[R] Strange lazy evaluation of default arguments

[Bert Gunter]

This is exactly as expected. See section 4.3.3 of the R Language definition
or google around on "R Lazy Evaluation" for details.

You should not "expect" R's semantics to be the same as other languages
with which you may be familiar. Spending time with a good tutorial or two
should help you sort out points of similarity and differences.

Cheers,
Bert



Bert Gunter

"The trouble with having an open mind is that people keep coming along and
sticking things into it."
-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )

On Sat, Sep 2, 2017 at 10:22 AM, Matthias Gondan <matthias-gondan at gmx.de>
wrote:

> Dear R developers,
>
> sessionInfo() below
>
> Please have a look at the following two versions of the same function:
>
> 1. Intended behavior:
>
> > Su1 = function(u=100, l=u, mu=0.53, sigma2=4.3^2)
> + {
> +   print(c(u, l, mu)) # here, l is set to u’s value
> +   u = u/sqrt(sigma2)
> +   l = l/sqrt(sigma2)
> +   mu = mu/sqrt(sigma2)
> +   print(c(u, l, mu))
> + }
> >
> > Su1()
> [1] 100.00 100.00   0.53
> [1] 23.2558140 23.2558140  0.1232558
>
> In the first version, both u and l are correctly divided by 4.3.
>
> 2. Strange behavior:
>
> > Su2 = function(u=100, l=u, mu=0.53, sigma2=4.3^2)
> + {
> +   # print(c(u, l, mu))
> +   u = u/sqrt(sigma2)
> +   l = l/sqrt(sigma2) # here, l is set to u’s value
> +   mu = mu/sqrt(sigma2)
> +   print(c(u, l, mu))
> + }
> >
> > Su2()
> [1] 23.2558140  5.4083288  0.1232558
> In the second version, the print function is commented out, so the
> variable u is
> copied to l (lowercase L) at a later place, and L is divided twice by 4.3.
>
> Is this behavior intended? It seems strange that the result depends on a
> debugging message.
>
> Best wishes,
>
> Matthias
>
>
> > sessionInfo()
> R version 3.4.1 (2017-06-30)
> Platform: x86_64-w64-mingw32/x64 (64-bit)
> Running under: Windows >= 8 x64 (build 9200)
>
> Matrix products: default
>
> locale:
> [1] LC_COLLATE=German_Germany.1252  LC_CTYPE=German_Germany.1252
> LC_MONETARY=German_Germany.1252
> [4] LC_NUMERIC=C                    LC_TIME=German_Germany.1252
>
> attached base packages:
> [1] stats     graphics  grDevices utils     datasets  methods   base
>
> loaded via a namespace (and not attached):
> [1] compiler_3.4.1 tools_3.4.1
>
>
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>
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