[R] Question about expression parser for "return" statement

[Wolf, Steven]

Just to add on a bit, please note that the return is superfluous.  If you write this:


normalDensityFunction = function(x, Mean, Variance) {

    # no  "return" value given at all

    (1/sqrt(2*pi*Variance))*exp(-(1/2)*((x - Mean)^2)/Variance)

}

normalDensityFunction(2,0,1)

...you get the right answer again.

This is not "best practices", and Duncan will probably give you 10 reasons why you should never do it this way.  But if the parentheses behavior bothers you enough, you can subvert it.  This probably won't work so well if you try to make any more complicated output.

Caveat Emptor.

-SW


--
Steven Wolf, PhD
Assistant Professor
Department of Physics
STEM CoRE -- STEM Collaborative for Research in Education
http://www.ecu.edu/cs-acad/aa/StemCore
East Carolina University
Phone: 252-737-5229




On Sun, 2016-11-13 at 13:35 -0500, Duncan Murdoch wrote:

On 13/11/2016 7:58 AM, Duncan Murdoch wrote:


On 13/11/2016 6:47 AM, Duncan Murdoch wrote:


On 13/11/2016 12:50 AM, Dave DeBarr wrote:


I've noticed that if I don't include parentheses around the intended return
value for the "return" statement, R will assume the first parenthetical
expression is the intended return value ... even if that parenthetical
expression is only part of a larger expression.

Is this intentional?



Yes, return is just a function call that has side effects.  As far as
the parser is concerned,

return ((1/sqrt(2*pi*Variance))*exp(-(1/2)*((x - Mean)^2)/Variance))

is basically the same as

f((1/sqrt(2*pi*Variance))*exp(-(1/2)*((x - Mean)^2)/Variance))



By the way, out of curiosity I took a look at the source of CRAN
packages to see if this actually occurs.  It turns out that "return" is
used as a variable name often enough to make automatic tests tricky, so
I don't know the answer to my question.  However, I did turn up a number
of cases where people have code like this:

     if (name == "") return;

(from the bio.infer package), which never calls return(), so doesn't
actually do what the author likely intended



I searched the R sources and the sources of CRAN packages, and found
this is a reasonably common problem:  it's in 111 packages, including
one in base R.  I'll be emailing the maintainers to let them know.

I'll see about putting a check for this into R CMD check.

Duncan Murdoch




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