R Data Frame
https://www.datamentor.io/r-programming/data-frame/
Check if a variable is a data frame or not
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.
Functions of data frame
> 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
How to create a Data Frame in R?
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.
How to access Components of a 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$Height > 82,] # selects rows with Height greater than 82
Girth Height Volume
6 10.8 83 19.7
17 12.9 85 33.8
18 13.3 86 27.4
31 20.6 87 77.0
> trees[10:12,2]
[1] 75 79 76
We can see in the last case that the returned type is a vector since we extracted data from a single column.
This behavior can be avoided by passing the argument drop=FALSE as follows.
> trees[10:12,2, drop = FALSE]
Height
10 75
11 79
12 76
How to modify a Data Frame in R?
Data frames can be modified like we modified matrices through reassignment.
> x
SN Age Name
1 1 21 John
2 2 15 Dora
> x[1,"Age"] <- 20; x
SN Age Name
1 1 20 John
2 2 15 Dora
Adding Components
Rows can be added to a data frame using the rbind() function.
> rbind(x,list(1,16,"Paul"))
SN Age Name
1 1 20 John
2 2 15 Dora
3 1 16 Paul
Similarly, we can add columns using cbind().
> cbind(x,State=c("NY","FL"))
SN Age Name State
1 1 20 John NY
2 2 15 Dora FL
Since data frames are implemented as list, we can also add new columns through simple list-like assignments.
> x
SN Age Name
1 1 20 John
2 2 15 Dora
> x$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
