7.14: List comprehensions
- Page ID
- 23759
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)What is a List Comprehension?
A list comprehension is an elegant way to create a new list by applying operations to an existing list or iterable object (like a string). It's a core feature of Python that draws inspiration from mathematical set-builder notation.
Basic Structure
The general syntax of a list comprehension looks like this:
new_list = [expression for item in iterable if condition]
Let's break it down:
- expression: This defines the element that will be included in the new list. It can be a simple variable reference, a transformation of the original item, or a more complex calculation.
- item: A placeholder representing each element in the original iterable object.
- iterable: The existing list, sequence, or any object that can be iterated over.
- if condition: An optional filter; only elements that meet the condition will be included in the new list.
Example
Let's square the numbers in a list:
numbers = [1, 2, 3, 4, 5]
# Traditional way (using a for loop)
squares = []
for number in numbers:
squares.append(number * number)
# Using a list comprehension
squares = [number * number for number in numbers]
print(squares) # Output: [1, 4, 9, 16, 25]
Here's an example using list comprehensions to make a list of squared numbers that are odd:
numbers = [1, 2, 3, 4, 5]
# Using a list comprehension
odd_squares = [number * number for number in numbers if number % 2 != 0]
print(odd_squares) # Output: [1, 9, 25]
This code first creates a list of numbers [1, 2, 3, 4, 5]. Then, it uses a list comprehension to create a new list odd_squares. The list comprehension iterates over the original list numbers, and for each number, it checks if the number is odd using the modulo operator (%). If the number is odd, it squares the number and adds it to the odd_squares list. Otherwise, it does nothing.
The final output of the code is the list [1, 9, 25], which contains the squares of all the odd numbers in the original list.
Advantages of List Comprehensions
- Conciseness: List comprehensions can condense several lines of traditional loop-based code into a single, readable expression.
- Clarity List comprehensions often clearly convey the intent of the list transformation.
- Potential Efficiency: Under certain conditions, list comprehensions may be more efficient than manually constructed loops.
When to Use List Comprehensions
List comprehensions are best suited for simple to moderately complex list transformations. For very complex scenarios, a traditional for loop might be more readable and maintainable.
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