CODEWITHSUNDEEP
tcl
Kuangzheng Xia
Kuangzheng Xia

Reputation: 15

TCL pass variable to proc

Pass argument to proc, expected result is puts $cust_id in proc will print the 123 instead of $cust_id

proc hello {cust_id} {
  puts $cust_id
}

set cust_id 123
puts $cust_id
hello {$cust_id}

Output is

123
$cust_id

Upvotes: 1

Views: 838

Answers (1)

Donal Fellows
Donal Fellows

Reputation: 137767

When you call hello, you give it a value and it prints that value that it was given (because you pass it to puts inside the body). When you call:

puts $cust_id

You are telling Tcl to read the cust_id variable and use that as the argument word to puts. But if you do:

hello {$cust_id}

then you are disabling substitutions (that's the literal meaning of putting something in braces in Tcl, and always was) so you get $cust_id passed into hello (and printed).

You can pass variables to procedures. You do it by giving them the name of the variable, and then using upvar to bind that to a local name. Like this:

proc hello {varName} {
    upvar $varName someLocalName

    puts $someLocalName
}

set cust_id 123
hello cust_id

Note that this is exactly the pattern used by the set command above. It is not special (other than that it is provided for you by the Tcl runtime; it's standard library, not language per se).

Yes, the upvar name is special (it converts a variable name into a variable) and it, along with uplevel, is one of the key features of Tcl that other languages don't have.

Upvotes: 3

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