In terms of probability distribution they use? I know that runif gives fractional numbers and sample gives whole numbers, but what I am interested in is if sample also use the 'uniform probability distribution'?
Consider the following code and output:
> set.seed(1)
> round(runif(10,1,100))
[1] 27 38 58 91 21 90 95 66 63 7
> set.seed(1)
> sample(1:100, 10, replace=TRUE)
[1] 27 38 58 91 21 90 95 67 63 7
This strongly suggests that when asked to do the same thing, the 2 functions give pretty much the same output (though interestingly it is round
that gives the same output rather than floor
or ceiling
). The main differences are in the defaults and if you don't change those defaults then both would give something called a uniform (though sample
would be considered a discrete uniform and by default without replacement).
Edit
The more correct comparison is:
> ceiling(runif(10,0,100))
[1] 27 38 58 91 21 90 95 67 63 7
instead of using round
.
We can even step that up a notch:
> set.seed(1)
> tmp1 <- sample(1:100, 1000, replace=TRUE)
> set.seed(1)
> tmp2 <- ceiling(runif(1000,0,100))
> all.equal(tmp1,tmp2)
[1] TRUE
Of course if the probs
argument to sample
is used (with not all values equal), then it will no longer be uniform.
sample
samples from a fixed set of inputs, and if a length-1 input is passed as the first argument, returns an integer output(s).
On the other hand, runif
returns a sample from a real-valued range.
> sample(c(1,2,3), 1)
[1] 2
> runif(1, 1, 3)
[1] 1.448551
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