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

[Liaw, Andy]

As Bert said, redefining functions like "[" is surely inadvisable, because
of possibility of breaking codes that depend on the intended behavior.  This
is a language _feature_.

If the problem is indexing beyond array extent, just check for it:  Are any
values that are going to be used for indexing larger than the length of the
object?  E.g.,

if (any(idx > length(x))) stop("index out of bound")
result <- x[i]

If this is still too much work for you, Perhaps R is not for you...

Andy

> From: 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
> 
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