Question

I need to remove the last number in a groups of vectors, i.e.:

v <- 1:3
v1 <- 4:8

should become:

v <- 1:2
v1 <- 4:7

Answer 1

You can use negative offsets in head (or tail), so head(x, -1) removes the last element:

R> head( 1:4, -1)
[1] 1 2 3
R> 

This also saves an additional call to length().

Edit: As pointed out by Jason, this approach is actually not faster. Can’t argue with empirics. On my machine:

R> x <- rnorm(1000)
R> microbenchmark( y <- head(x, -1), y <- x[-length(x)], times=10000)
Unit: microseconds
                expr    min      lq median     uq     max
1   y <- head(x, -1) 29.412 31.0385 31.713 32.578 872.168
2 y <- x[-length(x)] 14.703 15.1150 15.565 15.955 706.880
R>