R help - Nov 2016 - Question about expression parser for "return" statement

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.

Duncan Murdoch
>
> Duncan Murdoch
>
>>
>> I'm guessing it is intentional; but since there is no warning about
>> ignoring the rest of the expression, it could lead to hard-to-find
bugs.
>>
>> Thanks,
>> Dave
>>
>> Here's an example ...
>>
>> dnorm(2, 0, 1)
>> normalDensityFunction = function(x, Mean, Variance) {
>>     # no parentheses surrounding the entire "return" value
>>     return (1/sqrt(2*pi*Variance))*exp(-(1/2)*((x - Mean)^2)/Variance)
>> }
>> normalDensityFunction(2, 0, 1)    # incorrect answer
>> normalDensityFunction = function(x, Mean, Variance) {
>>     # parentheses surrounding the entire "return" value
>>     return ((1/sqrt(2*pi*Variance))*exp(-(1/2)*((x -
Mean)^2)/Variance))
>> }
>> normalDensityFunction(2, 0, 1)    # correct answer
>>
>> 	[[alternative HTML version deleted]]
>>
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>> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>

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

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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