Tcl/Tk Notes

Part I TCL Overview

1 Everything Is a String

In Tcl, evrything is a string, and you have to explicitly ask for evaluation of variables and nested commands.

2 Substitution

Variable

The use of dollar sign($) is one kind of substitution. Variables can be assigned values by set command.

Command Substitution

A nested command is delimited by square brackets, [ and ]. The Tcl
takes everything between the brackets and evaluates it as a command. It
Reqrites the outer command by replacing the square brackets and
everything between them with the result of nested command.

Math Expression

The expr command is used to evaluate math expressions. The Tcl interpreter treats expr just like any other command, and it leaves the expression parsing up to the expr implementation.

Backslash Substitution

    Backslash(/) is used to quote characters that have special meaning to the interpreter.
   You can also specify characters that are hard to type directly by giving their octal or hexadecimal value.

Another common use of backslashes is to continue long commands on
multiple lines. A backslash as the last character in a line is
converted into a space.

3 Double Quotes vs. Curly Braces

Double quotes, like braces, are used to group words together. The
difference between double quotes and curly braces is that quotes allow
substitutions to occur in the group, while curly braces prevent
substitutions.

In practice, grouping with curly braces is used when substitutions on
the argument need to be delayed until a later time( or never done at
all). Examples include contrl flow statements and procedure
declararions. Double quotes are usefulin simple cases like the puts command.

Another common use of quotes is with the format command
that is similar to C printf function. The only way to effectively group
special characters in format command into a single argument is with
quotes.
e.g. puts [format "Item: %s/t%5.3f" $name $value]

4 Procedures

    Tcl uses the proc command to define procedures. The syntax is :
        proc name arglist body
    The return
command in Tcl procedure is optional because the interpreter will
return the value of the last command in the body as the value of the
procedure.

5 A While Loop Example

   The loop around boolean expression $i<=10
is crucial, because it delays substitution until while command
implementation tests the expression. The following expression is an
infinite loop:
        set i; while $i<=10 {incr i}

   The Tcl interpreter will substitute for $i before while is called, so while gets a constant expression 1<=10
that will always be ture.You can avoid this kind of errors by adopting
a consistent coding style that always groups expressions and command
bodies with curly braces.
    The command in the example uses set with a single argument. When used in this way the set command returns the current value of the named variable.

6 Grouping And Command Substitution

Grouping
decisions are made before substitutions are performed. This means that
the values of variables or command results do not affect grouping.
A
single round if substitutions is performed before command invocation.
That is, the result of a substitution is not interpreted a second time.
This rule is important if you have a variable value or a command result
that contains special characters such as spaces, dollar-signs, square
brackets or braces. Because only a single round of substitution is
done, you don't have to worry about special characters in values
causing extra substitutions.

7 Comments

Tcl
uses # for comments. Unlike many languages, the # character must occur
at the begining of a command. An easy trick to append a comment to the
end of a command is to proceed the # with a semicolon in order to
terminate the previous command.
A backslash(/)
continues a comment line onto the next line of the script. In addition,
a semi-colon inside a comment is not significant. only a newline
terminates comments.