[Tut] How to Apply a Function to a List

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How to Apply a Function to a List

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Problem Formulation and Solution Overview

This article will show you how to apply a function to a List in Python.

To make it more interesting, we have the following running scenario:

As a Python assignment, you have been given a https://blog.finxter.com/python-lists/List of Integers and asked to apply a function to each List element in various ways.

Question: How would we write code to apply a function to a https://blog.finxter.com/python-lists/List in Python?

We can accomplish this task by one of the following options:

• Method 1: Use a Generator Expression
  
  
• Method 2: Use List Comprehension
  
  
• Method 3: Use a
  Code:

  lambda
  ,
  Code:

  map()
  and https://blog.finxter.com/python-lists/List
  
  
• Method 4: Use a
  Code:

  for
  Loop
  
  
• Bonus: Calculate Commissions on each List Element
  

Preparation

These examples use functions from the

Code:

math

library.

Add the following code to the top of each script. This snippet will allow the code in this article to run error-free.

Code:

import math

Method 1: Use a Generator Expression

This example uses a Generator Expression. This expression performs any operations in memory first and returns an iterable object. An efficient option as upon completion, memory is cleared, and variables erased.

Code:

nums = [18, 43, 54, 65, 31, 21, 27] nums = (math.pow(num,2) for num in nums) print(nums)

The above code declares a List of Integers and saves it to the variable

Code:

nums

.

Next, a Generator Expression is called and applies the

Code:

math.pow()

function from Python’s built-in

Code:

math

library to each list element. The results save back to

Code:

nums

.

If output to the terminal at this point, an iterable Generator Object similar to the following displays.

Code:

<generator object at 0x000002468D9B59A0>

To turn the Generator Object into a https://blog.finxter.com/python-lists/

Code:

List

, run the following code.

Code:

print(list(nums))

The content of

Code:

nums

is as follows.

[324.0, 1849.0, 2916.0, 4225.0, 961.0, 441.0, 729.0]



Note: The

Code:

math.pow()

function accepts two (2) integers as arguments:

Code:

x

(the value) and

Code:

y

(the power), and returns the value of

Code:

x

raised to the power of

Code:

y

.

Method 2: Use List Comprehension

This example uses List Comprehension to perform an operation on each List element.

Code:

nums = [18, 43, 54, 65, 31, 21, 27] nums = [math.sqrt(num) for num in nums] print(nums)

The above code declares a List of Integers and saves it to the variable

Code:

nums

.

Next, List Comprehension is called and applies thehttps://blog.finxter.com/a-guide-to-pyth...-function/

Code:

math.sqrt()

function from Python’s built-in

Code:

math

library to each https://blog.finxter.com/python-lists/List element. The results save back to

Code:

nums

.

If output to the terminal, the following displays.

Code:

[4.242640687119285, 6.557438524302, 7.3484692283495345, 8.06225774829855, 5.5677643628300215, 4.58257569495584, 5.196152422706632]

Note: The

Code:

math.sqrt()

function accepts an integer as an argument and returns the square root of said argument.

Method 3: Use a Lambda and map()

This example uses Python’s

Code:

lambda

function combined with

Code:

map()

and https://blog.finxter.com/python-lists/List to apply a mathematical operation to each List element.

Code:

nums = [18, 43, 54, 65, 31, 21, 27] nums = list(map(lambda x: math.degrees(x), nums)) print(nums)

The above code declares a https://blog.finxter.com/python-lists/List of numbers and saves it to the variable

Code:

nums

.

Next, List is called and passed an argument

Code:

map()

, which in turn passes the

Code:

lambda

function to apply the

Code:

math.degrees()

function from Python’s built-in

Code:

math

library to each https://blog.finxter.com/python-lists/

Code:

L

i

Code:

st

element. The result returns to

Code:

nums

.

If output to the terminal, the following displays.

Code:

[1031.324031235482, 2463.71851906254, 3093.9720937064453, 3724.225668350351, 1776.169164905552, 1203.2113697747288, 1546.9860468532227]

Code:

Note

: The

Code:

math.degrees()

function accepts an angle as an argument, converts this argument from radians to degrees and returns the result.

Method 4: Use a For Loop

This example uses a

Code:

for

Loop to apply a mathematical operation to each List element.

Code:

nums = [18, 43, 54, 65, 31, 21, 27] i = 0 while i < len(nums): nums[i] = round(math.sqrt(nums[i]), 2) i += 1 print(nums)

The above code declares a https://blog.finxter.com/python-lists/List of Integers and saves it to the variable

Code:

nums

. Then, a counter variable,

Code:

i

is declared, set to 0.

Next, a

Code:

while

loop is instantiated and iterates through each List element, applying the https://blog.finxter.com/a-guide-to-pyth...-function/

Code:

math.sqrt()

function, and limiting the decimal places to two (2). The results save back to the appropriate element in

Code:

nums

.

Upon completion of the iteration, the output is sent to the terminal.

Code:

[4.24, 6.56, 7.35, 8.06, 5.57, 4.58, 5.2]

Bonus: Calculate Commissions on each List Element

This bonus code extracts two (2) columns from a

Code:

real-estate.csv

file, the street and price columns and converts each into a List.

Then, the street column is converted from UPPERCASE

Code:

uppercase()

to Title Case by applying the https://blog.finxter.com/python-string-title/

Code:

title()

function. Next, Sales Commissions are calculated and applied to each price element using

Code:

round()

.

Code:

import pandas as pd df = pd.read_csv('real-estate.csv', usecols=['street', 'price']).head(5) street = list(df['street']) street = [item.title() for item in street] prices = list(df['price']) commis = [round(p*.06,2) for p in prices] print(street) print(prices)

The output it as follows.

Code:

['3526 High St', '51 Omaha Ct', '2796 Branch St', '2805 Janette Way', '6001 Mcmahon Dr'][59222, 68212, 68880, 69307, 81900]

Finxter Challenge!
Convert these Lists into a Dictionary format.

Summary

This article has provided four (4) ways to apply a function to each

Code:

List

element to select the best fit for your coding requirements.

Good Luck & Happy Coding!

Programming Humor – Python

[i]“I wrote 20 short programs in Python yesterday. It was wonderful. Perl, I’m leaving you.” — xkcd[/i]