[R] Strange lazy evaluation of default arguments

[Matthias Gondan]

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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