From bolker@zoo.ufl.edu  Fri Nov  8 14:05:42 2002
From: bolker@zoo.ufl.edu (Ben Bolker)
Date: Fri, 8 Nov 2002 09:05:42 -0500 (EST)
Subject: behavior of =
Message-ID: <Pine.LNX.4.44.0211080855020.23711-100000@bolker.zoo.ufl.edu>


  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?

----
318 Carr Hall                                bolker@zoo.ufl.edu
Zoology Department, University of Florida    http://www.zoo.ufl.edu/bolker
Box 118525                                   (ph)  352-392-5697
Gainesville, FL 32611-8525                   (fax) 352-392-3704

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behavior of =
Ben Bolker