[R] Strange lazy evaluation of default arguments

Sat Sep 2 19:53:14 CEST 2017

Dear Bill,

All makes perfect sense (including the late evaluation). I actually discovered the problem by looking at old code which used your proposed solution. Still I find it strange (and, hnestly, I don’t like R’s behavior in this respect), and I am wondering why u is not being copied to L just before u is assigned a new value. Of course, this would require the R interpreter to track all these dependencies in both ways incl. more complicated ones in which L might depend on more than just u.

In the future, I’ll avoid dependencies between parameters.

Su4 <- function(u=100, l=100, mu=0.53, sigma2=4.3^2) # instead of l=u

And maybe also „in-place“ changes of values…

Best regards,

Matthias

Von: William Dunlap
Gesendet: Samstag, 2. September 2017 19:41
An: Rui Barradas
Cc: Matthias Gondan; r-help at r-project.org
Betreff: Re: [R] Strange lazy evaluation of default arguments

Another way to avoid the problem is to not redefine variables that are arguments.  E.g.,

> Su3 <- function(u=100, l=u, mu=0.53, sigma2=4.3^2, verbose)
  {
    if (verbose) {
      print(c(u, l, mu))
    }
    uNormalized <- u/sqrt(sigma2)
    lNormalized <- l/sqrt(sigma2)
    muNormalized <- mu/sqrt(sigma2)
    c(uNormalized, lNormalized, muNormalized)
  }
> Su3(verbose=TRUE)
[1] 100.00 100.00   0.53
[1] 23.2558140 23.2558140  0.1232558
> Su3(verbose=FALSE)
[1] 23.2558140 23.2558140  0.1232558

Not redefining variables at all makes debugging easier, although it may waste space.

Bill Dunlap
TIBCO Software
wdunlap tibco.com

On Sat, Sep 2, 2017 at 10:33 AM, <ruipbarradas at sapo.pt> wrote:
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

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