[R] terminate R program when trying to access out-of-bounds array element?

[Rich FitzJohn]

Hi,

You could try redefining "[", so that if any element subsetted
returned an NA, it would throw an error, e.g.: 
(Warning: Largely untested! - this will almost certainly cause
problems in other classes that use [ to subset.  Possibly defining
this as "[.default" would be better...)

"[" <- function(x, ...) {
  res <- (base::"[")(x, ...)
  if ( any(is.na(res)) )
    stop("An element was NA in a subset")
  res
}

> x <- 1:5
> x[4]
[1] 4
> x[7]
Error in x[7] : An element was NA in a subset

However, you'll probably find this is a little over-zealous, e.g.:
> y <- c(1:3, NA, 4)
> y[5]
[1] 4
> y[4]
Error in y[4] : An element was NA in a subset

If you just want to check for an NA at printing, defining a function
like this might be more appropriate:
print.or.stop <- function(x) {
  if ( any(is.na(x)) )
    stop("An element was NA in a subset")
  print(x)
}

You could write a more complicated "[" function that does a bunch of
testing, to see if the element extracted is going to be out of the
extent of the vector (rather than a "genuine" NA), but since there are
a number of ways elements can be extracted from vectors (numeric,
logical and character indices can all be used to index vectors, and
these have recycling rules, etc), this is probably much more work than
a few checks in your code where an NA would actually indicate an
error.

Cheers,
Rich

On 4/14/05, Vivek Rao <rvivekrao at yahoo.com> wrote:
> I want R to stop running a script (after printing an
> error message) when an array subscript larger than the
> length of the array is used, for example
> 
> x = c(1)
> print(x[2])
> 
> rather than printing NA, since trying to access such
> an element may indicate an error in my program. Is
> there a way to get this behavior in R? Explicit
> testing with the is.na() function everywhere does not
> seem like a good solution. Thanks.
> 
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-- 
Rich FitzJohn
rich.fitzjohn <at> gmail.com   |    http://homepages.paradise.net.nz/richa183
                      You are in a maze of twisty little functions, all alike