[R] seq(2,5,-2) not an error but NULL

[Duncan Murdoch]

On 3/27/2006 8:28 AM, Robin Hankin wrote:
> Hi.
> 
> seq() is a complex beast indeed.   'by' being the wrong
> sign is a special case of the behaviour seen in the
> following code snippets, the first of which is correctly
> rejected by seq(), the second of which should arguably
> return a three element complex vector.
> 
> 
>  > seq(from=1,to=3,by=1+1i)
> Error in n < 0 : invalid comparison with complex values
> 
>  > seq(from=1,to=4+3i,by=1+1i)
> Error in n < 0 : invalid comparison with complex values

I don't think seq() could reasonably be expected to handle "to" and "by" 
arguments with complex values.  Trying to divide the (to-from) 
difference by (by) to find how many steps to take would usually result 
in enough rounding error that the result wouldn't be real-valued.  It's 
enough of a miracle that it correctly handles

seq(from=1, by=1+1i, len=4)

Duncan Murdoch


> 
> best wishes
> 
> Robin
> 
> 
> 
> On 27 Mar 2006, at 13:23, Duncan Murdoch wrote:
> 
>> On 3/27/2006 4:41 AM, Christian Hoffmann wrote:
>>> Hi,
>>>
>>> This may belong more to r-develop, but general discussion may be  
>>> useful
>>> (for the how many-th time ?)
>>>
>>> seq(2,5,-2)
>>> seq(5,2,2)
>>>
>>> both result in
>>>
>>> Error in seq.default(2, 5, -2) : wrong sign in 'by' argument
>>>
>>> But often, if not always, mathematicians and programmers want a
>>> behaviour e.g. in for loops, where this statement results in an empty
>>> statement, that is
>>>
>>> for (ii in seq(2,5,-2)) print(ii)
>>>
>>> were equivalent to
>>>
>>> for (ii in NULL) print(ii).
>>>
>>> The relevant part in seq.default is now
>>>
>>>              if (n < 0)
>>>                  stop("wrong sign in 'by' argument")
>>>
>>> but could be changed by option to
>>>
>>>    return(NULL)
>>>
>>> I think there should be an option to seq requiring this behaviour,  
>>> or a
>>> specific function, may be even a special operator, e.g. %;%:
>>>
>>> 3;5 resulting in NULL.
>>>
>>> What do you think?
>>
>> If you want optional behaviour, the easiest way is to write your own
>> wrapper function.  E.g.
>>
>> emptyseq <- function(from, to, by) {
>>    if ((to-from)*by < 0) return(NULL)
>>    else return(seq(from, to, by))
>> }
>>
>> I don't think this is a desirable default, though.  We already have a
>> special way to handle the most common case, i.e.
>>
>> seq(1, length(x), 1)
>>
>> should be written as
>>
>> seq(along=x))
>>
>> to handle the length(x) == 0 case the way you're requesting.
>>
>> But I'm not so sure that seq(2,5,-2) should really be NULL; it looks
>> much more like an error to me.  You say mathematicians and programmers
>> want this behaviour, but I really can't think of any examples other  
>> than
>> the one above.
>>
>> As a general principle, I think it's better to throw an error on
>> ambiguous or apparently erroneous code rather than always returning an
>> answer.  If the code can be made unambiguous, it should be.  (R  
>> doesn't
>> always follow this principle; for example, recycling of vectors of
>> lengths bigger than 1 is probably an error at least as often as it's
>> intended.)
>>
>> Duncan Murdoch
>>
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> 
> --
> Robin Hankin
> Uncertainty Analyst
> National Oceanography Centre, Southampton
> European Way, Southampton SO14 3ZH, UK
>   tel  023-8059-7743
> 
> 
>