[R] Strange lazy evaluation of default arguments

S Ellison S.Ellison at LGCGroup.com
Tue Sep 5 16:17:10 CEST 2017

```Mathias,
If it's any comfort, I appreciated the example; 'expected' behaviour maybe, but a very nice example for staff/student training!

S Ellison

> -----Original Message-----
> From: R-help [mailto:r-help-bounces at r-project.org] On Behalf Of Matthias
> Gondan
> Sent: 02 September 2017 18:22
> To: r-help at r-project.org
> Subject: [R] Strange lazy evaluation of default arguments
>
> 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
>
>
> 	[[alternative HTML version deleted]]
>
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> and provide commented, minimal, self-contained, reproducible code.

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