Writing Tcl Scripts

1
Writing Tcl Scripts

• Outline
• Syntax
• Data Types
• Control Structures
• Variable Scoping
• Goal
• Understand Tcl syntax, data types, control
  structures, scoping
• Reading
• Ch. 10-13, Practical Programming in Tcl and Tk

2
Basics

• Tcl script
• sequence of commands
• commands separated by newlines, semicolons
• in place of command is comment line
• Tcl command
• one or more words separated by spaces
• first word is command name, others are arguments
• cmd arg1 arg2 arg3 . . .
• Examples
• set a 22 set b 33
• set a 22
• set b 33

3
Division of Responsibility
Command
Chops command into words, makes substitutions.
Does not interpret values of words.
Tcl Parser
Words
Command Procedure
Interprets words, produces string result.
Result
4
Arguments

• Parser assigns no meaning to arguments (quoting
  by default, evaluation is special)
• C x 4 y x10
• y is 14
• Tcl set x 4 set y x10
• y is x10
• Different commands assign different meanings to
  their arguments
• set a 122
• expr 24/3.2
• eval set a 122
• button .b -text Hello -fg red
• string length Abracadabra

5
Variable Substitution

• Syntax varName
• Variable name is letters, digits, underscores
• May occur anywhere within a word
• Sample command Result
• set b 66 66
• set a b b
• set a b 66
• set a bbb 666666
• set a b.3 66.3
• set a b4 no such variable

6
Command Substitution

• Syntax script
• Execute script, substitute result
• May occur anywhere within a word
• Sample Command Result
• set b 8 8
• set a expr b2 10
• set a b-3 is expr b-3 b-3 is 5

7
Controlling Word Structure

• Words break at white space and semicolons except
• double-quotes prevent breaks
• set a This is one big word with spaces
• curly braces prevent breaks and substitutions
• set a nested braces
• backslashes quote special characters
• set a word\ with\ \\ and\ space
• Substitutions do not change word structure
• set a two words - a is one word
• set b a - b is still one word

8
Syntax Summary

• Script commands separated by newlines,
  semicolons
• Command words separated by white space
• at beginning of command is comment
• causes variable substitution
• causes command substitution
• Only one level of substitution done by parser
• quotes white space and semicolons
• still do substitutions
• \ quotes next character
• also provides some special characters like \n,
  \t, etc.
• quotes all characters - no substitutions

9
Expressions

• C-like (int and double) plus strings
• Used in expr, conditional commands
• Commands perform substitutions within expression
  arguments
• can and should put braces around arguments
• Sample command Result
• set b 5 5
• expr (b4)-3 17
• expr blt2 0
• expr b fac 4 120
• set a Bill Bill
• expr a lt Anne 0

10
Lists

• Zero or more elements separated by white space
• red green blue
• Braces and backslashes for grouping
• zero one twoA twoB twoC three
• one\ word two three
• List-related commands
• concat, foreach, lappend, lindex, linsert, list,
  llength, lrange, lreplace, lsearch, lsort
• Example
• lindex a b c d e f 2
• returns c d e
• Lists parse as commands each element becomes
  one word

11
Associative Arrays

• Syntax
• arrayname(index)
• Strings as array indices
• set x(fred) 44
• set x(2) expr x(fred)6
• result 50
• Array command - searching, size queries
• array names x
• returns fred 2
• foreach i array names x
• set x(i) 0

12
Control Structures

• C-like appearance
• Just commands that take Tcl scripts as arguments
• Example
• if x lt 3
• puts stdout x is too small!
• set x 3
• Commands
• if, for, foreach, while, case, break, continue,
  eval

13
Control Structures (cont.)

• Should usually put braces around control command
  arguments
• set i 0
• for set i 0 i lt 10 incr i 3
• lappend aList i
• set aList
• gt 0 3 6 9
• If no braces, then i lt 10 --gt 0 lt 10, always
  true, infinite loop

14
Procedures

• proc command defines procedure
• proc sub1 x expr x-1
• Procedures behave just like built-in commands
• sub1 3 returns 2
• Arguments can have defaults
• proc decr x y 1 expr x-y
• Can have variable number of arguments
• proc foo a b args . . .

name
body
list of argument names
gets list of extra args
15
Variable Scope

• Variables in proc are local by default
• Must explicitly reference global variables
• global command makes global variables locally
  accessible
• only in procedures where it appears
• proc foo a
• global x
• set x a
• proc bar a
• global x
• puts stdout x is x
• foo 5
• bar 3
• gt x is 5