passing arrays to functions in tcl. Only upvar ?

Question

As far as I understand, in tcl if you want to pass a named array to a function, you have to access the upper scope of the caller via the upvar command within the callee body. Is this the only way to pass an array in tcl ?

Answer 1

There are other ways, like converting it into a list first (via array get and array set).

Answer 2

As Michael indicated, there are several ways, plus a wiki page that discusses it. Just to have some of that information here, some options are:

By Upvar

proc by_upvar {&arrName} {
    upvar 1 ${&arrName} arr
    puts arr(mykey)
    set arr(myotherkey) 2
}
set myarr(mykey) 1
by_upvar myarr
info exists myarr(myotherkey) => true

• results in changes to the array being seen by the caller

By array get/set

proc by_getset {agv} {
    array set arr $agv
    puts arr(mykey)
    set arr(myotherkey) 2
    return [array get arr]
}
set myarr(mykey) 1
array set mynewarr [by_upvar myarr]
info exists myarr(myotherkey) => false
info exists mynewarr(myotherkey) => true

• results in changes to the array being seen by the caller
• similar mechanism can be used to return an array

Answer 3

If you're only passing in the value of the array, you could pass in a dictionary instead (hint: array get serializes an array into a dictionary value) and use the dict command to access values in it. But if you want access to the live value, upvar is definitely easiest. It's also a very fast technique; it compiles down to an extra traversal of a pointer during variable access after upvar itself finishes.