How to generate a series （1,2,2,3,3,3,4,4,4,4）

Question

I am confused that how to generate a series （like this 1,2,2,3,3,3,4,4,4,4） in R. I use the for loop and rep function like this,

for (i in 1:10)
{rep(i,i)}

but I can't paste them together.

Answer 1

rep(1:4, 1:4)
[1] 1 2 2 3 3 3 4 4 4 4

Similarly for another series we get

rep(1:5, 1:5) * (-1)^rep(0:4, 1:5)
[1]  1 -2 -2  3  3  3 -4 -4 -4 -4  5  5  5  5  5