How to insert elements into a vector?

You can do some magic with indexes:

First create vector with output values:

probs <- rep(TRUE, 15)
ind <- c(5, 10)
val <- c( probs, rep(FALSE,length(ind)) )
# > val
#  [1]  TRUE  TRUE  TRUE  TRUE  TRUE  TRUE  TRUE  TRUE  TRUE  TRUE  TRUE  TRUE
# [13]  TRUE  TRUE  TRUE FALSE FALSE

Now trick. Each old element gets rank, each new element gets half-rank

id  <- c( seq_along(probs), ind+0.5 )
# > id
#  [1]  1.0  2.0  3.0  4.0  5.0  6.0  7.0  8.0  9.0 10.0 11.0 12.0 13.0 14.0 15.0
# [16]  5.5 10.5

Then use order to sort in proper order:

val[order(id)]
#  [1]  TRUE  TRUE  TRUE  TRUE  TRUE FALSE  TRUE  TRUE  TRUE  TRUE  TRUE FALSE
# [13]  TRUE  TRUE  TRUE  TRUE  TRUE

These are all very creative approaches. I think working with indexes is definitely the way to go (Marek's solution is very nice).

I would just mention that there is a function to do roughly that: append().

probes <- rep(TRUE, 15)
probes <- append(probes, FALSE, after=5)
probes <- append(probes, FALSE, after=11)

Or you could do this recursively with your indexes (you need to grow the "after" value on each iteration):

probes <- rep(TRUE, 15)
ind <- c(5, 10)
for(i in 0:(length(ind)-1)) 
    probes <- append(probes, FALSE, after=(ind[i+1]+i))

Incidentally, this question was also previously asked on R-Help. As Barry says:

"Actually I'd say there were no ways of doing this, since I dont think you can actually insert into a vector - you have to create a new vector that produces the illusion of insertion!"

probes <- rep(TRUE, 1000000)
ind <- c(50:100)
val <- rep(FALSE,length(ind))

new.probes <- vector(mode="logical",length(probes)+length(val))
new.probes[-ind] <- probes
new.probes[ind] <- val

Some timings: My method user system elapsed 0.03 0.00 0.03

Marek method user system elapsed 0.18 0.00 0.18

R append with for loop user system elapsed 1.61 0.48 2.10