I have a data.frame (or a matrix or any other tabular data structure object for that matter):
df = data.frame(field1 = c(1,1,1),field2 = c(2,2,2),field3 = c(3,3,3))
And I want to copy part of its columns - given in the vector below:
fields = c("field1","field2")
to a new data.table that already has 1 or more columns:
dt = data.table(fieldX = c("x","x","x"))
I'm looking for something more efficient (and elegant) than:
for(f in 1:length(fields))
{
dt[,fields[f]] = df[,fields[f]]
}
You can use cbind:
cbind(dt, df[fields])
However, the most efficient way is still probably going to be to use data.table's assign by reference:
dt[, (fields) := df[fields]]
t = data.table(c1=1,c2=2); x = list(); x[1] <- 10; x[2] <- 100; invisible(foreach (i=1:2) %dopar% { input = x[[i]]; col = LETTERS[i]; t[ ,eval(col):= input] })I think you want cbind
cbind(dt, df[, 1:2])
# fieldX field1 field2
# 1 x 1 2
# 2 x 1 2
# 3 x 1 2
