0 to the index of the n-th last list element (included). In the loop body, use the indices to retrieve the windows by slicing lst[i:i+n]. Slicing is memory-efficient because it doesn’t create copies of the original list.

Here’s the full code:

lst = ['a', 'b', 'c', 'd', 'e', 'f', 'g']
n = 3

for i in range(len(lst)-n+1):
    batch = lst[i:i+n]
    print('Window: ', batch)

The output is:

'''
OUTPUT:
Window:  ['a', 'b', 'c']
Window:  ['b', 'c', 'd']
Window:  ['c', 'd', 'e']
Window:  ['d', 'e', 'f']
Window:  ['e', 'f', 'g']
'''

A variant of this is to iterate over consecutive windows of size 2: pairs!

Method 2: Itearing over a List in Pairs

Problem Formulation: How to iterate over a list in pairs of elements?

This is a variant of the previous problem where the window size is n=2. We use the same approach:

? Solution: To iterate over a Python list lst in pairs, iterate over all list indices i from 0 to the index of the second last list element (included). In the loop body, use the indices to retrieve the windows by slicing lst[i:i+2]. Slicing is memory-efficient because it doesn’t create copies of the original list.

Here’s the full code:

lst = ['a', 'b', 'c', 'd', 'e', 'f', 'g']

for i in range(len(lst)-1):
    pair = lst[i:i+2]
    print('Pair: ', pair)

The output is:

'''
OUTPUT:
Pair:  ['a', 'b']
Pair:  ['b', 'c']
Pair:  ['c', 'd']
Pair:  ['d', 'e']
Pair:  ['e', 'f']
Pair:  ['f', 'g']
'''

Method 3: Slicing Over Non-Overlapping Windows (Batches)

Python iterate batch window (non-overlapping)

A variant of the general problem addressed in Method 1 is to iterate over the list in batches of size n. However, the sliding windows are not allowed to overlap!

Problem Formulation: How to iterate over the list in batches of size n whereas each batch must have unique elements (non-overlapping batches)?

Solution: You can do this by modifying the range() function that defines the loop variable i to also include a step size of n. You then use slicing lst[i:i+n] to access the n next elements from each index i. This results in non-overlapping windows (batches) of list slices.

lst = ['a', 'b', 'c', 'd', 'e', 'f']
n = 3 # batch size

for i in range(0, len(lst)-n+1, n):
    batch = lst[i:i+n]
    print('Batch: ', batch)

Here’s the output:

'''
OUTPUT:
Batch:  ['a', 'b', 'c']
Batch:  ['d', 'e', 'f']
'''

Background Slicing

Background: Slicing is a concept to carve out a substring from a given string. Use slicing notation s[start:stop:step] to access every step-th element starting from index start (included) and ending in index stop (excluded). All three arguments are optional, so you can skip them to use the default values (start=0, stop=len(lst), step=1). For example, the expression s[2:4] from string 'hello' carves out the slice 'll' and the expression s[:3:2] carves out the slice 'hl'.

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