Method 1: Iterating over Consecutive (Sliding) Windows
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)
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'.
Where to Go From Here?
Enough theory. Let’s get some practice!
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