use string vector elements to refer to variable names in r [duplicate]

Question

I have list of variable names in a vector of strings v and a data table my.dt that contains all these variables.

> v

[1] "var1" "var2" "var3"

I want to use those variables whose names are in the vector v, such that i create new variable that is cbind of these 3, or any number of names that appear in v, like:

  new <- cbind(my.dt[,"var1"],my.dt[,"var2"],my.dt[,"var3"])
new1 <- rowSums(new, na.rm=TRUE) * ifelse(rowSums(is.na(new)) == ncol(new), NA, 1)

How can i get this, having in mind that number of variables is not fixed, so i dont want to refer to each element like v[1], v[2] etc.

Answer 1

Based on your comment, you are working on a data.table. You will need to add with = FALSE to the code as follows.

library(data.table)

my.dt <- data.table( ID = c("b","b","b","a","a","c"), a = 1:6, b = 7:12, c = 13:18 ) 
v <- c("a", "ID") 

my.dt[, v, with = FALSE]
#    a ID
# 1: 1  b
# 2: 2  b
# 3: 3  b
# 4: 4  a
# 5: 5  a
# 6: 6  c

Notice that if you are working on a data frame, you don't need with = FALSE.

my.dt <- data.frame( ID = c("b","b","b","a","a","c"), a = 1:6, b = 7:12, c = 13:18 ) 
v <- c("a", "ID") 

my.dt[, v]
#   a ID
# 1 1  b
# 2 2  b
# 3 3  b
# 4 4  a
# 5 5  a
# 6 6  c