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

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