R: Performing binary function to a column in a data frame.

Question

Say I have a data frame with the contents:

Trial Person 
1     John   
2     John   
3     John   
4     John
1     Bill 
2     Bill
3     Bill
4     Bill

and I want to transform this to

Trial Person Day
1     John   1
2     John   1
3     John   2
4     John   2
1     Bill   1
2     Bill   1
3     Bill   2
4     Bill   2

I can very easily make it

Trial Person Day
1     John   TRUE
2     John   TRUE
3     John   FALSE
4     John   FALSE
1     Bill   TRUE
2     Bill   TRUE
3     Bill   FALSE
4     Bill   FALSE

by doing d$day=d$trial<3 but how can I get to what I want?

Answer 1

Ok, so I found a solution, if I do

(d$trial>=3)+1

It converts the boolean to an integer and it works ... however, is there a better way to do this?

Answer 2

If you want to be explicit with the assignment (and hard-coding the cutoff of 3), you can use

d$Day <- ifelse(d$trial<3, 1, 2)

This is a bit more transparent. Otherwise, as you discovered, doing an arithmetic operation will convert the logical value to numeric. You can do it yourself by using as.numeric or as.integer:

as.integer(FALSE)  #0
as.integer(TRUE)   #1

Answer 3

Get the data:

x <- read.table(textConnection(
"Trial Person 
1     John   
2     John   
3     John   
4     John
1     Bill 
2     Bill
3     Bill
4     Bill"), header=TRUE)

I think that your current approach is the right one (note: you don't need as.numeric, because it's automatically cast when doing addition in this case):

(x$Trial >= 3) + 1

Otherwise, here's a way to do it with plyr.

library(plyr)
ddply(x, .(Person), transform, Day=rep(c(1,2), each=2))

Answer 4

More generally, if you're trying to convert a vector of the form c(1,2,3,4,5,6) to c(1,1,2,2,3,3), as if you had two trials per day, then you might want to express this using integer division:

> x <- 1:6
> x
[1] 1 2 3 4 5 6
> (x-1) %/% 2 + 1
[1] 1 1 2 2 3 3