From mschwartz@medanalytics.com  Fri Nov  8 14:53:13 2002
From: mschwartz@medanalytics.com (Marc Schwartz)
Date: Fri, 8 Nov 2002 08:53:13 -0600
Subject: behavior of =
In-Reply-To: <3DCBC6F9.B83C534C@research.bell-labs.com>
Message-ID: <004b01c28736$90df4410$0201a8c0@MARC>

> -----Original Message-----
> From: owner-r-devel@stat.math.ethz.ch 
> [mailto:owner-r-devel@stat.math.ethz.ch] On Behalf Of John Chambers
> Sent: Friday, November 08, 2002 8:15 AM
> To: bolker@zoo.ufl.edu
> Cc: R development mailing list
> Subject: Re: behavior of =
> 
> 
> It's a real confusion, but don't blame it on the use of "=" 
> as an assignment operator.
> 
> It's all the fault of having named arguments, which goes way 
> back to the dawn of time.
> 
> What you said (as opposed to what, of course, you meant) was 
> that the argument called "x" to "[" had the value of min(x).
> 
> The clue is that after evaluating the expression, x is 
> unchanged (distinctly NOT the case for the infamous C equivalent).
> 
> R> x <- runif(20)
> R> y <- 1:20
> R> y[x = min(x)]
> numeric(0)
> R> x
>  [1] 0.1452749 0.4936352 0.7872399 0.4377552 0.4072236 
> 0.4416220 0.4748017  [8] 0.8523369 0.8870081 0.7401091 
> 0.1480467 0.3367802 0.3825770 0.3596608 [15] 0.1040230 
> 0.6873980 0.2974596 0.9955219 0.3217589 0.2199942
> 
> Unfortunately, the implementation of "[" doesn't check 
> argument names (plausibly a bug).  To see the situation more 
> clearly, use the sort function, whose first argument is "x".
> 
> R> sort(x=min(x))
> [1] 0.1040230
> which is puzzling, but:
> 
> R> sort(y=5)
> Error in sort(y = 5) : unused argument(s) (y ...)
> 
> All the same, it's a useful example for a "what went wrong" FAQ.
> 
> 
> Ben Bolker wrote:
> > 
> >   I probably didn't follow the discussion of allowing "=" as an 
> > assignment operator closely enough, but I was a little bit 
> horrified 
> > to discover today (using 1.6.0; I haven't upgraded to 1.6.1 
> yet) that
> > 
> > x <- runif(20)
> > y <- 1:20
> > y[x=min(x)]
> > 
> > gives numeric(0) (because min(x) is non-integer).
> > 
> > x <- sample(1:20,20,TRUE)
> > y[x=min(x)]
> > 
> > is even worse -- it gives the "wrong" answer. I know exactly what's 
> > going on here (the = in the subsetting statement above was 
> a typo for 
> > ==), and I should have known better, but I felt like I was 
> programming 
> > in C again!
> > 
> > I know that the designers tried to build in safeguards (from the man
> > page):
> > 
> >    the `=' is only allowed at the top level (that is, in 
> the complete
> >    expression typed by the user) or as one of the 
> subexpressions in a
> >    braced list of expressions.
> > 
> > These safeguards don't prevent the mishap shown above.  Is 
> there any 
> > way to detect this case syntactically (check for assignments inside 
> > subsetting operations??), or will I just have to train my 
> students to 
> > watch out for this possibility?

FWIW, presuming that you and your students may be using an external
editor that supports syntax highlighting for coding, you can set up the
highlighting such that the assigment operators are specifically colored
to make them more readily visible. I recall a discussion here akin to
this earlier this year when the "_" assignment operator subject came up.

I am presently using Multi-Edit 9 under WinXP Pro, but am in the process
of switching to Linux and am still wrestling with (X)ESS vs.gVim vs
running Multi-Edit using Wine, but that's another issue.

In either case, I have set up the syntax highlighting in Multi-Edit such
that the assignment operators are bolded and colored red to stand out
against surrounding code. This has helped me greatly in avoiding exactly
the type of error that you are discussing and has saved me much time in
debugging code.

Marc


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behavior of =
Marc Schwartz