dataframe - How to loop through columns in data.table in R -


this problem bothered me whole weekend. want count number of '1's in column based group of column. want loop through many columns, , example giving here simple case. think dt1 , dt2 should give me same result, dt1 doesn't work. can tell me reason 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),                   = sample.name]  dt2 <- dt[, sum(class1 == 1),           = sample.name]

starting character vector of column names, can use get have 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:            2 #2:           b  0 #3:           c  1  dt[, lapply(round.test, function(col) sum(get(col) == 1)), .(sample.name)]  #   sample.name v1 v2 #1:            2  2 #2:           b  0  2 #3:           c  1  1

or can use .sdcols pass in column names, , access columns via .sd:

dt[, lapply(.sd, function(col) sum(col == 1)), by=.(sample.name), .sdcols=round.test]  #   sample.name class1 class2 #1:                2      2 #2:           b      0      2 #3:           c      1      1

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