Make old and new variables backward compatible - scripting

Question

v1- old var, v2 - new var.. Currently if anyone sets v1 it will error out so I am making it backward compataible. So if old var v1 is defined anywhere I want to make those 2 variables the same i.e v2 = v1 (Also value of v1 should be in v2)
set v1 test
For this I have a hash with old and new names
set oldvars(v1) v2
foreach ele [array names oldvars] {
if {([info exists ele] || $ele == "") && $oldvars($ele)!= ""} {
o2n $ele $oldvars($ele) //o2n is a proc defined below
}
}
proc o2n {gvar gval}
global v1
puts "gvar is $gvar, gval is $gval" //prints gvar - v1 and gval - v2
set $gval [set $$gvar] **//Error - Cant read $v1 no such variable. Above line does print gvar = v1 so $$gvar should be $v1 =test**
puts "$gval [set $gvar]" // Has the value of v1 i.e test
}
puts "v2 is $v2" **: Error : Can't read v2 no such variable**

Answer 1

If we assume
set a1 1
set b1 2
set c1 3
set oldvars(a1) a2
set oldvars(b1) b2
set oldvars(c1) c2
All you have to do to copy values from x1 variables to corresponding x2 variables is
foreach {x1 x2} [array get oldvars] {
set $x2 [set $x1]
}
The assignment looks a little off, but what it means is ”assign to the variable whose name is in x2 the value of the variable whose name is in x1.
If you want to do this via a procedure, you need to take into account that the procedure’s code executes in another scope. The uplevel command helps with that, and as a sideeffect the assignment code becomes simpler:
proc o2n {x1 x2} {
upvar 1 $x1 v1 $x2 v2
set v2 $v1
}
foreach {x1 x2} [array get oldvars] {
o2n $x1 $x2
}
Checking existence
If you keep references to variables in the form of variables that store those variables’ names, testing for existence also looks different from the usual. If you have
foreach ele [array names oldvars] {
the variable ele is guaranteed to exist (it is created by foreach as long as the list isn’t empty – but then the body of foreach won’t be executed anyway), but it isn’t the variable you wanted to verify: it only holds the name of the variable you wanted to verify. So you’ll need to write the test like this:
info exists $ele
(you also need to be in the same scope where the variable referenced by ele exists).
This looks very strange since we usually are very careful to verify the name of the variable, not the value. But in this case the value of the variable is the name we want to verify, so.
Inside the procedure it’s easier to write the test. The upvar command creates names in the current scopes that are linked to variables in another scope. If those variables don’t exist, the local names will in effect be names of unset variables.
upvar 1 $x1 v1 $x2 v2
if {[info exists v1]} {
Documentation:
array,
foreach,
if,
info,
proc,
set,
upvar