R Under development (unstable) (2023-08-12 r84939 ucrt) -- "Unsuffered Consequences"
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> # Tests for the drop.special function, which is not exported
> # 
> library(survival)
> library(splines)
> dfun <- survival:::drop.special
> 
> # In a terms structure, the factors attribute is a matrix with row and column
> # names.  The predvars and dataClasses attributites, if present, index to the
> # row names; as do values of the specials attribute.  The term.labels attribute
> # aligns with the column names.
> 
> #  For most model formula the row and column names nicely align, but not always.
> # [.terms, unfortunately, implicitly assumes that they do align.  This means
> # that the result of subscripting is not always proper.  
> 
> # The coxph routine needs to drop terms such as strata, fraily, and cluster
> # from the terms structure BEFORE calling model.matrix, otherwise a huge
> # matrix could result, from which most of the columns would need to be
> # dropped.  (A matched case-control fit will have n/2 strata).
> 
> # The assumptions are correct for form1, fail for the others
> form0 <- terms(Surv(time, status) ~ age + ns(wt.loss) + strata(sex), 
+                specials="strata")
> form1 <- terms(Surv(time, status) ~ age + ns(wt.loss) + strata(sex) -1, 
+                specials="strata")
> form2 <- terms(Surv(time, status) ~ age + offset(ph.ecog) + ns(wt.loss) + 
+                    strata(sex), specials= "strata")
> form3 <- terms(Surv(time, status) ~ age + strata(sex)/ph.ecog + ns(wt.loss), 
+                specials= "strata")
> form4 <- terms(~ age + meal.cal*ph.ecog - ph.ecog + strata(sex) + ns(wt.loss), 
+                specials = "strata")
> form5 <- terms(~ age + ph.ecog - ph.ecog + strata(sex) + ns(wt.loss), 
+                specials = "strata")
> 
> test0 <- terms(model.frame(form0, data= lung))
> test1 <- terms(model.frame(form1, data= lung))
> test2 <- terms(model.frame(form2, data= lung))
> test3 <- terms(model.frame(form3, data= lung))
> test4 <- terms(model.frame(form4, data= lung))
> test5 <- terms(model.frame(form5, data= lung))
> 
> ccheck <- function(term) {
+     aa <- attributes(term)
+     cname <- colnames(aa$factors)
+     rname <- rownames(aa$factors)
+     sindx <- aa$specials$strata
+     offset <- aa$offset
+     vname <- sapply(aa$variables, deparse)[-1]
+     pname <- sapply(aa$predvars,  function(x) deparse(x, nlines=1))[-1]
+ 
+     # predvars contains an expanded form of ns(wt.loss), so only use the first
+     #  8 chars.  The goal is to see if it is the right terms, in the right order
+     test <- c(vname= identical(rname, vname),
+               labels = identical(cname, aa$term.labels),
+               data = identical(rname, names(aa$dataClasses)),
+               predvars= identical(substr(pname, 1,8), substr(rname, 1,8)),
+               strata = any(grepl("strata", aa$term.labels)))
+     
+     test
+ }
> 
> # the untangle.specials was the first attempt to get subscripts right
> u0 <- untangle.specials(test0, 'strata')
> u1 <- untangle.specials(test1, 'strata')
> u2 <- untangle.specials(test2, 'strata')
> u3 <- untangle.specials(test3, 'strata')
> u4 <- untangle.specials(test4, 'strata')
> u5 <- untangle.specials(test5, 'strata')
> 
> # The survival package, in various places, had used a [.terms to remove
> #   the strata, via a newTerms <- Terms[-u0$terms] assignment where
> #   Terms = terms(model-frame)
> # The newer drop.specials function, assigned as 'dfun' above, addresses
> #   the issue.
> # All is well for test0 and test1: [.terms works as expected
> # In the remainder, some combination of a/b/c occurs with [.terms
> #   a. the wrong variable gets dropped (serious), 
> #   b. the dataClasses attribute has too many elements (not serious).,
> #   c.  Predvars may be out of order (serious).
> # 
> #
> ans1 <- c(TRUE, TRUE, TRUE, TRUE, FALSE) # correct answer for all but test3
> ans2 <- rep(TRUE, 5) # correct answer for test3
> names(ans1) <- names(ans2) <- c("vname", "labels", "data", "predvars", "strata")
> 
> all.equal(ccheck(test0[-u0$terms]), ans1) 
[1] TRUE
> all.equal(ccheck(test1[-u1$terms]), ans1)
[1] TRUE
> all.equal(ccheck(test2[-u2$terms]), ans1)
[1] "2 element mismatches"
> all.equal(ccheck(test3[-u3$terms]), ans2)
[1] "2 element mismatches"
> all.equal(ccheck(test4[-u4$terms]), ans1)
[1] "2 element mismatches"
> all.equal(ccheck(test5[-u5$terms]), ans1)
[1] "2 element mismatches"
> 
> # The dropterms function works in all 6 cases
> all.equal(ccheck(dfun(test0, attr(test0, "specials")$strata)), ans1)
[1] TRUE
> all.equal(ccheck(dfun(test1, attr(test1, "specials")$strata)), ans1)
[1] TRUE
> all.equal(ccheck(dfun(test2, attr(test2, "specials")$strata)), ans1)
[1] TRUE
> all.equal(ccheck(dfun(test3, attr(test3, "specials")$strata)), ans2)
[1] TRUE
> all.equal(ccheck(dfun(test4, attr(test4, "specials")$strata)), ans1)
[1] TRUE
> all.equal(ccheck(dfun(test5, attr(test5, "specials")$strata)), ans1)
[1] TRUE
> 
> 
> 
> 
> proc.time()
   user  system elapsed 
   0.90    0.10    1.01