[R] Strange lazy evaluation of default arguments

[ruipbarradas at sapo.pt]

Hello,

One way of preventing that is to use ?force.
Just put

    force(l)

right after the commented out print and before you change 'u'.

Hope this helps,

Rui Barradas



Citando Matthias Gondan <matthias-gondan at gmx.de>:

> 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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