ch30.htm

linux-unix130.linux.and.unix.ebooks130 linux and unix ebookslinuxLearning Linux - Collection of 12 E

      puts [expr 2 * $i]

      set i [expr $i + 1]

}

</FONT></PRE>

<P>This <TT>while</TT> loop performs the same function as the example that was presented

in the section describing the <TT>for</TT> loop. It calculates 2 x <TT>i</TT> each

time through the loop and prints the result to the screen. Notice that in this example

you have to handle incrementing the counter yourself. With the <TT>for</TT> loop,

the counter incrementing was taken care of by the <TT>for</TT> command.

<H4 ALIGN="CENTER"><A NAME="Heading16<FONT COLOR="#000077">The switch Command</FONT></H4>

<P>The <TT>switch</TT> command provides the same function as an <TT>if</TT> statement

that has multiple <TT>elseif</TT> clauses associated with it. The <TT>switch</TT>

command compares a value (this value is usually stored in a variable) with any number

of patterns, and if it finds a match it executes the Tcl code associated with the

matching pattern.<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF">switch $thing {

      car {puts &quot;thing is a car&quot;}

      truck {puts &quot;thing is a truck&quot;}

      default {puts &quot;I don't know what this thing is&quot;}

}

</FONT></PRE>



<DL>

	<DT><FONT COLOR="#0066FF"></FONT></DT>

</DL>





<DL>

	<DD>

<HR>

<A NAME="Heading17<FONT COLOR="#000077"><B>NOTE:</B> </FONT>The Tcl <TT>switch</TT>

	command is equivalent to the <TT>case</TT> statement found in Pascal and some other

	languages.

<HR>



</DL>



<P>This <TT>switch</TT> command compares the value that is stored in the <TT>thing</TT>

variable (which must be set prior to these statements, of course) with the string

<TT>car</TT> and the string <TT>truck</TT> to see if it matches either of them. If

the value of the <TT>thing</TT> variable is equal to <TT>car</TT>, then <TT>thing

is a car</TT> is displayed on the screen. If the value of the <TT>thing</TT> variable

is equal to <TT>truck</TT>, then <TT>thing is a truck</TT> is displayed on the screen.

If neither of these cases are true, the default clause is executed and <TT>I don't

know what this thing is</TT> displays on the screen.



<DL>

	<DT></DT>

</DL>





<DL>

	<DD>

<HR>

<A NAME="Heading18<FONT COLOR="#000077"><B>TIP:</B> </FONT>Whenever you need

	to check to see if a variable is equal to one of a number of values, you should use

	a <TT>switch</TT> command instead of an <TT>if</TT> command with multiple <TT>elseif</TT>

	clauses. This makes your code much easier to read and understand.

<HR>



</DL>



<H4 ALIGN="CENTER"><A NAME="Heading19<FONT COLOR="#000077">Comments</FONT></H4>

<P>It is always a good idea to include comments in any Tcl code you write--or code

you write in any other language, for that matter. This becomes especially important

if any of the following situations is possible:



<UL>

	<LI>Someone else needs to look at or maintain your code.

	<P>

	<LI>Your programs get large.

	<P>

	<LI>You won't be looking at code that you have written for long periods of time after

	you write it.

</UL>



<P>Chances are that at least one of these situations will come up with Tcl code you

have written.</P>

<P>Comments cannot be placed in the middle of a command. They must occur between

commands. The pound sign (#) is used to inform Tcl to expect a comment.<FONT COLOR="#0066FF"></FONT>

<PRE><FONT COLOR="#0066FF"># This is a valid comment

set a 1 ;   # This is a valid comment

set a 1           # This is an invalid comment

</FONT></PRE>

<P>The third comment shown here is invalid because it occurs in the middle of a command.



<DL>

	<DT></DT>

</DL>





<DL>

	<DD>

<HR>

<A NAME="Heading20<FONT COLOR="#000077"><B>NOTE:</B> </FONT>Recall that Tcl

	interprets everything up to a newline character or a semicolon to be part of the

	command.

<HR>



</DL>



<H3 ALIGN="CENTER"><A NAME="Heading21<FONT COLOR="#000077">The Tk Language

Extensions</FONT></H3>

<P>Earlier in this chapter, a simple example of Tk displayed <TT>Hello there</TT>

in a button in the <TT>wish</TT> window. Tk is much more powerful than that example

showed. Along with the button widget are many other widgets provided by Tk. These

include menus, scrollbars, and list boxes. This section gives you an overview of

some of the other Tk widgets and gives short examples explaining how these widgets

can be used.

<H4 ALIGN="CENTER"><A NAME="Heading22<FONT COLOR="#000077">Frames</FONT></H4>

<P>Frame widgets are containers for other widgets. They do not have any interesting

behavior like the other Tk widgets. The only visible properties of frame widgets

that you can set are their color and their border appearance. You can give three

different border appearances to a frame widget: flat, raised, and sunken. You can

experiment with the different frame widgets to see how they look.</P>

<P>The flat border frame widget is not too exciting. It looks exactly the same as

the default <TT>wish</TT> window (because the default border appearance is flat).

