Data Frame

Data frame is a two dimensional data structure in R. It is a special case of a list which has each component of equal length. Each component form the column and contents of the component form the rows. We can check if a variable is a data frame or not using the class() function.

> x
 SN Age Name
1  1  21 John
2  2  15 Dora

> typeof(x)    # data frame is a special case of  list
[1] "list"

> class(x)
[1] "data.frame"

In this example, x can be considered as a list of 3 components with each component having a two element vector. Some useful functions to know more about a data frame are given below.

> names(x)
[1] "SN"   "Age"  "Name"

> ncol(x)
[1] 3

> nrow(x)
[1] 2

> length(x)    # returns length of the list, same as ncol()
[1] 3

Creating a Data Frame

We can create a data frame using the data.frame() function. For example, the above shown data frame can be created as follows.

> x <- data.frame("SN"=1:2,"Age"=c(21,15),"Name"=c("John","Dora"))

> str(x)    # structure of x
'data.frame':   2 obs. of  3 variables:
$ SN  : int  1 2
$ Age : num  21 15
$ Name: Factor w/ 2 levels "Dora","John": 2 1

Notice above that the third column, Name is of type factor, instead of a character vector. By default, data.frame() function converts character vector into factor. To suppress this behavior, we can pass the argument stringsAsFactors=FALSE.

> x <- data.frame("SN"=1:2,"Age"=c(21,15),"Name"=c("John","Dora"),stringsAsFactors=FALSE)

> str(x)    # now the third column is a character vector
'data.frame':   2 obs. of  3 variables:
$ SN  : int  1 2
$ Age : num  21 15
$ Name: chr  "John" "Dora"

Many data input functions of R like, read.table(), read.csv(), read.delim(), read.fwf() also read data into a data frame.

Accessing Components in Data Frame

Components of data frame can be accessed like a list or like a matrix.

Accessing like a list

We can use either [, [[ or $ operator to access columns of data frame.

> x["Name"]
 Name
1 John
2 Dora

> x$Name
[1] "John" "Dora"

> x[["Name"]]
[1] "John" "Dora"

> x[[3]]
[1] "John" "Dora"

Accessing with [[ or $ is similar. However, it differs for [ in that, indexing with [ will return us a data frame but the other two will reduce it into a vector.

Accessing like a matrix

Data frames can be accessed like a matrix by providing index for row and column. To illustrate this, we use datasets already available in R. Datasets that are available can be listed with the command library(help = "datasets"). We will use the trees dataset which contains Girth, Height and Volume for Black Cherry Trees. A data frame can be examined using functions like str() and head().

> str(trees)
'data.frame':   31 obs. of 3 variables:
$ Girth : num  8.3 8.6 8.8 10.5 10.7 10.8 11 11 11.1 11.2 ...
$ Height: num  70 65 63 72 81 83 66 75 80 75 ...
$ Volume: num  10.3 10.3 10.2 16.4 18.8 19.7 15.6 18.2 22.6 19.9 ...

> head(trees,n=3)
 Girth Height Volume
1   8.3     70   10.3
2   8.6     65   10.3
3   8.8     63   10.2

 

We can see that trees is a data frame with 31 rows and 3 columns. We also display the first 3 rows of the data frame. Now we proceed to access the data frame like a matrix.

> trees[2:3,]    # select 2nd and 3rd row
 Girth Height Volume
2   8.6     65   10.3
3   8.8     63   10.2

> trees[trees$Height > 82,]    # selects rows with Height greater than 82
  Girth Height Volume
6   10.8     83   19.7
17  12.9     85   33.8
18  13.3     86   27.4
31  20.6     87   77.0

> trees[10:12,2]
[1] 75 79 76

We can see in the last case that the returned type is a vector since we extracted data from a single column. This behavior can be avoided by passing the argument drop=FALSE as follows.

> trees[10:12,2, drop=FALSE]
  Height
10     75
11     79
12     76

Modifying a Data Frame

Data frames can be modified like we modified matrices through reassignment.

> x
 SN Age Name
1  1  21 John
2  2  15 Dora

> x[1,"Age"] <- 20; x
 SN Age Name
1  1  20 John
2  2  15 Dora

Adding Components

Rows can be added to a data frame using the rbind() function.

> rbind(x,list(1,16,"Paul"))
 SN Age Name
1  1  20 John
2  2  15 Dora
3  1  16 Paul

Similarly, we can add columns using cbind().

> cbind(x,State=c("NY","FL"))
 SN Age Name State
1  1  20 John    NY
2  2  15 Dora    FL

Since data frames are implemented as list, we can also add new columns through simple list-like assignments.

> x
 SN Age Name
1  1  20 John
2  2  15 Dora

> x$State <- c("NY","FL"); x
 SN Age Name State
1  1  20 John    NY
2  2  15 Dora    FL

Deleting Component

Data frame columns can be deleted by assigning NULL to it.

> x$State <- NULL
> x
 SN Age Name
1  1  20 John
2  2  15 Dora

Similarly, rows can be deleted through reassignments.

> x <- x[-1,]
> x
 SN Age Name
2  2  15 Dora