[R] Question about expression parser for "return" statement
Duncan Murdoch
murdoch.duncan at gmail.com
Sun Nov 13 12:47:10 CET 2016
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))
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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