[R] strange thing with sd

[Andreas Pauling]

Dear R people

I came across a strange thing: 


sd(rep(0.01,      15))  #0
sd(rep(0.001,     15))  #4.489023e-19
sd(rep(0.00001,   15))  #0
sd(rep(0.00000001,15))  #1.712427e-24

sd(rep(0.01,      13))  #1.805557e-18
sd(rep(0.001,     13))  #4.513894e-19
sd(rep(0.00001,   13))  #0
sd(rep(0.00000001,13))  #0

sd(rep(5.01,	  15))  #0
sd(rep(5.001,	  15))  #4.489023e-19
sd(rep(5.00001,   15))  #1.838704e-15
sd(rep(5.00000001,15))  #9.19352e-16

sd(rep(5.01,	  13))  #9.244454e-16
sd(rep(5.001,	  13))  #9.244454e-16
sd(rep(5.00001,   13))  #1.848891e-15
sd(rep(5.00000001,13))  #0

Why gives sd sometimes zero and sometimes values close to zero
and why does it depend on the value and on how many times it is
repeated?
Shouldn't it give always zero?
Is there a way to control this?

I use R Version 1.8.1 under UNIX.

Thanks for any suggestions!!

Andreas