procs

procedures are supported by TCL :

Procedures return to the calling program through "return" statement

Every procedure must have a corresponding return statement. This however is not strictly required.

Once they are created, procedures could be used just as any other TCL command

Procedures are declared with the help of keyword "proc"

Each procedure may have a set of formal parameters and some variables - local and global

Syntax:

proc proc_name { formal parameters } {

  statement1

  statement2

  :

  return

}

Example:

proc plus { a b } {

  expr $a + $b

  return

}

Syntax:

proc name argList body

where,

name - is the procedure name

argList - List of arguments. If a procedure takes more than one argument, they should be specified within braces.

body - body of the procedure

Example:

% proc proc_a { a b c } { body of procedure }

% proc proc_b b { body of procedure }

Once created, a procedure could be invoked/ called just by its name

Example: proc_a { $a $b $c }, proc_b { $b } w.r.t. The above example

Every procedure has its own set of local variables.

Any two procedures may have variables with the same names. The values of the variables do not get affected since they are not visible outside the procedure block.

"global" could be used to declare the scope of some variables as global

"global" should be used inside a procedure

Syntax:

global varName ?varName ...?

where

varName - is the name of variable whose scope has to be

extended beyond a procedure. More than one variable could be 

specified.

"global" binds variable names "varName"... to global variables. References to these names will refer to global variables instead of local variables for the duration of the current procedure.

Example 1:

	proc proc_a { } {
	global globvar
	puts "Variable globvar is associated to $globvar"
	set globvar "Variable updated inside proc_a"
	puts $globvar
	}
	proc proc_b { } {
	set globvar "Variable updated inside proc_b"
	puts $globvar
	}
	set globvar "outside proc"
	proc_a
	puts "Value of globvar now after proc_a execution: $globvar"
	proc_b
	puts "Value of globvar now after proc_b execution: $globvar"

view raw proc_global.tcl hosted with ❤ by GitHub

Example 2: Simple tree traversal iterative procedure.

	set child(a) "b c"
	set child(b) "d e"
	set child(c) "f g h"
	set child(d) "i j"
	set child(e) "k l"
	set child(f) "m"
	set child(g) "n o p"
	set child(h) "q"
	set child(i) r
	set child(j) s
	set child(k) "t u"
	proc iter_tree { a } {
	global child;
	if [info exists child($a)] {
	foreach aa $child($a) {
	puts "$aa is child of $a"
	iter_tree $aa
	}
	}
	}
	iter_tree a

view raw recursive_example.tcl hosted with ❤ by GitHub

Example 3 :

	#Procedure whose default variable is set 1
	#This script will increment the variable with the given amount of increment if provided.
	#if optional increment value is not provided, it will increment by 1
	proc increment { var {increment 1} } {
	return [expr $var+$increment]
	}

view raw proc_incr.tcl hosted with ❤ by GitHub