Now, if you’ve ever worked with nested lists or arrays, you know how frustrating it can be to extract all those ***** elements into a single flat list. Don’t Worry! With our trusty friend the list comprehension, we can flatten that sucker in no time.
Here’s an example: let’s say we have this nested list of numbers:
# Define a nested list of numbers
vec = [[1,2,3], [4,5,6], [7,8,9]]
# Create an empty list to store the flattened list
flat_list = []
# Loop through each sublist in the nested list
for sublist in vec:
# Loop through each element in the sublist
for element in sublist:
# Append the element to the flat list
flat_list.append(element)
# Print the flattened list
print(flat_list)
# Output: [1, 2, 3, 4, 5, 6, 7, 8, 9]
# Alternatively, we can use list comprehension to flatten the nested list in one line
flat_list = [element for sublist in vec for element in sublist]
# Print the flattened list
print(flat_list)
# Output: [1, 2, 3, 4, 5, 6, 7, 8, 9]
To flatten it into a single flat list using a list comprehension, all you need to do is write the following line of code:
# The following script flattens a nested list into a single flat list using a list comprehension.
# Original script:
flat_list = [num for elem in vec for num in elem]
# The original script is missing a closing bracket at the end.
# The variable "vec" is not defined, so it cannot be used in the list comprehension.
# The variable "elem" is used as both the iterator and the element in the nested list, which can cause confusion.
# The list comprehension is not properly indented, making it difficult to read.
# Define a nested list with multiple sublists.
nested_list = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
# Use a list comprehension to flatten the nested list into a single flat list.
flat_list = [num for sublist in nested_list for num in sublist]
# Print the flattened list.
print(flat_list)
# Output:
# [1, 2, 3, 4, 5, 6, 7, 8, 9]
# Explanation:
# The list comprehension iterates through each sublist in the nested list.
# For each sublist, it then iterates through each element and adds it to the flat list.
# This results in a single flat list containing all the elements from the nested list.
This may look intimidating at first glance, but let’s break it down.
First, we have our list comprehension syntax: `[expression for item in iterable if condition]`. In this case, the expression is simply `num`, which means that we want to extract each individual number from the nested list. The first ‘for’ loop (`elem in vec`) iterates over each sublist within `vec`, and the second ‘for’ loop (`num in elem`) iterates over each element within those sublists.
So, for example, when we run this code on our sample input:
# The following script extracts each individual number from a nested list.
# The first 'for' loop iterates over each sublist within the list, and the second 'for' loop iterates over each element within those sublists.
# Sample input
vec = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
# The 'for' loop iterates over each sublist within the list
[num for elem in vec
# The 'for' loop iterates over each element within the sublist
for num in elem]
# Output: [1, 2, 3, 4, 5, 6, 7, 8, 9]
We get a flat list containing all the numbers from our nested list. Pretty cool, right?
But wait! There’s more! List comprehensions can contain complex expressions and nested functions as well:
# This script uses list comprehension to create a new list containing rounded values of pi, with increasing precision.
# First, we import the math module to access the value of pi.
import math
# Next, we use list comprehension to iterate through a range of numbers from 1 to 5, representing the desired precision of the rounded values.
# The "round" function is used to round the value of pi to the specified precision.
# The "str" function is used to convert the rounded value to a string, as list comprehension can only create lists of strings.
# The "for" loop is used to iterate through each value in the range and add it to the new list.
[str(round(math.pi, i)) for i in range(1, 6)]
# The resulting list contains the rounded values of pi with increasing precision, from 1 decimal place to 5 decimal places.
# The "round" function takes two arguments - the number to be rounded and the desired precision.
# The "str" function is used to convert the rounded value to a string, as list comprehension can only create lists of strings.
# The "for" loop is used to iterate through each value in the range and add it to the new list.
# The "range" function is used to create a range of numbers from 1 to 5, representing the desired precision of the rounded values.
# The "math.pi" expression accesses the value of pi from the math module.
# The "i" variable represents each value in the range and is used as the precision argument for the "round" function.
In this example, we’re using the `round()` function to round the value of pi to two decimal places for each index in our range (starting from 1 and ending at 5). The resulting list comprehension gives us a flat list containing strings representing rounded values of pi.
Flattening lists using list comprehensions is not only powerful, but also incredibly versatile. Give it a try with your own nested lists or arrays you won’t be disappointed!