R is a statistical programming language, designed primarily for data analysis and visualisation. Key to this is in-built support for tables of data. A table of data is called a data.frame. A table has columns, which are given names, and there are rows, which contain data for each column.

For example, let us create a table that contains animals and their sounds;

pets <- data.frame(animal = c("cat", "dog", "bird"),
                   sound = c("meow", "woof", "tweet"))

This will create a data.frame with two columns; animal and sound. There are three rows. You can print pets to the screen by typing print(pets), or, just by typing pets, e.g.

pets

  animal sound
1    cat  meow
2    dog  woof
3   bird tweet

You can access a column using $ followed by the column name, e.g.

pets$animal

[1] "cat"  "dog"  "bird"

pets$sound

[1] "meow"  "woof"  "tweet"

You can also use the index of the column, e.g.

pets[1]

  animal
1    cat
2    dog
3   bird

pets[[1]]

[1] "cat"  "dog"  "bird"

The second version (pets[[1]]) returns the vector that holds the data, while the first version (pets[1]) returns a data.frame that contains only the first column.

To get a row, you need to add a comma (,) after the index, e.g.

pets[1,]

  animal sound
1    cat  meow

pets[2,]

  animal sound
2    dog  woof

You can access a specific value using row index, column index, e.g.

pets[1, 1]

[1] "cat"

pets[1, 2]

[1] "meow"

Reading data tables from files

There are many methods in R that let you read data into a data.frame. We will cover them in much more detail in upcoming workshops.

One such function is read.csv. This reads table data from a comma-separated file. Each line in the file is a row, with columns separated by commas.

For example, cats is a data set available as a CSV file that contains data about cats. You can find a (slightly modified) version of this dataset here.

read.csv can read CSV files from the filesystem (via their filename), or, if you have a URL, directly from the internet. To read this CSV file into a data.frame you should type;

cats <- read.csv("https://chryswoods.com/intermediate_r/data/cats.csv")

EXERCISE

Read the cats data set into a data.frame. What are the three columns in this data?

Hint - type cats$ and wait for a popup to come up.

Answer

Use the calculate_mean and calculate_max functions you wrote in the last page to calculate the mean and maximum body weight and heart weight of the cats.

Answer

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