How to loop through columns in data.table in R

Question

This problem bothered me the whole weekend. I want to count the number of '1's in a column based the group of another column. I want to loop through many columns, and the example I am giving here is just a simple case. I think DT1 and DT2 should give me the same result, but obviously DT1 doesn't work. Can anyone tell me what is the reason for this? Thank you!

DT <- data.table(Sample.name = c("A","B","C","A","A","B"),
                 Class1 = c(1, 0, 1, 0, 1, 0),
                 Class2 = c(1, 1, 1, 0, 1, 1))

round.test <- colnames(DT)
round.test <- round.test[c(2, 3)]
round.test <- noquote(round.test)


DT1 <- DT[, sum((round.test[1]) == 1),
                  by = Sample.name]

DT2 <- DT[, sum(Class1 == 1),
          by = Sample.name]

Answer 1

Starting with a character vector of column names, you can use get to have the column value:

round.test <- colnames(DT)
round.test <- round.test[c(2, 3)]

DT[, sum(get(round.test[1]) == 1), .(Sample.name)]

#   Sample.name V1
#1:           A  2
#2:           B  0
#3:           C  1

DT[, lapply(round.test, function(col) sum(get(col) == 1)), .(Sample.name)]

#   Sample.name V1 V2
#1:           A  2  2
#2:           B  0  2
#3:           C  1  1

Or you can use .SDcols to pass in the column names, and access the columns via .SD :

DT[, lapply(.SD, function(col) sum(col == 1)), by=.(Sample.name), .SDcols=round.test]

#   Sample.name Class1 Class2
#1:           A      2      2
#2:           B      0      2
#3:           C      1      1