In R, can I concatenate, then call variable column=concatenated string? - r

Question

I am trying to identify the time of primary ambulance arrival for a number of patients in my dataframe=data.
The primary ambulance is either the 1st, 2nd, 3rd or 4th vehicle on scene (data$prim.amb.num=1, 2, 3, or 4 for each patient/row).
data$time_v1, data$time_v2, data$time_v3 and data$time_v4 have a time or a missing value, which corresponds to the 1st, 2nd, 3rd and 4th vehicles, where relevant.
What I would like to do is make a new variable=prim.amb.time with the time that corresponds to primary ambulance arrival time. Suppose for patient=1, the ambulance was the first. Then I want data[1,"prim.amb.time"]=data[1,"time_v1"].
I can figure out the correct time_v* with the following:
paste("time_v", data$prim.amb.num, sep="")
But I'm stuck as to how to pass the resulting information to call the correct column.
My hope was to simply have something like:
data$prim.amb.time<-data$paste("time_v", data$prim.amb.num, sep="")
but of course, this doesn't work. I'm not even sure how to Google for this; I tried various combinations of this title but to no avail. Any suggestions?

Answer 1

Although I liked the answer by #mhermans, if you want a one-liner, one solution is to use ?apply as follows:
#From #mhermans
zz <- textConnection("patient.id prime.amb.num time_v1 time_v2 time_v3 time_v4
1000 1 30 40 60 100
1001 3 40 50 60 80
1002 2 10 30 40 45
1003 1 24 40 45 60
")
d <- read.table(zz, header = TRUE)
close(zz)
#Take each row of d and pull out time_vn where n = d$prime.amb.num
d$prime.amb.time <- apply(d, 1, function(x) {x[x['prime.amb.num'] + 2]})
> d
patient.id prime.amb.num time_v1 time_v2 time_v3 time_v4 prime.amb.time
1 1000 1 30 40 60 100 30
2 1001 3 40 50 60 80 60
3 1002 2 10 30 40 45 30
4 1003 1 24 40 45 60 24
EDIT - or with paste:
d$prime.amb.time <-
apply(
d,
1,
function(x) {
x[paste('time_v', x['prime.amb.num'], sep = '')]
}
)
#Gives the same result

Answer 2

Set up example data:
# read in basic example data for four patients, wide format
zz <- textConnection("patient.id prime.amb.num time_v1 time_v2 time_v3 time_v4
1000 1 30 40 60 100
1001 3 40 50 60 80
1002 2 10 30 40 45
1003 1 24 40 45 60
")
d <- read.table(zz, header = TRUE)
close(zz)
In the example dataset I'm thus assuming your data looks like this:
patient.id prime.amb.num time_v1 time_v2 time_v3 time_v4
1 1000 1 30 40 60 100
2 1001 3 40 50 60 80
3 1002 2 10 30 40 45
4 1003 1 24 40 45 60
Given that data structure, it is perhaps easier to work with a dataset with a vehicle per row, instead of a patient per row. This can be accomplised by using reshape() to convert from a wide to a long format.
dl <- reshape(d, direction='long', idvar="patient.id", varying=list(3:6))
# ordering & rename var for aesth. reasons:
dl <- dl[order(dl$patient.id, dl$time),]
dl$vehicle.id <- dl$time
dl$time <- NULL
dl
This gives a long dataset, with a row per vehicle:
patient.id prime.amb.num time_v1 vehicle.id
1000.1 1000 1 30 1
1000.2 1000 1 40 2
1000.3 1000 1 60 3
1000.4 1000 1 100 4
1001.1 1001 3 40 1
1001.2 1001 3 50 2
1001.3 1001 3 60 3
1001.4 1001 3 80 4
1002.1 1002 2 10 1
1002.2 1002 2 30 2
1002.3 1002 2 40 3
1002.4 1002 2 45 4
1003.1 1003 1 24 1
1003.2 1003 1 40 2
1003.3 1003 1 45 3
1003.4 1003 1 60 4
Getting the arrival time of the first ambulance per patient then become a simple oneliner:
dl[dl$prime.amb.num == dl$vehicle.id,]
which gives
patient.id prime.amb.num time_v1 vehicle.id
1000.1 1000 1 30 1
1001.3 1001 3 60 3
1002.2 1002 2 30 2
1003.1 1003 1 24 1