How to select rows comparing multiple columns with the same value?

Question

Suppose I have the following data and a vector of column names

dt <- data.table(id = letters[1:10], amount = 1:10, id2 = c(rep('a',5),rep('b',5)),test=rep('a',10))
cols <- c('id','id2','test')

what I'm trying to do is select rows where all columns in the vector have the same specific value like

dt[id=='a' & id2=='a' & test == 'a']

but using the vector cols. Is there a way to do it?

Note: I need to find a way to do it using data.table or base R without making comparisons between the columns like

dt[id==id2 & id==test & id2==test]

Answer 1

You can take help of .SDcols -

library(data.table)

dt[dt[, rowSums(.SD == 'a') == length(cols), .SDcols = cols]]

#   id amount id2 test
#1:  a      1   a    a

This can also be written as -

dt[rowSums(dt[, ..cols] == 'a') == length(cols), ]