Chapter 1, “Introducing Tcl and Tk,” consists of a short history of Tcl and Tk, a section highlighting their salient features, and a description of their distinguishing characteristics. If your computer doesn’t already have Tcl and Tk installed, then don’t skip “Getting Tcl and Tk,” or you won’t be able to work through the example programs, which would certainly defeat the purpose of buying this book.

Chapter 2, “Running TCL Programs, describes how to use the Tcl interpreter interactively and in batch mode. Tcl is an interpreted language, so Tcl commands must be executed by an interpreter, tclsh, instead of being compiled and then executed directly. You can use tclsh interactively, allowing you to enter commands and see their results immediately. You can also save your Tcl commands to a file, called a command file (imaginative name, yes?) or script, and have the Tcl interpreter execute the script. After you become familiar with Tcl’s simple syntax, I think you will find it much more efficient to save your scripts in a file and execute them by passing the command file to the interpreter.

Chapter 3, “Doing Mathematics,” introduces the fundamental elements of the Tcl language, such as comments, variables, expressions, and commands. You also get your first exposure to the two most difficult elements of Tcl: command substitution and grouping. Command substitution describes the manner in which programming statements are built from Tcl’s built-in commands and the results of those commands. Grouping refers to the way in which the operators "" and {} affect how command substitution works. This chapter also teaches you how to perform mathematical operations using Tcl’s expr command.

After learning how to perform basic math using Tcl, Chapter 4, “Strings, Strings, Everywhere Strings!,” shows you how to perform string operations. Tcl has a rich set of commands and functionality for manipulating strings, an unsurprising fact when you consider that Tcl is a string-based programming language. The final section continues the discussion of Tcl control structures I started in the previous chapter by introducing two looping commands: while and for.

Lists are one of Tcl’s two native or built-in data structures. In Chapter 5, “Working with Lists,” you spend some quality time with lists. Tcl has a broad set of commands for dealing with lists, and this chapter will get you up to speed with them. I also finish up the discussion of control structures by introducing the switch command, another command used for conditional execution, and the foreach command, a looping control structure that specializes in iterating over list items. The chapter ends with the two commands you can use to interrupt loop execution: break and continue.

Chapter 6, “Creating and Using Arrays,” is devoted mostly to arrays. Tcl arrays, like Perl’s hashes, are associative, meaning that they are indexed by strings, rather than integers or other numeric types. In addition to learning how to create and use arrays, this chapter also shows you commands and techniques for handling errors. Error handling combines well with material on arrays because common mistakes that occur when using arrays (such as accessing out-of-bounds or non-existent array indices) raise errors that need to be handled gracefully.

Procedures are covered in depth in Chapter 7, “Writing Tcl Procedures.” Procedures enable you to replace a commonly used sequence of commands with a single new command. Known as subroutines or functions in other programming languages, Tcl procedures can be called with or without arguments. You will also learn about variable and procedure scope, which determines when and where variables and procedures are visible. Together, procedures and an understanding of variable and procedure scope give you the tools you need to start implementing your Tcl scripts in a more modular and easy-to-maintain manner.

Most non-trivial programs involve interacting with the host filesystem. In Chapter 8, “Accessing Files and Directories,” you’ll learn how to open, close, delete, and rename files. The chapter also shows you how to perform file I/O using the puts (output) and gets (input) commands and how to use the format command to “pretty print” output. Finally, you’ll learn how to navigate the filesystem programmatically and work with file and directory names in a platform-neutral manner.

Chapter 9, “Understanding Tk Programming,” starts the discussion of graphical programming, introducing programming in Tk. As an introductory chapter, this chapter is light on code and long on text, as it discusses topics including event-driven programming and widget attributes and operations. Covering this information here simplifies my job in the rest of the chapters because most Tk programming assumes familiarity with material presented in this chapter. The chapter closes with a description of each of the widgets available to Tk programming.

Unless you’ve been living in an unelectrified cave for the last decade, you are accustomed to clicking buttons. Chapter 10, “Button Widgets,” describes how to program Tk buttons. After providing more information about the first of Tk’s three geometry managers, pack, this chapter looks at Tk’s button widgets. In addition to learning how to use buttons, I’ll show you how to use color in a Tk application and how to bind buttons to commands and events

Chapter 11, “Windows, Frames, and Messages,” shows you how to use the grid geometry manager; I think you’ll like grid better than pack. I’ll also introduce you to three more Tk widgets: frames, toplevels (that is, top-level windows), and messages.

Chapter 12, “Entry and Spinbox Widgets,” introduces the entry and spinbox widgets. Tk’s entry widget is a specialized type of text-entry field best suited to high-speed, head-down data entry, but applicable for many types of data entry in which you want to control or validate the data that is input. The spinbox widget, often referred to as a spinner in other GUI toolkits, is based on the entry widget.

In Chapter 13, “Listbox Widgets,” you learn how to use Tk’s listbox widget. A listbox displays a series of read-only text lines. The list is vertically scrollable and can be scrolled horizontally as necessary. You can select zero, one, or more items in a list, so the listbox widget has methods for determining which items are selected (and for selecting items programmatically). You can add and delete items from a listbox, but items themselves cannot be edited. As usual, you can also control the colors, relief, and other visual attributes of listbox widgets.

Chapter 14, “Scrollbar, Scale, and Text Widgets,” discusses three Tk widgets: scrollbars, scales, and text boxes. Scrollbars allow you or your users to scroll the viewable area of a window. A scale widget is a slider whose value changes as the slider is moved. Text widgets provide areas for displaying and editing text. As you will see later in the chapter, Tk’s text widget is a full-featured text display and manipulation tool. The price of this feature set is that the text widget is complex.

Chapter 15, “The Canvas Widget,” concludes the book with a tour of the canvas widget. The canvas is a general purpose widget you can use to display drawing primitives, such as arcs, lines, polygons, and other shapes; images in a variety of formats; text; and even other embedded widgets. I will show you how to use many of the canvas widget’s other features in this chapter.