Pandas for Everyone: Python Data Analysis, First Edition

9. Apply

9.1 Introduction

Learning about apply is fundamental in the data cleaning process. It also encapsulates key concepts in programming, mainly writing functions. apply takes a function and “applies” (i.e., runs it) across each row or column of a dataframe “simultaneously.” If you’ve programmed before, then the concept of an “apply” should be familiar. It is similar to writing a for loop across each row or column and calling the function—apply just does it simultaneously. In general, this is the preferred way to apply functions across dataframes, because it typically is much faster than writing a for loop in Python.

Objectives

This chapter will cover:

1. Functions

2. Applying functions across columns or rows of data

9.2 Functions