> I am looking at the Braun/Murdoch book, " A First Course in
> Statistical Programming in R", and I have a question about a function
> there. It's on page 52, Example 4.5; the sieve of Erastosthenes.
>
> There is a line:
> primes <- c()
>
> Is there a difference between using that and
> primes <- NULL
> please?
>
> When you put in primes <- c(), primes comes back as NULL.
They return the same thing
identical(c(), NULL) #TRUE
> Is one more efficient or is this just a matter of programming style,
please?
system.time(for(i in 1:1000000) c())
# user system elapsed
# 0.63 0.02 0.64
system.time(for(i in 1:1000000) NULL)
# user system elapsed
# 0.28 0.00 0.28
Using NULL appears to be quicker on my machine, but given that you can do
a million of these assignments in a fraction of a second, it makes no
practical difference. NULL is perhaps more intuitive than c() for
demonstrating that the variable is empty.
Regards,
Richie.
Mathematical Sciences Unit
HSL
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