baz baz -4 years ago 90
R Question

coding variable values into classes using R

I have a set of data in which I need to code values of certain variables (numeric) into 3 classes.

My data set is similar to this but has 60 more variables:

anim <- c(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15)
wt <- c(181,179,180.5,201,201.5,245,246.4,189.3,301,354,369,205,199,394,231.3)
data <- data.frame(anim,wt)

> data
anim wt
1 1 181.0
2 2 179.0
3 3 180.5
4 4 201.0
5 5 201.5
6 6 245.0
7 7 246.4
8 8 189.3
9 9 301.0
10 10 354.0
11 11 369.0
12 12 205.0
13 13 199.0
14 14 394.0
15 15 231.3


I need to code values of the variable "wt" up into 3 classes: (wt >= 179 & wt < 200) = 1; (wt >= 200 & wt < 300) = 2; (wt > 300) = 3

which should give me this

> data2
anim wt SWT
1 1 181.0 1
2 2 179.0 1
3 3 180.5 1
4 4 201.0 2
5 5 201.5 2
6 6 245.0 2
7 7 246.4 2
8 8 189.3 1
9 9 301.0 3
10 10 354.0 3
11 11 369.0 3
12 12 205.0 2
13 13 199.0 1
14 14 394.0 3
15 15 231.3 2

Answer Source

The cut method as outlined by @Greg is probably what you want here. One thing to note is that cut returns a factor by default, which you can suppress by supplying labels = FALSE to return the integer values:

cut(data$wt, c(178, 200, 300, Inf), labels = FALSE)

Alternatively, if your cutting does not lend itself to natural breaks, you can use ifelse(). You can "nest" the ifelse statements similar to Excel. I use "with" to cut down on the typing needed:

data$group2 <- with(data, ifelse(wt >= 179 & wt < 200, 1, 
  ifelse(wt >= 200 & wt < 300, 2, 3))
)
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