R help - May 2012 - Odd behaviour of identical()

Consider the following code:

test <- function(n)
{
  for(x in 1:n)
  {
    for(y in 1:n)
    {
      for(r in max(x-1,1):min(x+1,n))
      {
        for(s in max(y-1,1):min(y+1,n))
        {
          vec <- c(x-r,y-s)
          print(c("vec = ", vec))
          print(identical(vec,c(0,0)))
        } 
      }
    }
  } 
}

If you run test(2) you'll see a printout of the values of the vector vec
followed by a logical telling you whether vec is identical to c(0,0), which
it will be for certain iterations of the nested loop. However, the logical
is always FALSE. If I don't perform the loop but instead assign the values
directly to vec, this problem does not arise. Can anyone tell me what is
happening here? 

Thank you very much in advance for any help.


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I believe it's coming down to the difference between integers and
doubles (the computer data types, not the math-y meaning of those
terms) -- e.g.,

identical( c(0L, 0L), c(0,0) )

Note that sequences made by `:` provide integers when possible: is.integer(1:5)

You may want to use all.equal() instead.

Best,
Michael

On Tue, May 15, 2012 at 11:45 AM, math_daddy <math_daddy at hotmail.com>
wrote:
> Consider the following code:
>
> test <- function(n)
> {
> ?for(x in 1:n)
> ?{
> ? ?for(y in 1:n)
> ? ?{
> ? ? ?for(r in max(x-1,1):min(x+1,n))
> ? ? ?{
> ? ? ? ?for(s in max(y-1,1):min(y+1,n))
> ? ? ? ?{
> ? ? ? ? ?vec <- c(x-r,y-s)
> ? ? ? ? ?print(c("vec = ", vec))
> ? ? ? ? ?print(identical(vec,c(0,0)))
> ? ? ? ?}
> ? ? ?}
> ? ?}
> ?}
> }
>
> If you run test(2) you'll see a printout of the values of the vector
vec
> followed by a logical telling you whether vec is identical to c(0,0), which
> it will be for certain iterations of the nested loop. However, the logical
> is always FALSE. If I don't perform the loop but instead assign the
values
> directly to vec, this problem does not arise. Can anyone tell me what is
> happening here?
>
> Thank you very much in advance for any help.
>
>
> --
> View this message in context:
http://r.789695.n4.nabble.com/Odd-behaviour-of-identical-tp4630118.html
> Sent from the R help mailing list archive at Nabble.com.
>
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