<H4 ALIGN="CENTER"><A NAME="Heading23<FONT COLOR="#000077">Buttons</FONT></H4>

<P>Button widgets are used to get specific input from a user. A button can be turned

on or activated by the user of a Tk program by moving the mouse pointer over the

button and then pressing the left mouse button. Tk provides the following three kinds

of button widgets:



<UL>

	<LI>Button

	<P>

	<LI>Check button

	<P>

	<LI>Radio button

</UL>



<P>The button widget is used to initiate some specific actions. The button usually

has a name such as &quot;Load file&quot; that describes the action that results if

you press the button.</P>

<P>Check button widgets are used to allow users of a program to turn program options

on or off. When the check button is shaded the program option is on, and when the

check button is not shaded the program option is off.</P>

<P>Radio buttons are similar to check buttons except that they are defined in groups,

where only one member of a group of radio buttons is allowed to be on at one time.

This means that if one radio button in a group of radio buttons is on, none of the

other radio buttons in that group can be turned on. When the radio button is shaded

it is on, and when the radio button is not shaded it is off.

<H4 ALIGN="CENTER"><A NAME="Heading24<FONT COLOR="#000077">Menus and Menu Buttons</FONT></H4>

<P>Menu widgets are used to implement pull-down menus, cascading menus, and pop-up

menus. A menu is a top-level widget that contains a set of menu entries that have

values or commands associated with them. Five kinds of entries can be used in menus:



<UL>

	<LI>Cascade entries display a submenu when the mouse pointer passes over them. The

	cascade entry is similar in function to the menu button widget.

	<P>

	<LI>Command entries invoke a Tcl script when activated. The command entry is similar

	to the button widget in function.

	<P>

	<LI>Check button entries toggle a program option between on and off. When the check

	button is shaded the option is on, and when the check button is not shaded it is

	off. The check button entry is similar in function to the check button widget.

	<P>

	<LI>Radio button entries toggle a program option. The difference between the radio

	button entry and the check button entry is that radio buttons are typically defined

	in groups, with the restriction that only one of the radio buttons in the group can

	be active at once. The radio button entry is similar in function to the radio button

	widget.

	<P>

	<LI>Separator entries display a horizontal line in the menu. This is used for appearance

	purposes only. There is no behavior associated with a separator entry.

</UL>



<P>The main difference between the menu entries and the button widgets is that the

button widgets can exist by themselves, but the menu entries must exist within the

context of a menu widget.</P>

<P>Menu button widgets are similar to button widgets. The only real difference between

the two is that when menu buttons are invoked they bring up menus instead of executing

Tcl scripts as button widgets would. The menu button name usually describes the types

of menu entries contained in the menu that the menu button activates. This means

that you should find menu entries that perform some kind of file operations contained

within the File menu.</P>

<P>You can activate a menu by moving the mouse pointer to the menu button widget

and pressing the left mouse button. This activates the menu associated with the menu

button and displays the menu entries that are contained in that menu to the screen.

You can now move the mouse pointer down through the list of menu entries and select

the one you want.</P>

<P>The File menu contains two command entries (the Open entry and Quit entry), one

cascade entry (the Save As entry), and one separator entry. The menu that comes up

as a result of clicking the mouse pointer on the Save As cascade entry contains three

command entries: the Text entry, the Ver 1 file entry, and the Ver 2 file entry.

<H4 ALIGN="CENTER"><A NAME="Heading25<FONT COLOR="#000077">List Boxes</FONT></H4>

<P>The list box widget enables users of a Tk application to select items from a list

of one or more items. If the number of items to be displayed in the list box is larger

than the number of lines in the list box, you can attach scrollbars to make the extra

items accessible.

<H4 ALIGN="CENTER"><A NAME="Heading26<FONT COLOR="#000077">Scrollbars</FONT></H4>

<P>Scrollbar widgets are used to control what is displayed in other widgets. Scrollbar

widgets are attached to other widgets to allow users to scroll up and down through

the information contained in the widget. You typically put scrollbars on any widget

that is designed to contain an arbitrary number of lines of information (such as

a list box) or on widgets that contain more lines of information than the widget

can display, given its size.

<H3 ALIGN="CENTER"><A NAME="Heading27<FONT COLOR="#000077">Summary</FONT></H3>

<P>This chapter started off by introducing Tcl and the Tk tool kit and describing

the uses of both. Although this chapter contained a lot of information, it barely

scratched the surface of the programming tools provided by Tcl and the Tk tool kit.</P>

<P>Tcl has many more programming features than were described in this book. Some

of the most notable are arrays, lists, and procedures.</P>

<P>Not all of the Tk widgets were described here, either. Some of the widgets that

were not described are canvasses, scales, labels, messages, and <TT>textedit</TT>

widgets.</P>

<P>Tk is just one example of an extension to Tcl. There are many other extensions

available that extend the behavior of Tcl in different ways. Some of these extensions

are listed here:



<UL>

	<LI>Ak: An audio extension for Tcl, Ak provides numerous commands for sound recording

	and playback.

	<P>

	<LI>XF: This is an interactive graphical user interface developer for Tk.

	<P>

	<LI>Tcl-DP: This is a Tcl extension that helps programmers develop distributed applications

	using Tcl.

</UL>



<P>If you would like to learn more about Tcl and Tk, a good place to start is the

manual pages for Tcl, Tk, and any of the specific commands you want information about.

There are also a few books available that are devoted to Tcl and Tk programming.









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