[R] pooling categories in a contingency table

[Michael Friendly]

John Fox helped show me how to collapse the categories, pooling
frequencies, in an n-way table, using levels()<- on an equivalent
data frame:

    # create a table
     sex <- c("Male", "Female")
     age <- letters[1:6]
     education <- c("low", 'med', 'high')
     data <- expand.grid(sex=sex, age=age, education=education)

     counts <- rpois(36, 100) 
     data <- cbind(data, counts)
     t1 <- xtabs(counts ~ sex + age + education, data=data)

    # collapse age to 3 levels
     levels(data$age) <- c("A", "A", "B", "B", "C", "C")
     t2 <- xtabs(counts ~ sex + age + education, data=data)
     mosaicplot(t2,shade=T)

I'd like to generalize this to a function, collapse.table.  Here's what
I tried:

collapse.table <- function(table, ...) {
    nargs <- length(args <- list(...))
    if (!nargs) 
        return(table)
    if (inherits(table, "table")) {
        table <- as.data.frame.table(table)
        freq <- table[,"Freq"]
        tvars <- dimnames(table)
        }
    names <- names(args)
    print(cat("names: ", names, "\n"))
    for (i in 1:nargs) {
        vals <- args[[i]]
        nm <- names[[i]]
        print(cat("vals: ", vals, "\n"))
        print(cat("nm: ", nm, "\n"))
        levels(table$nm) <- vals
        }
    terms <- paste(tvars, sep = '+')
    result <- xtabs(freq ~ terms, data=table)
}

>From the output below, I see what the problem is, but don't know
how to fix it:

> collapse.table(t1, age=c("A", "A", "B", "B", "C", "C"))
names:  age 
NULL
vals:  A A B B C C 
NULL
nm:  age 
NULL
Error in "levels<-.default"(*tmp*, value = c("A", "A", "B", "B", "C",  : 
        attempt to set an attribute on NULL
> traceback()
3: "levels<-.default"(*tmp*, value = c("A", "A", "B", "B", "C", 
   "C"))
2: "levels<-"(*tmp*, value = c("A", "A", "B", "B", "C", "C"))
1: collapse.table(t1, age = c("A", "A", "B", "B", "C", "C"))

Can someone help?

-Michael

-- 
Michael Friendly              friendly at yorku.ca
York University               http://www.math.yorku.ca/SCS/friendly.html
Psychology Department
4700 Keele Street             Tel:  (416) 736-5115 x66249
Toronto, Ontario, M3J 1P3     Fax:  (416) 736-5814
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