How to Apply a Function to List Elements

Problem Formulation and Solution Overview

As a Pythonista, coding issues may occur where you need to apply a function against array/matrix elements.

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

The organization Happy Mortgages has six (6) different Mortgage Terms available: 30-Year, 20-Year, 15-Year, 10-Year, 7-Year, and 5-Year terms.

The US Federal Reserve has decided to increase the Mortgage Rate by 1.23%.

💬 Question: How would we update the Array/Matrix entries to increase the matrix/array elements accordingly?

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

• Method 1: Use List Comprehension
• Method 2: Use a map and a lambda
• Method 3: Use a for loop and enumerate

Consider the following related tutorial if you want to apply a function to column elements instead of the matrix or array.

Related Tutorial: How to Apply a Function to Column Elements?

Preparation

Before any data manipulation can occur, one (1) new library will require installation.

• The Pandas library enables access to/from a DataFrame.

To install this library, navigate to an IDE terminal. At the command prompt ($), execute the code below. For the terminal used in this example, the command prompt is a dollar sign ($). Your terminal prompt may be different.

$ pip install pandas

Hit the <Enter> key on the keyboard to start the installation process.

If the installation was successful, a message displays in the terminal indicating the same.

Feel free to view the PyCharm installation guide for the required library.

• How to install Pandas on PyCharm

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

import pandas as pd 

Method 1: Use List Comprehension

List Comprehension offers a single-line expression to change all the Mortgage Rates in one fell swoop!

m_terms = [30, 20, 15, 10, 7, 5]
m_rates = [4.6, 4.3, 3.6, 4.7, 3.8, 3.9]
m_rates = [round(x*.0123+x, 3) for x in m_rates]
print(m_rates)

Above is a list of Mortgage Terms (m_terms) available for the six (6) existing Mortgage Rates (m_rates).

In our code, List Comprehension loops through m_rates applying the Mortgage Rate increase to each element accordingly. The round() method trims the decimal places to three (3). The results save back to m_rates.

Output

Method 2: Use Map and a Lambda

This method is a bit more complex than Method 1. Here we use the map() and lambda functions to accomplish the same task.

m_terms = [30, 20, 15, 10, 7, 5]
m_rates = [4.6, 4.3, 3.6, 4.7, 3.8, 3.9]
m_rates = list(map(lambda x : round(x*.0123+x, 3), m_rates))
print(m_rates)

In this code, we loop through m_rates using map() and passing a lambda as a parameter. The Mortgage Rate increases using the lambda to adust each element accordingly.

The round() method trims the decimal places to three (3). The results save back to m_rates as a list.

Output

Method 3: Use a For Loop and enumerate()

The for loop is initiated with an index (counter) and an item (element value) for m_rates. This variable is wrapped inside enumerate() as an iterable.

m_terms = [30, 20, 15, 10, 7, 5]
m_rates = [4.6, 4.3, 3.6, 4.7, 3.8, 3.9]

for index, item in enumerate(m_rates):
   m_rates[index] = round(m_rates[index]*.0123+m_rates[index], 3)

print (m_rates)

This code loops through m_rates and applies the Mortgage Rate increase to each element.

The round() method trims the decimal places to three (3). Each element saves accordingly.

In case you need a quick refresher on the enumerate() function, have a look at this video tutorial:

Output

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