R help - Nov 2002 - language inconsistency puzzle (in the tradition of Bill V enables (Hi Bill) puzzle "what is x")

R allows you to dynamically extend a vector by assigning past the end. So if
a has length 10, then assigning to a[11] creates also a[11] with value
a2[11] NA. So a is now length 11, while a2 is still length 10. So the
occurrence of a[11] has a different meaning on the assigned-to side than on
the assigned-from side. E.g.
>a <- rep(1,10)
>b <- 11
>a2 <- a
>a[b] <- a2[b]
>a
[1] 1 1 1 1 1 1 1 1 1 1 NA
>a2
[1] 1 1 1 1 1 1 1 1 1 1

This doesn't seem inconsistent to me; unavoidable yes, given that you want
vectors to be dynamically extendible. Have I missed something?

Reid Huntsinger

-----Original Message-----
From: oehl_list at gmx.de [mailto:oehl_list at gmx.de]
Sent: Thursday, November 07, 2002 2:11 PM
To: r-help at stat.math.ethz.ch
Subject: [R] language inconsistency puzzle (in the tradition of Bill
Venables (Hi Bill) puzzle "what is x")

> # let's say we have two vectors a and b
> # lets make a copy of a 
> 
> a2 <- a
> 
> # not surprisingly
> identical(a[b], a2[b])
[1] TRUE
> 
> # now assigning same to same
> a[b] <- a2[b]
> 
> # and SURPRISE
> identical(a, a2)
[1] FALSE
> 
WHAT IS a AND b ?

Hint: this "works" under 1.5.1 and 1.6.1. 
However, the final value of a will differ between 1.5.1 and 1.6.1 under
certain circumstances. Explain why it is both, DOUBLY INCONSISTENT AND
UNAVOIDABLE.

BTW, the prototype does the same. 

The serious side of this: I think the situation deserves at least a WARNING.

Best


Jens Oehlschl?gel


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Reid,

Thanks that you took up the challenge of this serious puzzle (the issue
masked a bug in one of my functions under 1.5.1, which was unmasked when
changing
to 1.6.1, however the issue is still in 1.6.1).

You are getting close. Your example might be seen as inconsistent, but not
as DOUBLY inconsistent. And your example will not differ between R1.5.1 and
R1.6.1. 

What if I tell you, that in my example
> is.na(a2[b])
[1] FALSE

whereas yours
> a <- rep(1,10)
> b <- 11
> a2 <- a
> is.na(a2[b])
[1] TRUE

in other words, you tell R to replace non-existent element no. 11 by NA, and
dynamic extension makes <a> different from <a2>.
In my case <b> DOES NOT SELECT AN NON-EXISTENT ELEMENT OF <a> (resp.
<a2>).

Any ideas?


Jens


BTW: Patrick, you guessed right: not a character a and numeric b

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