In R there are usually many ways to perform a task. The ones that have been developed more recently are generally easier, having been developed to avoid the difficulty of the earlier ways. Subsetting a dataframe is a common task.
The original method is to use the bracket operator.
NewDF = OldDF[Rows I want, Columns I want]
Columns I want can be specified as a vector of column numbers or as a vector of column names enclosed in quotes.
Rows I want is usually expressed as a logical expression. Let’s take a look at the mtcars dataframe, which is always installed in base R inside the datasets package. First do an str() and a summary() to look at it.
str(mtcars)## 'data.frame': 32 obs. of 11 variables:
## $ mpg : num 21 21 22.8 21.4 18.7 18.1 14.3 24.4 22.8 19.2 ...
## $ cyl : num 6 6 4 6 8 6 8 4 4 6 ...
## $ disp: num 160 160 108 258 360 ...
## $ hp : num 110 110 93 110 175 105 245 62 95 123 ...
## $ drat: num 3.9 3.9 3.85 3.08 3.15 2.76 3.21 3.69 3.92 3.92 ...
## $ wt : num 2.62 2.88 2.32 3.21 3.44 ...
## $ qsec: num 16.5 17 18.6 19.4 17 ...
## $ vs : num 0 0 1 1 0 1 0 1 1 1 ...
## $ am : num 1 1 1 0 0 0 0 0 0 0 ...
## $ gear: num 4 4 4 3 3 3 3 4 4 4 ...
## $ carb: num 4 4 1 1 2 1 4 2 2 4 ...summary(mtcars)## mpg cyl disp hp
## Min. :10.40 Min. :4.000 Min. : 71.1 Min. : 52.0
## 1st Qu.:15.43 1st Qu.:4.000 1st Qu.:120.8 1st Qu.: 96.5
## Median :19.20 Median :6.000 Median :196.3 Median :123.0
## Mean :20.09 Mean :6.188 Mean :230.7 Mean :146.7
## 3rd Qu.:22.80 3rd Qu.:8.000 3rd Qu.:326.0 3rd Qu.:180.0
## Max. :33.90 Max. :8.000 Max. :472.0 Max. :335.0
## drat wt qsec vs
## Min. :2.760 Min. :1.513 Min. :14.50 Min. :0.0000
## 1st Qu.:3.080 1st Qu.:2.581 1st Qu.:16.89 1st Qu.:0.0000
## Median :3.695 Median :3.325 Median :17.71 Median :0.0000
## Mean :3.597 Mean :3.217 Mean :17.85 Mean :0.4375
## 3rd Qu.:3.920 3rd Qu.:3.610 3rd Qu.:18.90 3rd Qu.:1.0000
## Max. :4.930 Max. :5.424 Max. :22.90 Max. :1.0000
## am gear carb
## Min. :0.0000 Min. :3.000 Min. :1.000
## 1st Qu.:0.0000 1st Qu.:3.000 1st Qu.:2.000
## Median :0.0000 Median :4.000 Median :2.000
## Mean :0.4062 Mean :3.688 Mean :2.812
## 3rd Qu.:1.0000 3rd Qu.:4.000 3rd Qu.:4.000
## Max. :1.0000 Max. :5.000 Max. :8.000So suppose I want a smaller dataframe which contains only cars with mpg > 20 and contains only the columns mpg, cyl and disp.
SmallDF = mtcars[mtcars$mpg > 20, c("mpg","cyl","disp")]
