Filling in a new column based on a condition in a data frame

Question

I want to add a new column in my data frame so that for each row if LOC == 1 then V is equal to the value given for V1; if LOC==

Answer 1

Use matrix indexing:

idx <- cbind(seq_len(nrow(dat)), dat$LOC)
#      row  col
#     [,1] [,2]
#[1,]    1    1
#[2,]    2    1
#[3,]    3    2
#[4,]    4    1

dat[-1][idx]
#[1] 0.5 0.5 0.7 0.6

Answer 2

If LOC only contains 1 or 2 then this will work

 df$V <- ifelse(df$LOC == 1, df$V1, df$V2)