Tcl and Tk Programming for the Absolute Beginner

378 Pages·2007·6.95 MB·English

by  Kurt Wall

Tcl/Tk
Programming
for the Absolute
Beginner
KURT WALL
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To my wife, Kelly, who truly is flesh of my flesh and bone of
my bone.
A
CKNOWLEDGMENTS
T
he image of writing as a solitary person laboring in the dark of night to
produce a literary masterpiece is a grand fiction, usually perpetuated by
writers who know better. I might have worked in the dark of night, but
that’s the only part of the image that holds up to serious scrutiny. Writing a book
is a team effort. My agent, Marta Justak, helped me get the book and took some
lumps intended for me when I fell behind on the schedule. As usual, Marta, you’re
the best. Let’s do this again, but leave out the parts that weren’t fun.
The editorial team was first rate. Special thanks to Mitzi Koontz for graciously
accommodating my request to extend the schedule; Marta Justak, the copy editor,
eliminated a distressing number of typos and grammatical mistakes; Melba
Hopper, proofreader extraordinaire, gets extra credit for understanding the con-
ventions I adopted and perversely managed not to follow; and Rick Reynolds, my
technical editor, noted some algorithmic subtleties and downright blunders that
would have embarrassed me and misled you. Sharon Shock, the indexer, had the
thankless but vital task of creating the index. A good index makes the difference
between a book that sits on the shelf and a book that gets dog-eared and marked-
up. Thanks to you all—it’s a much better book as a result of your ministrations.
I appreciate the help and support of my colleagues at Panasas, the inhabitants of
the #tcl channel on Freenode, and my friends. Panasas tolerated my bleary-eyed
appearances at the office. The Tcl and Tk pros on #tcl answered oddball questions
about Tcl and Tk features. My friends provided support and encouragement and
said: “Ooh! Aaah!” at just the right times.
I would be remiss if I failed to thank my wife. Kelly gave me the space and time to
work on the book that I would otherwise have spent with her. She also told me not
to stay up too late working on the book, advice I would have done well to heed.
Poots the cat provided needed comic relief and company at 2:00 a.m.
All the support and assistance notwithstanding, I take full responsibility for any
errors and mistakes that remain.
A A
BOUT THE UTHOR
W
hen he isn’t writing a book (including numerous Linux books), KKurt
Wall might be found cooking, drinking coffee, working at his day job,
playing in his garden, trying not to hurt himself while learning to build
furniture, or sitting in front of his computers. Kurt dislikes writing about himself
in the third person.
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Table of Contents
Chapter 1 Introducing Tcl and Tk............................................. 1
What Is Tcl?.............................................................................................................................. 1
What Is Tk?............................................................................................................................... 2
What Makes Tcl and Tk Different?.................................................................................... 3
Why Use Tcl and Tk?.............................................................................................................. 5
Getting Tcl and Tk.................................................................................................................. 6
Installing Tcl and Tk on Linux................................................................................... 6
Installing Tcl and Tk on Windows............................................................................ 7
Installing Tcl and Tk on OS X................................................................................... 13
Installing Tcl and Tk from Source.......................................................................... 18
Chapter 2 Running Tcl Programs........................................... 23
Invoking the Interpreter.................................................................................................... 23
Executing Tcl Commands Interactively......................................................................... 25
Creating Tcl Command Files............................................................................................. 27
Chapter 3 Doing Mathematics.................................................. 33
Guessing Numbers............................................................................................................... 33
Language Fundamentals.................................................................................................... 34
Comments..................................................................................................................... 35
Commands.................................................................................................................... 36
Command Substitution............................................................................................. 38
Grouping........................................................................................................................ 41
Grouping with Double Quotes................................................................................. 42
Grouping with Braces................................................................................................. 42
Variables........................................................................................................................ 43
Procedures..................................................................................................................... 46
Getting User Input............................................................................................................... 48
Basic Mathematical Operators......................................................................................... 50
Conditional Execution: The if Command.................................................................... 52
Analyzing the Guessing Numbers Program.................................................................. 56
Looking at the Code.................................................................................................... 56
viii Tcl/Tk Programming for the Absolute Beginner
Understanding the Code........................................................................................... 57
Modifying the Code..................................................................................................... 57
Chapter 4 Strings, Strings, Everywhere Strings!................ 59
Mad Libs.................................................................................................................................. 59
The string Command........................................................................................................ 60
Comparing Strings.............................................................................................................. 62
The compare Option.................................................................................................... 63
The equal Option........................................................................................................ 65
The match Option........................................................................................................ 66
Inspecting Strings................................................................................................................ 68
The length and bytelength Options..................................................................... 68
The index Option........................................................................................................ 68
The first and last Options.................................................................................... 69
