R Data Frame: How to Create, Append, Select & Subset

Data Science, AI and ML
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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.).

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 1 book TRUE 2.5 ## 2 2 pen TRUE 8.0 ## 3 3 textbook TRUE 10.0 ## 4 4 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