# let's look at it
SmallDF## mpg cyl disp
## Mazda RX4 21.0 6 160.0
## Mazda RX4 Wag 21.0 6 160.0
## Datsun 710 22.8 4 108.0
## Hornet 4 Drive 21.4 6 258.0
## Merc 240D 24.4 4 146.7
## Merc 230 22.8 4 140.8
## Fiat 128 32.4 4 78.7
## Honda Civic 30.4 4 75.7
## Toyota Corolla 33.9 4 71.1
## Toyota Corona 21.5 4 120.1
## Fiat X1-9 27.3 4 79.0
## Porsche 914-2 26.0 4 120.3
## Lotus Europa 30.4 4 95.1
## Volvo 142E 21.4 4 121.0You can use column numbers instead of names.
SmallDF = mtcars[mtcars$mpg > 20, c(1,2,3)]
# let's look at it
SmallDF## mpg cyl disp
## Mazda RX4 21.0 6 160.0
## Mazda RX4 Wag 21.0 6 160.0
## Datsun 710 22.8 4 108.0
## Hornet 4 Drive 21.4 6 258.0
## Merc 240D 24.4 4 146.7
## Merc 230 22.8 4 140.8
## Fiat 128 32.4 4 78.7
## Honda Civic 30.4 4 75.7
## Toyota Corolla 33.9 4 71.1
## Toyota Corona 21.5 4 120.1
## Fiat X1-9 27.3 4 79.0
## Porsche 914-2 26.0 4 120.3
## Lotus Europa 30.4 4 95.1
## Volvo 142E 21.4 4 121.0Note that the logical expression mtcars$mpg > 20 defines a vector of logical values. You could do this separately in advance and use the logical vector to specify the rows you want.
RowsIWant = mtcars$mpg > 20
# Let's look at it
RowsIWant## [1] TRUE TRUE TRUE TRUE FALSE FALSE FALSE TRUE TRUE FALSE FALSE
## [12] FALSE FALSE FALSE FALSE FALSE FALSE TRUE TRUE TRUE TRUE FALSE
## [23] FALSE FALSE FALSE TRUE TRUE TRUE FALSE FALSE FALSE TRUE# Now use it to get the rows you want
SmallDF = mtcars[RowsIWant,c(1,2,3)]
# Look at it
SmallDF## mpg cyl disp
## Mazda RX4 21.0 6 160.0
## Mazda RX4 Wag 21.0 6 160.0
## Datsun 710 22.8 4 108.0
## Hornet 4 Drive 21.4 6 258.0
## Merc 240D 24.4 4 146.7
## Merc 230 22.8 4 140.8
## Fiat 128 32.4 4 78.7
## Honda Civic 30.4 4 75.7
## Toyota Corolla 33.9 4 71.1
## Toyota Corona 21.5 4 120.1
## Fiat X1-9 27.3 4 79.0
## Porsche 914-2 26.0 4 120.3
## Lotus Europa 30.4 4 95.1
## Volvo 142E 21.4 4 121.0Very frequently, you will want all of the columns. In this case, you need to include the comma, but follow it with nothing.
SmallDF = mtcars[RowsIWant,]
SmallDF## mpg cyl disp hp drat wt qsec vs am gear carb
## Mazda RX4 21.0 6 160.0 110 3.90 2.620 16.46 0 1 4 4
## Mazda RX4 Wag 21.0 6 160.0 110 3.90 2.875 17.02 0 1 4 4
## Datsun 710 22.8 4 108.0 93 3.85 2.320 18.61 1 1 4 1
## Hornet 4 Drive 21.4 6 258.0 110 3.08 3.215 19.44 1 0 3 1
## Merc 240D 24.4 4 146.7 62 3.69 3.190 20.00 1 0 4 2
## Merc 230 22.8 4 140.8 95 3.92 3.150 22.90 1 0 4 2
## Fiat 128 32.4 4 78.7 66 4.08 2.200 19.47 1 1 4 1
## Honda Civic 30.4 4 75.7 52 4.93 1.615 18.52 1 1 4 2
## Toyota Corolla 33.9 4 71.1 65 4.22 1.835 19.90 1 1 4 1
## Toyota Corona 21.5 4 120.1 97 3.70 2.465 20.01 1 0 3 1
## Fiat X1-9 27.3 4 79.0 66 4.08 1.935 18.90 1 1 4 1
## Porsche 914-2 26.0 4 120.3 91 4.43 2.140 16.70 0 1 5 2
## Lotus Europa 30.4 4 95.1 113 3.77 1.513 16.90 1 1 5 2
## Volvo 142E 21.4 4 121.0 109 4.11 2.780 18.60 1 1 4 2If you leave out the comma, you’ll get an error message. Try it for yourself and note the error message you get. You will forget that comma many times in the near future. Making note of the error message now will help you to recover quickly.