[R] turning off automatic coersion from list to matrix

[HENRIKSON, JEFFREY]

Hello,

I am having trouble understanding how R is coercing between matrices and
lists in the following example.  I have an aggregate behavior I like:

aggregate(a[,"num"],by=list(product=a[,"product"],region=a[,"region"]),
sum)

Now in reality I have more columns than just product and region, and
need to pick different combinations.  So I want to abstract this into a
function.  Example use:

myagg(a,c("product","region"))
  
But I am having trouble because by= requires a list and apply and sapply
seem to cast things back to the "matrix" type automatically.  Can I turn
this off?  Eg:

data.class(sapply(c("product","region"),function(i) {a[,i]}))
[1] "matrix"

whereas this would be acceptable to by=

data.class(list(product=a[,"product"],region=a[,"region"]))
[1] "list"


Regards,


Jeff Henrikson