r - Subset based on variable column name

Question

I'm wondering how to use the subset function if I don't know the name of the column I want to test. The scenario is this: I have a Shiny app where the user can pick a variable on which to filter (subset) the data table. I receive the column name from the webapp as input, and I want to subset based on the value of that column, like so:

subset(myData, THECOLUMN == someValue)

Except where both THECOLUMN and someValue are variables. Is there a syntax for passing the desired column name as a string?

Seems to want a bareword that is the column name, not a variable that holds the column name.

Answer 1

Both subset and with are designed for interactive use and warnings against their use within other functions will be found in their help pages. This stems from their strategy of evaluation arguments as expressions within an environment constructed from the names of their data arguments. These column/element names would otherwise not be "objects" in the R-sense.

If THECOLUMN is the name of an object whose value is the name of the column and someValue is the name of an object whose value is the target, then you should use:

dfrm[ dfrm[[THECOLUMN]] == someValue , ]

The fact that "[[" will evaluate its argument is why it is superior to "$" for programing. If we use joran's example:

 d <- data.frame(x = letters[1:5],y = runif(5))
 THECOLUMN= "x"
 someValue= "c"

d[ d[[THECOLUMN]] == someValue , ]
#   x         y
# 3 c 0.7556127

So in this case all these return the same atomic vector:

d[[ THECOLUMN ]]
d[[ 'x' ]]
d[ , 'x' ]
d[, THECOLUMN ]
d$x  # of the three extraction functions: `$`, `[[`, and `[`,
     # only `$` is unable to evaluate its argument