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
Credits
About the Author
Acknowledgment
About the Reviewers
www.PacktPub.com
Preface

Determining if a key exists


Now that we have a named dictionary with key/value pairs, it becomes necessary to determine if the given key exists. To accomplish this, Tcl provides the dict exists command. The syntax is as follows:

dict exists DictionaryValue key… key…

How to do it…

In the following example, we will create a dictionary containing a set of key/value pairs and then determine whether or not a specific key exists. Return values from the commands are provided for clarity. Enter the following command:

% set names [dict create 1 John 2 Mary 3 Paul]
1 John 2 Mary 3 Paul
% dict exists $names 3
1

How it works…

The dict exists command returns a Boolean value to indicate if the specified key exists in the dictionary referenced in name. A return value of 1 indicates that the key exists, while a return of 0 indicates that it does not. Be aware that this command will return an error if dictionaryValue does not reference an existing dictionary.