[R] outer
Peter Holzer
holzer at stat.math.ethz.ch
Thu May 10 09:59:49 CEST 2001
Hello Sean
When using outer you need vectorized functions. I think in your case it
would be enough to change the last line of your function to
ifelse(r==0, y, gcd(y,r))
Cheers, Peter
Sean Carmody writes:
> I've just begun playing around with defining functions in R and
> I'm not really sure what's going on in the following situation.
>
> I've defined the following function:
>
> gcd <- function(x,y) {
> x <- as.integer(x); y <- as.integer(y)
> q <- as.integer(x / y)
> r <- x - q * y
> if (r==0) y else gcd(y,r)
> }
>
> As expected, it gives the following results:
>
> > gcd(3,2)
> [1] 1
>
> > gcd(2,4)
> [1] 2
>
> etc. However, the function behaves strangely with outer:
>
> > outer(1:5,1:5,gcd)
> [,1] [,2] [,3] [,4] [,5]
> [1,] 1 2 3 4 5
> [2,] 1 2 3 4 5
> [3,] 1 2 3 4 5
> [4,] 1 2 3 4 5
> [5,] 1 2 3 4 5
>
> If i change the second line of the definition of gcd to
>
> y <- as.integer(x); x <- as.integer(y)
>
> then the effect of gcd should not change, and the following
> results still hold:
>
> > gcd(3,2)
> [1] 1
>
> > gcd(2,4)
> [1] 2
>
> however, now outer gives a different result:
>
> > outer(1:5,1:5,gcd)
> [,1] [,2] [,3] [,4] [,5]
> [1,] 1 1 1 1 1
> [2,] 2 2 2 2 2
> [3,] 3 3 3 3 3
> [4,] 4 4 4 4 4
> [5,] 5 5 5 5 5
>
> Somehow something strange seems to be happening to the
> internal variable r when outer is used, but I'm not sure what
> or why. Any thoughts would be appreciated.
>
> Sean.
>
>
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