The range Option........................................................................................................ 70
The replace Option.................................................................................................... 71
The is Option............................................................................................................... 73
Modifying Strings................................................................................................................ 76
Repeating Strings........................................................................................................ 76
Switching Case............................................................................................................. 76
Trimming Strings........................................................................................................ 77
Appending Strings...................................................................................................... 78
Looping Commands............................................................................................................ 78
Looping with the while Command........................................................................ 79
Iterative Loops: The for Command........................................................................ 81
Analyzing Mad Libs............................................................................................................. 83
Looking at the Code.................................................................................................... 83
Understanding the Code........................................................................................... 84
Modifying the Code..................................................................................................... 85
Chapter 5 Working with Lists.................................................. 87
Playing Blackjack................................................................................................................. 87
What Is a Tcl List?................................................................................................................ 88
Creating Lists........................................................................................................................ 89
Appending Lists.................................................................................................................... 90
Merging Lists......................................................................................................................... 90
Accessing List Elements..................................................................................................... 91
Accessing Specific List Elements............................................................................. 92
Modifying Lists..................................................................................................................... 94
Inserting New Elements............................................................................................ 94
Replacing Elements.................................................................................................... 95
Searching and Sorting Lists............................................................................................... 96
Contents ix
Searching 101............................................................................................................... 96
Sorting............................................................................................................................ 99
Additional List Operations.............................................................................................. 102
Strings to Lists............................................................................................................ 102
Lists to Strings............................................................................................................ 103
Looping with the foreach Command.......................................................................... 104
Conditional Execution: The switch Command......................................................... 105
Interrupting Loop Execution.......................................................................................... 108
Analyzing Playing Blackjack........................................................................................... 110
Looking at the Code.................................................................................................. 110
Understanding the Code......................................................................................... 112
Modifying the Code................................................................................................... 113
Chapter 6 Creating and Using Arrays................................... 115
What’s the Word?.............................................................................................................. 115
The Differences Between Arrays and Lists.................................................................. 116
Working with Arrays........................................................................................................ 117
Getting Information about Arrays........................................................................ 118
Converting Lists to Arrays....................................................................................... 120
Converting Arrays to Lists....................................................................................... 121
Retrieving Array Elements...................................................................................... 122
Searching Arrays........................................................................................................ 123
Grace Under Pressure....................................................................................................... 124
Dealing with Exceptions: The catch Command............................................... 124
Raising Errors: The error Command................................................................... 126
Examining Variables......................................................................................................... 128
Analyzing What’s the Word?.......................................................................................... 129
Looking at the Code.................................................................................................. 129
Understanding the Code......................................................................................... 130
Modifying the Code................................................................................................... 131
Chapter 7 Writing Tcl Procedures........................................ 133
Fortune Teller..................................................................................................................... 133
What Is a Procedure?........................................................................................................ 134
Defining Procedures......................................................................................................... 135
Defining Procedures with Default Values.......................................................... 136
Defining Procedures with Variable Arguments................................................ 137
Understanding Variable and Procedure Scope.......................................................... 139
Analyzing Fortune Teller................................................................................................. 142
Looking at the Code.................................................................................................. 142
Understanding the Code......................................................................................... 143
Modifying the Code................................................................................................... 144