How To Create A Subset Of A Dataframe In R

What is a Data Frame?

A data frame is a list of vectors which are of equal length. A matrix contains only one type of data, while a data frame accepts different data types (numeric, character, factor, etc.).

In this tutorial, you will learn-

What is a Data Frame?
How to Create a Data Frame
Append a Column to Data Frame
Select a Column of a Data Frame
Subset a Data Frame

How to Create a Data Frame

We can create a dataframe in R by passing the variable a,b,c,d into the data.frame() function. We can R create dataframe and name the columns with name() and simply specify the name of the variables.

data.frame(df, stringsAsFactors = TRUE)

Arguments:

df: It can be a matrix to convert as a data frame or a collection of variables to join
stringsAsFactors: Convert string to factor by default

We can create a dataframe in R for our first data set by combining four variables of same length.

# Create a, b, c, d variables a <- c(10,20,30,40) b <- c('book', 'pen', 'textbook', 'pencil_case') c <- c(TRUE,FALSE,TRUE,FALSE) d <- c(2.5, 8, 10, 7) # Join the variables to create a data frame df <- data.frame(a,b,c,d) df

Output:

##     a         b     c     d ## 1  10        book  TRUE   2.5 ## 2  20         pen  FALSE  8.0 ## 3  30    textbook  TRUE   10.0 ## 4  40 pencil_case  FALSE  7.0

We can see the column headers have the same name as the variables. We can change column name in R with the function names(). Check the R create dataframe example below:

# Name the data frame names(df) <- c('ID', 'items', 'store', 'price') df

Output:

##   ID       items store price ## 1 10        book  TRUE   2.5 ## 2 20         pen FALSE   8.0 ## 3 30    textbook  TRUE  10.0 ## 4 40 pencil_case FALSE   7.0

# Print the structure str(df)

Output:

## 'data.frame':    4 obs. of  4 variables: ##  $ ID   : num  10 20 30 40 ##  $ items: Factor w/ 4 levels "book","pen","pencil_case",..: 1 2 4 3 ##  $ store: logi  TRUE FALSE TRUE FALSE ##  $ price: num  2.5 8 10 7

By default, data frame returns string variables as a factor.

Slice Data Frame

It is possible to SLICE values of a Data Frame. We select the rows and columns to return into bracket precede by the name of the data frame.

A data frame is composed of rows and columns, df[A, B]. A represents the rows and B the columns. We can slice either by specifying the rows and/or columns.

From picture 1, the left part represents the rows, and the right part is the columns. Note that the symbol : means to. For instance, 1:3 intends to select values from 1 to 3.

Slice Data Frame

In below diagram we display how to access different selection of the data frame:

The yellow arrow selects the row 1 in column 2
The green arrow selects the rows 1 to 2
The red arrow selects the column 1
The blue arrow selects the rows 1 to 3 and columns 3 to 4

Note that, if we let the left part blank, R will select all the rows. By analogy, if we let the right part blank, R will select all the columns.

Slice Data Frame

We can run the code in the console:

## Select row 1 in column 2 df[1,2]

Output:

## [1] book ## Levels: book pen pencil_case textbook

## Select Rows 1 to 2 df[1:2,]

Output:

##   ID items store price ## 1 10  book  TRUE   2.5 ## 2 20   pen FALSE   8.0

## Select Columns 1 df[,1]

Output:

## [1] 10 20 30 40

## Select Rows 1 to 3 and columns 3 to 4 df[1:3, 3:4]

Output:

##   store price ## 1  TRUE   2.5 ## 2 FALSE   8.0 ## 3  TRUE  10.0

It is also possible to select the columns with their names. For instance, the code below extracts two columns: ID and store.

# Slice with columns name df[, c('ID', 'store')]

Output:

##   ID store ## 1 10  TRUE ## 2 20 FALSE ## 3 30  TRUE ## 4 40 FALSE

Append a Column to Data Frame

You can also append a column to a Data Frame. You need to use the symbol $ to append dataframe R variable and add a column to a dataframe in R.

# Create a new vector quantity <- c(10, 35, 40, 5)  # Add `quantity` to the `df` data frame df$quantity <- quantity df

Output:

##   ID       items store price quantity ## 1 10        book  TRUE   2.5       10 ## 2 20         pen FALSE   8.0       35 ## 3 30    textbook  TRUE  10.0       40 ## 4 40 pencil_case FALSE   7.0        5

Note: The number of elements in the vector has to be equal to the no of elements in data frame. Executing the following statement to add column to dataframe R

quantity <- c(10, 35, 40) # Add `quantity` to the `df` data frame df$quantity <- quantity

Gives error:

Error in `$<-.data.frame`(`*tmp*`, quantity, value = c(10, 35, 40))   replacement has 3 rows, data has 4

Select a Column of a Data Frame

Sometimes, we need to store a column of a data frame for future use or perform operation on a column. We can use the $ sign to select the column from a data frame.

# Select the column ID df$ID

Output:

## [1] 1 2 3 4

Subset a Data Frame

In the previous section, we selected an entire column without condition. It is possible to subset based on whether or not a certain condition was true.

We use the subset() function.

subset(x, condition) arguments: - x: data frame used to perform the subset - condition: define the conditional statement

We want to return only the items with price above 10, we can do:

# Select price above 5 subset(df, subset = price > 5)

Output:

ID       items store price 2 20         pen FALSE     8 3 30    textbook  TRUE    10 4 40 pencil_case FALSE     7