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when to use pairlist instead list

2 messages · William Dunlap, Brian Ripley

Original
William Dunlap
# 
I was writing some assertion tests for modelling-related
code I had written and was surprised to see one test
fail because the "specials" attribute of the output of
terms() is a "pairlist" instead of a "list".  In 2.12.0
I get:

  > dput(attr(terms(y~Spec(x1)+x2, specials=c("Spec")), "specials"))
  list(Spec = 2L)
  >  all.equal(attr(terms(y~Spec(x1)+x2, specials=c("Spec")),
"specials"), list(Spec=2L))
  [1] "Modes: pairlist, list"
  >  all.equal(attr(terms(y~Spec(x1)+x2, specials=c("Spec")),
"specials"), pairlist(Spec=2L))
  [1] TRUE
  >  identical(attr(terms(y~Spec(x1)+x2, specials=c("Spec")),
"specials"), pairlist(Spec=2L))
  [1] TRUE

I was wondering if there was a reason for using pairlist
instead of list here or it it was just an historical
artifact.  In general, when should one use pairlists?

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com
Brian Ripley
#
On Mon, 6 Dec 2010, William Dunlap wrote:

            

      
      
      
    

        
Probably never directly.  But indirectly, a lot as for example 
argument lists are pairlists.

Looking at the code, I think this one is simply history.  For 
completeness I will correct ?terms.object.