R help - Feb 2011 - Removing elements from a vector matching a criteria, BUG in which() function

Dear all,

I found a bug in the which() function.

When trying to remove elements with the which function,
if the criteria is not matched, numeric(0) is returned instead of the
array itself.

This is very weird.
> a = c(1,2,3,4,5)
> a[!a==6]
[1] 1 2 3 4 5
> a[-which(a==6)]
numeric(0)
> a[-which(a==5)]
[1] 1 2 3 4
> a[!a==5]
[1] 1 2 3 4

Is this correct? I believe this is a bug.

I have to rewrite a lot of my R code to use
a = a[!criteria]
and not
a = a[-which(criteria)]

R.

I think your rewrite is overdue, because returning the array from the which
function seems counterintuitive.
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Rumen Kostadinov <rkostadi@gmail.com> wrote:

Dear all, I found a bug in the which() function. When trying to remove elements
with the which function, if the criteria is not matched, numeric(0) is returned
instead of the array itself. This is very weird. > a = c(1,2,3,4,5) >
a[!a==6] [1] 1 2 3 4 5 > a[-which(a==6)] numeric(0) > a[-which(a==5)] [1]
1 2 3 4 > a[!a==5] [1] 1 2 3 4 Is this correct? I believe this is a bug. I
have to rewrite a lot of my R code to use a = a[!criteria] and not a =
a[-which(criteria)] R._____________________________________________
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	[[alternative HTML version deleted]]

If by "bug" you mean "function behaving exactly as
documented."

which() returns only the matches, the TRUE values. If there are no
matches, it doesn't return anything.

If I understand what you are trying to do, and I may not,
a[which(a != 5)]
is really what you want, and it is precisely to preserve
that behavior that which() does what it does.

Sarah

---

which                   package:base                   R Documentation

Description:

     Give the ?TRUE? indices of a logical object, allowing for array
     indices.

Value:

     If ?arr.ind == FALSE? (the default), an integer vector with
     ?length? equal to ?sum(x)?, i.e., to the number of ?TRUE?s in ?x?;
     Basically, the result is ?(1:length(x))[x]?.

---
On Sun, Feb 13, 2011 at 11:59 AM, Rumen Kostadinov <rkostadi at gmail.com>
wrote:
> Dear all,
>
> I found a bug in the which() function.
>
> When trying to remove elements with the which function,
> if the criteria is not matched, numeric(0) is returned instead of the
> array itself.
>
> This is very weird.
>
>> a = c(1,2,3,4,5)
>> a[!a==6]
> [1] 1 2 3 4 5
>> a[-which(a==6)]
> numeric(0)
>> a[-which(a==5)]
> [1] 1 2 3 4
>> a[!a==5]
> [1] 1 2 3 4
>
> Is this correct? I believe this is a bug.
>
> I have to rewrite a lot of my R code to use
> a = a[!criteria]
> and not
> a = a[-which(criteria)]
>
> R.
>

-- 
Sarah Goslee
http://www.functionaldiversity.org