Tcl Procedures

The proc command is used to define a procedure. There are optional arguments that can be defined for a procedure as values that will be passed to the procedure when the procedure is called. Each parameter becomes a variable in the procedure. The commands contained within the curly braces are run when the procedure is called. The format of a procedure is:

proc name {?arg1? ?arg2? etc…} {body}

An example procedure named curve_info that takes curve_reference and color as its arguments:

proc curve_info {curve_reference color} {
   puts "Curve reference = $curve_reference";
   puts "Curve color = $color";
}
curve_info p1w1c1 5;
Curve reference = p1w1c1
Curve color = 5

The name args is a special procedure parameter when you don't know how many arguments will be used. This is useful when creating a context menu where multiple items may be passed.

proc node_info {args} {
   foreach node $args {
      puts "Node = $node";
   }
}
node_info node1 node2 node3;
Node reference = node1
Node reference = node2
Node reference = node3

Variables are treated as one argument and do not require the args parameter.

proc node_info {node_list} {
   foreach node $node_list {
      puts "Node = $node";
   }
}
set node_list "node1 node2 node3";
node_info $node_list;
Node reference = node1
Node reference = node2
Node reference = node3

The return command is used to exit a procedure. This is handy for error control.

proc node_info {args} {
   if {[llength $args] == 0} {
      puts "No nodes are selected";
      return;
   }
   foreach node $args {
      puts "Node = $node";
   }
}
curve_info;
No curves are selected

curve_info node1 node2 node3;
Node = node1
Node = node2
Node = node3

The return command can also be used to assign the results of a procedure to a variable.

proc add_three_numbers {num1 num2 num3} {
   set sum [expr $num1 + $num2 + $num3];
   return $expression;
}
set sum [add_three_numbers 2 4 5];
puts $sum;
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As mentioned above, all variables defined within a procedure are local to that procedure. This means that, unless otherwise defined, a variable definition only exists, and is only accessible, from within the procedure it is created in.

set element_list "1 2 3 4 5";
proc element_info {} {
   puts $element_list;
   set node_list "10 20 30 40";
   puts $node_list;
}
element_info;
can't read "element_list": no such variable
10 20 30 40

puts $element_list;
1 2 3 4 5

puts $node_list;
can't read "node_list": no such variable

This also allows the programmer to define the same local variable within different procedures without any conflict.

proc node_info_1 {} {
   puts $node_list;
   set node_list "10 20 30 40";
   puts $node_list;
}

proc node_info_2 {} {
   puts $node_list;
   set node_list "10 20 30 40";
   puts $node_list;
}
node_info_1;
can't read "node_list": no such variable
10 20 30 40

node_info_2;
can't read "node_list": no such variable
10 20 30 40