Question

I am trying to calculate a simple ratio using data.table. Different files have different tmax values, so that is why I need ifelse. When I debug this, the dt looks good. The tmaxValue is a single value (the first “t=60” encountered in this case), but t0Value is all of the “t=0” values in dt.

summaryDT <- calculate_Ratio(reviewDT[,list(Result, Time), by=key(reviewDT)])

calculate_Ratio <- function(dt){
tmaxValue <- ifelse(grepl("hhep", inFile, ignore.case = TRUE), 
    dt[which(dt[,Time] == "t=240min"),Result],
    ifelse(grepl("hlm",inFile, ignore.case = TRUE), 
        dt[which(dt[,Time] == "t=60"),Result],
        dt[which(dt[,Time] == "t=30"),Result]))
t0Value <- dt[which(dt[,Time] == "t=0"),Result]
return(dt[,Ratio:=tmaxValue/t0Value])
}

What I am getting out is theResult for tmaxValue divided by all of the Result‘s for all of the t0Value‘s, but what I want is a single ratio for each unique by.

Thanks for the help.

Answer 1

You didn’t provide a reproducible example, but typically using ifelse is the wrong thing to do.

Try using if(...) ... else ... instead.

ifelse(test, yes, no) acts very weird: It produces a result with the attributes and length from test and the values from yes or no.
…so in your case you should get something without attributes and of length one – and that’s probably not what you wanted, right?

[UPDATE] …Hmm or maybe it is since you say that tmaxValue is a single value…
Then the problem isn’t in calculating tmaxValue? Note that ifelse is still the wrong tool for the job…