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Strange lazy evaluation of default arguments

9 messages · Rui Barradas, William Dunlap, Matthias Gondan +3 more

Matthias Gondan
# 
Dear R developers,

sessionInfo() below

Please have a look at the following two versions of the same function:

1. Intended behavior:
+ {
+   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))
+ }
[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:
+ {
+   # 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))
+ }
[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
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
Rui Barradas
# 
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>:
In the second version, the print
William Dunlap
# 
Another way to avoid the problem is to not redefine variables that are
arguments.  E.g.,
{
    if (verbose) {
      print(c(u, l, mu))
    }
    uNormalized <- u/sqrt(sigma2)
    lNormalized <- l/sqrt(sigma2)
    muNormalized <- mu/sqrt(sigma2)
    c(uNormalized, lNormalized, muNormalized)
  }
[1] 100.00 100.00   0.53
[1] 23.2558140 23.2558140  0.1232558
[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:

            

      
      
      
    

        

      
      
    

            

      
      
      
    

        

      
      
    

        

  
  


  


      

    

    

      
      

        


  

        

      Matthias Gondan
    

    

      
      #
    

  


  

      
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.,

            

      
      
      
    

        
? {
? ? if (verbose) {
? ? ? print(c(u, l, mu))
? ? }
? ? uNormalized <- u/sqrt(sigma2)
? ? lNormalized <- l/sqrt(sigma2)
? ? muNormalized <- mu/sqrt(sigma2)
? ? c(uNormalized, lNormalized, muNormalized)
? }

            

      
      
      
    

        
[1] 100.00 100.00 ? 0.53
[1] 23.2558140 23.2558140 ?0.1232558

            

      
      
      
    

        
[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


? ? ? ? [[alternative HTML version deleted]]

______________________________________________
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https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.

______________________________________________
R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.

  
  


  


      

    

    

      
      

        


  

        

      Jeff Newmiller
    

    

      
      #
    

  


  

      
Yes, this is intended behavior, and it has everything to do with where the parameters are first referenced and nothing to do with debugging.

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      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:

            

      
      
      
    

        

      
      
    

        

  
  


  


      

    

    
2 days later

    

      
      

        


  

        

      S Ellison
    

    

      
      #
    

  


  

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

S Ellison

            

      
      
      
    

        

      
      
    

        
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      Matthias Gondan
    

    

      
      #
    

  


  

      
Dear S Ellison,

Thanks for the flowers! Indeed, I was actually considering to use it in my own teaching material, as well. With rounded numbers, of course.

Though I am still slightly disturbed about this feature. I thought, now it is the time to switch to Python, but that?s even worse, see here:

def add_elem(List=[]):
    List.append('elem')
    return List

            

      
      
      
    

        
[2, 'elem']

            

      
      
      
    

        
['elem']

            

      
      
      
    

        
['elem', 'elem'] <<<<<<<<<<< why on earth does this happen? [it?s documented behavior, but still?]

So, I?ll stick with R. Still 25 years or so until retirement, but I?ll survive, even without crossreferenced default arguments.

Best wishes,

Matthias


Von: S Ellison
Gesendet: Dienstag, 5. September 2017 16:17
An: Matthias Gondan; r-help at r-project.org
Betreff: RE: [R] Strange lazy evaluation of default arguments

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

S Ellison

            

      
      
      
    

        

      
      
    

        
*******************************************************************
This email and any attachments are confidential. Any use...{{dropped:12}}

  
  


  


      

    

    

      
      

        


  

        

      Jeff Newmiller
    

    

      
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You don't need to cut off your nose to spite your face... you are the one writing the code that breaks the dependency, so you have the option to not write your code that way (e.g. by using force() as Rui suggests).