how do i pass parameters to subset()?

Question

I am building a gui which lets me select a subset of a data.frame by clicking on the various factor names. After having received user input, how do i pass it to the subset function?

eg: I have a dataframe df with factors MORNING and EVENING in column timeofday and RECEIVE and SEND in column optype. From the GUI I know that the user wants to a subset containing only RECEIVE operations, so i have the following strings as well:

RequestedFactor1 which equals "optype"
RequestedRelationship1 equals "=="
RequestedValue1 which equals "RECEIVE"

What can i do to those strings to pass them to subset, so that I will receive the same output as if i had called subset(df,optype=="RECEIVE") ?

TIA

Answer 1

For this you can use an eval-parse construct, but again I warn that this is actually tricky business. Please read the help files about these two very carefully. So in your case this becomes :

subset(df,eval(parse(text=paste(RF1,RR1,RV1)))) 

An example to illustrate some tricky parts :

> RF1 <- "optype"

> RR1 <- "=="

> RV1 <- "\"RECEIVE\""

> optype <- c("RECEIVE","Not")

> ifelse(eval(parse(text=paste(RF1,RR1,RV1))),1,0)
[1] 1 0

Mind the escaped quote-marks (\\"). This is necessary as you want to test against a string, and not the RECEIVE object. Alternatively you can do :

> RF1 <- "optype"

> RR1 <- "=="

> RV1 <- "Text"

> optype <- c("RECEIVE","Not")

> Text <- "RECEIVE"

> ifelse(eval(parse(text=paste(RF1,RR1,RV1))),1,0)
[1] 1 0

Answer 2

The comparison operators in R are actually special functions, so you can use do.call to run the functions, no need for eval and parse and the potential headaches that can come from there. eg:

rf1 <- 'Species'
rr1 <- '=='
rv1 <- 'setosa'

subset(iris, do.call(rr1, list( get(rf1), rv1 ) ) )

You need to "get" the variable so that you have the variable value rather than the name, the rest can be the character strings.