[Tut] How to Get a List Slice with Arbitrary Indices in Python?

[Tut] How to Get a List Slice with Arbitrary Indices in Python? - Printable Version

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[Tut] How to Get a List Slice with Arbitrary Indices in Python? - xSicKxBot - 06-18-2020

How to Get a List Slice with Arbitrary Indices in Python?

To extract elements with specific indices from a Python list, use slicing

Code:
list[start:stop:step]

. If you cannot use slicing because there’s no pattern in the indices you want to access, use the list comprehension statement

Code:
[lst[i] for i in indices]

, assuming your indices are stored in the variable

Code:
indices

.

People always want to know the most Pythonic solution to a given problem. This tutorial shows you the most Pythonic solution(s) to the following problem:

Problem: How to extract elements with specific indices from a Python list?

Example: You’ve got the following elements.

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

You want to create a new list of elements with

Code:
indices = [0, 2, 6]

in the original list:

Code:
['a', 'c', 'g']

You can get a quick overview of the most Pythonic methods in our interactive Python shell:

https://repl.it/@finxter/accesslistelements?lite=true

Exercise: Run the code shell. Now, try to access the element with index 7 as well in each of the given methods!

Method 1: List Comprehension

A simple, readable, and efficient way is to use list comprehension that’s a compact way of creating lists. The simple formula is

Code:
[expression + context]

Expression: What to do with each list element?
Context: What elements to select? The context consists of an arbitrary number of
Code:
for
and
Code:
if
statements.

Here’s the code that creates a new list that contains the elements at specific indices (e.g., 0, 2, and 6) in the original list:

Code:
lst = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']
indices = [0, 2, 6]
out = [lst[i] for i in indices]
print(out)
# ['a', 'c', 'g']

In most cases, this will be the best solution because you don’t need any library and it’s still short and readable. However, if you need to do this multiple times, it may be better to import the NumPy library:

Method 2: NumPy Array Indexing

Python’s library for numerical computations, NumPy, is one of the most powerful tools in your toolbelt—especially if you work as a data scientist. Here’s how you can use NumPy to access arbitrary indices, given a sequence of specific indices:

Code:
import numpy as np
lst = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']
a = np.array(lst)
indices = [0, 2, 6] out = a[indices]

The output is:

Code:
print(out)
# ['a' 'c' 'g']

You see that NumPy indexing is far more powerful than Python indexing—it allows you to use arbitrary sequences as indices. Especially, if you need to do multiple of those numerical operations, you may want to import the NumPy library once and gain much in readability and conciseness.

Related resources:

Method 3: Itemgetter

The following method can be seen sometimes—using the

Code:
itemgetter

function from the

Code:
operator

module:

Code:
from operator import itemgetter
lst = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']
indices = [0, 2, 6]
out = list(itemgetter(*indices)(lst))
print(out)
# ['a', 'c', 'g']

The code performs the following steps:

Call the
Code:
itemgetter()
function and pass the arguments 0, 2, and 6. We use the asterisk operator
Code:
*
to unpack the values from the
Code:
indices
variable into the
Code:
itemgetter()
function. Learn more about the asterisk operator in our detailed blog article.
This returns a new itemgetter function object.
Call the new itemgetter function object by passing the original list
Code:
lst
.
The new itemgetter function will now get a tuple of the items at positions 0, 2, and 6 in the original list lst.
Convert the tuple to a list using the
Code:
list(...
) built-in Python function.

Method 4: Manual Indices

Just for comprehensibility, I also want to point out the “naive” way of accessing a few elements in the original list and put them into a new list:

Code:
lst = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']
out = [lst[0], lst[2], lst[6]]
print(out)
# ['a', 'c', 'g']

This is a perfectly valid and efficient approach if you have only a few elements to access. For more than, say, five indices, it quickly becomes unhandy though.

Method 5: Simple Loop

Here’s another approach that’s often used by coders who come from other programming languages such as Java or C++: using simple loops.

Code:
lst = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']
indices = [0, 2, 6]
out = []
for i in indices: out.append(lst[i])
print(out)
# ['a', 'c', 'g']

While there’s nothing wrong with this, it looks somehow “brutal” to an advanced coder. If you’d use this method, it would be like shouting into the world that you’re not a very sophisticated Python coder.

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