R Language Subsetting Data frames
Example
Subsetting a data frame into a smaller data frame can be accomplished the same as subsetting a list.
> df3 <- data.frame(x = 1:3, y = c("a", "b", "c"), stringsAsFactors = FALSE)
> df3
## x y
## 1 1 a
## 2 2 b
## 3 3 c
> df3[1] # Subset a variable by number
## x
## 1 1
## 2 2
## 3 3
> df3["x"] # Subset a variable by name
## x
## 1 1
## 2 2
## 3 3
> is.data.frame(df3[1])
## TRUE
> is.list(df3[1])
## TRUE
Subsetting a dataframe into a column vector can be accomplished using double brackets [[ ]] or the dollar sign operator $.
> df3[[2]] # Subset a variable by number using [[ ]]
## [1] "a" "b" "c"
> df3[["y"]] # Subset a variable by name using [[ ]]
## [1] "a" "b" "c"
> df3$x # Subset a variable by name using $
## [1] 1 2 3
> typeof(df3$x)
## "integer"
> is.vector(df3$x)
## TRUE
Subsetting a data as a two dimensional matrix can be accomplished using i and j terms.
> df3[1, 2] # Subset row and column by number
## [1] "a"
> df3[1, "y"] # Subset row by number and column by name
## [1] "a"
> df3[2, ] # Subset entire row by number
## x y
## 2 2 b
> df3[ , 1] # Subset all first variables
## [1] 1 2 3
> df3[ , 1, drop = FALSE]
## x
## 1 1
## 2 2
## 3 3
Note: Subsetting by j (column) alone simplifies to the variable's own type, but subsetting by i alone returns a data.frame, as the different variables may have different types and classes. Setting the drop parameter to FALSE keeps the data frame.
> is.vector(df3[, 2])
## TRUE
> is.data.frame(df3[2, ])
## TRUE
> is.data.frame(df3[, 2, drop = FALSE])
## TRUE
