r - Create string with variable names and values for data.table lookup -

i'm trying construct lookup string match values in r data.table. have data.table called mydatatable columns v1:v4 , list called mylist 3 elements c("a", "b", "c"). here's (admittedly inelegant) code:

# create first value matchstr <- paste('v1', '=="', mylist[1], '"', sep="")  # construct rest of match string (i in 2:length(mylist)) {     matchstr <- paste(matchstr, ' & v', i, '=="', mylist[i], '"', sep="") } matchstr <- paste(matchstr, ",", sep="") 

my match string looks this:

matchstr [1] "v1==\"a\" & v2==\"b\" & v3==\"c\"," 

if use cat output string, looks this:

cat(matchstr) v1=="a" & v2=="b" & v3=="c", 

i want use lookup string data.table, this:

mydatatable[v1=="a" & v2=="b" & v3=="c",] 

if text shown, result expect. if try this:

mydatatable[matchstr] 

i get

error in `[.data.table`(wordtable, matchstr) :  when data.table (or character vector), x must keyed (i.e. sorted, and, marked sorted) data.table knows columns join , take advantage of x being sorted. call setkey(x,...) first, see ?setkey. 

if try

mydatatable[cat(matchstr)] 

i get

v1=="a" & v2=="b" & v3=="c",null data.table (0 rows , 0 cols) 

how can work, preferably without awkward string manipulation. thanks!

this horrible way go this, can done:

txt <- "v1==\"a\" & v2==\"b\" & v3==\"c\"" # note - i've removed last comma end of string  # example data: set.seed(13) dt <- data.table(replicate(3,sample(letters[1:3],5,replace=true))) dt  #   v1 v2 v3 #1:  c   b #2:   b  c #3:  b  c  c #4:   c  b #5:  c   b  dt[eval(parse(text=txt))] #   v1 v2 v3 #1:   b  c 

keep in mind:

library(fortunes) fortune(106) 

if answer parse() should rethink question.
-- thomas lumley r-help (february 2005)

as @arun notes, preferred way use more standard data.table functionality:

setkey(dt,v1,v2,v3) mylist <- list("a", "b", "c") dt[mylist]  #   v1 v2 v3 #1:   b  c