Skip to contents

Lab 5: Writing and debugging a function in R

Lab5.Rmd

Overview

In this lab, you will learn how to write a simple function in R and use the browser() command to debug it. Debugging is a crucial step in the development process of any software. It involves finding and fixing errors or bugs in your code. The browser() command allows you to stop the execution of your code at a specific point and inspect the values of variables and objects.

Loading data

We will use the NHANES library which provides a small subset of the National Health and Nutrition Examination data (https://wwwn.cdc.gov/nchs/nhanes/Default.aspx).

Install and load the “NHANES” dataset and find the help page that contains a codebook.

  1. Do not include an evaluated install.package() in your Rmd program. What can happen if you do?

  2. What is in the “Description” section of the help page for the “NHANES” object?

Viewing the data

Note that the following command is for interactive use only, so to show it only you can use the eval = FALSE command to the code chunk.

What can happen if you have an evaluated View() command in your .Rmd file?

View(NHANES)

Summarizing the data

Use the summary() function to summarize the dataset. How many missing values are there for weight and height?

Writing a Function

Let’s write a function that takes two arguments: 1. df is a dataframe with two columns of numeric data 2. i is an integer vector of indices corresponding to rows of this dataframe that will be used in the calculation

The function filters the dataframe to only rows i that have no missing values, then returns the difference in the mean of the first column minus the mean of the second column.

Hint: start by doing this calculation on the weight and height columns of the NHANES dataset, then copy your working code into a function.

Debugging with the Browser Command

If your function is working without error, create an error intentionally. For example, pass a character value for the argument i.

Now we want to use the browser() command to debug the function. The browser() command will stop the execution of the function at a specific point and allow us to inspect the values of variables and objects.

  1. Insert the browser() command as the first line inside your function, and pass this new function definition to the R console.

  2. Use ls() or the “Environment” tab to see what variables are visible in your new environment within the function. What variables do you see? What are they? How has this changed from before you entered the function?

  3. Once you have figured out what caused the error, exit from browser using Q, remove the browser command from your function, and re-define it without containing browser().

Write another function

Write another function that takes the same input arguments, but returns the difference in mean weight of males minus mean weight of females.