Book Image

Tcl/Tk 8.5 Programming Cookbook

Book Image

Tcl/Tk 8.5 Programming Cookbook

Overview of this book

With Tcl/Tk, you can create full-featured cross-platform applications in a simple and easy-to-understand way without any expensive development package; the only tools required are a simple text editor and your imagination. This practical cookbook will help you to efficiently interact with editors, debuggers, and shell type interactive programs using Tcl/Tk 8. This cookbook will comprehensively guide you through practical implementation of Tcl/Tk 8.5 commands and tools. This book will take you through all the steps needed to become a productive programmer in Tcl/Tk 8. Right from guiding you through the basics to creating a stand-alone application, it provides complete explanation of all the steps along with handy tips and tricks. The book begins with an introduction to the Tcl shell, syntax, variables, and programming best practices in the language. It then explores procedures and the flow of events with control constructs followed by advanced error trapping and recovery. From Chapter 4, a detailed study of string expressions and handling enables you to handle various string functions and use lists to expand the string functionality. The book then discusses in-depth the Tcl Dictionary and how to utilize it to store and retrieve data. File operations and Tk GUI handling are covered extensively along with a developing a real-world address book application to practice the concepts learned.
Table of Contents (20 chapters)
Tcl/Tk 8.5 Programming Cookbook
About the Author
About the Reviewers

Breaking out of a procedure

As with the continue keyword, break is not in and of itself a control construct. The break keyword allows you to terminate the processing of a loop, whenever a specific condition is encountered. I routinely use this as a method of avoiding an endless loop by setting a maximum value to be detected and to break out of the loop.

How to do it…

In the following recipe, we will create a Tcl script, to be called from the command line, that increments the value of x and without the break keyword, would create the endless loop as mentioned. Once the upper limit has been reached the loop will break and the output will be an error message.

Create a text file named break.tcl that contains the following commands:

for {set x 1} {$x > 0} {incr x} {
if {$x == 5} {
puts "Upper limit reached"
puts "x = $x"

Now invoke the script using the following command line:

% tclsh85 break.tcl
x = 1
x = 2
x = 3
x = 4
Upper limit reached

How it works…

The action was invoked a total...