Presented By: Sunil Ramesh Tcl Programming: Theory & Practice.

Presented By:Sunil Ramesh

Tcl Programming:Theory & Practice

2

Origins of Tcl

John Ousterhout, UC Berkeley

John McCarthy, AI Lab, Stanford

Alonzo Church, Princeton

⋋-Calculus - 1930

LISP - 1950

TCL - 1990

C - 1970

Dennis Ritchie, Bell Labs

Theory Part 1Lambda Calculus

Anything which can be computed can be computed using the lambda calculus

- Church’s Thesis

4

Church’s Lambda Calculus 1

• Highly theoretical basis for programming languages• Based on “Lambda” – λ

– λ is what is used to define an “anonymous” function– Grammar

• λargVarNameList.expression• argVarNameList is the list of variables• expression denotes the return value of lambda

• About State– The only state allowed is the current list of function aliases*

• Example:f = λx.x2

g = λx.f(x)+1– Thus, g(x) = x2 + 1, and g(5) = 26

*It is said that Church was initially opposed to this idea, but it was forced on him by his student, Alan Turing.

5

Church’s Lambda Calculus 2

• Two kinds of statements– Function declarations – lambda abstraction

• Ex: λx.x+2 - a function with 1 argument that returns the sum of the argument and 2

• Ex: λx,y.x*y - a function with two arguments that returns the product of two given arguments

– Function applications – lambda application• Ex: (λx.x+2)(3) – will return 5• Ex: (λx,y.x+y)(3,5) – will return 8

– Complex applications• (λx.x+(λx,y.x+y)(4,3)) 1 - will return 8

• If you are interested in more:– Representation of numbers: Church Numerals

http://www.cs.usm.maine.edu/class/cos370/handouts/lambda/node9.html

6

LISP - Lambda Calculus Realized

• Church’s proposition finally realized in an implementation called LISP – list processor

• Uses prefix notation!• Function add2:

– Lambda calculus: λx.x+2– Scheme (dialect of LISP): (define (f x) (+ x 2))

• Alternatively (and closer to λ): (define f (lambda (x) (+ x 2)))• “f is a function: takes one argument: returns the sum of the

argument and 2– Optimizations:

• (lambda (x) (+ x 2)) and (lambda (y) (+ y 2)) equivalent• Usage:

– (f 2) -> will evaluate to 4– ((lambda (x) (+ x 2)) 3) -> will evaluate to 5

Theory Part 2List Languages

LISP, Scheme, and Tcl are common examples of List-centric languages

We will use Scheme to illustrate the salient features and derivations of the List abstraction.

8

• Pair – a construct with exactly 2 elements

Pairs

car cdr

• Constructing a pair– (define a (cons 1 2)) will yield 21

• Referencing “left” and “right” elements– (car a) will yield 1

– (cdr a) will yield 2

a

9

Pairs Implementation

• Pairs are implemented using λ-calculus, too!• Pair construction is a contract “C”

– Cons: T1, T2 -> C such that:– Car: C -> T1– Cdr: C -> T2

• Let’s see some code…

(define cons (lambda (x y)

(define select (lambda (m)

(cond ((= m 0) x)

((= m 1) y)))) select))

(define (car z) (z 0))

(define (cdr z) (z 1))

10

Lists

• Implementation of pairs has no bearing on “type” of the elements of the pair.

• Can an element be a pair itself? Sure!• Lists are realized easily:

1 2 3 4a

• (define a (cons 1 (cons 2 (cons 3 (cons 4 nil)))))• How to retrieve values?

1 = (car a) 2 = (car (cdr a)) …… Built-in, called (cadr a)

11

Abstraction: Clumsiness to Grace

• The above list can also be declared by (Scheme):– (define a (list 1 2 3 4))

• In Tcl:– Declaration: set a [list 1 2 3 4]

– Accessing: The “value” of the list will be in “$a”

– More later…

1 2 3 4a

12

List Operations

• Length of a list (Scheme) - llength(define (llength lst)

(if (null? lst)0(+ 1 (llength (cdr lst)))))

• Index in a list (Scheme) - lindex(define (lindex lst idx)

(if (= 0 idx)(car list)(lindex (cdr lst) (- idx 1))))

• These same functions (and more) are built-in in Tcl

Practice Part 1Introduction to Tcl

14

Constants

• In Tcl, all free constants are strings• Upshot

– abc is the same as “abc”

– “5” is the same as 5

• <, >, == ,<= and >= compare numerically– If any of the arguments are non-numbers, errors are

thrown

• Strings can also be compared literally:– string compare string1 string2

15

Variables

• A Tcl variable is a C-style name, which satisfies the regular expression:– {[A-Za-z_][A-Za-z_0-9]*}

• Examples:– iterator

– loopInit

– _ErrorCount76

• Non-examples:– 78Cool

– ~notAVariable

16

Variables, Contd.

• “set” command– Usage: set <varname> <value>

– In most other languages, <varname> = <value>

– Artifact of basis on LISP-like PLs (define varname value)

• Example:– set a 5

– puts $a puts = putString – exactly like C’s• Will print out 5

• What’s with the $ ?– $<varname> is the value of a variable with name varname

• Weird…– set a a will set the string “a” to the variable a

17

Output to screen

• puts lets the programmer send output to the screen• Example:

– puts 5

– puts $a

– puts “The value of variable a is $a”• Variables can be freely placed in a puts string

• Any way to suppress the new line?– puts –nonewline “Enter some number:”

– Caveat: Most Operating System implementations use \n to trigger a flush of the buffer. In most cases, after a puts –nonewline, adding flush stdout is necessary

18

Comments

• Comments are C-shell and Perl style, and start with a #. The rest of the line is part of the comment

• Example 1: comment on its own line

# I will now set the variable a to 5

set a 5• Comments after a statement need a slight change• Example 2: comment after a statement

set a 5 ;# I just set the variable a to 5• We are essentially ending the current statement

with the ; The rest of the line is the comment.

19

Math expressions

• expr is used for evaluating math expressions• Usage: expr <expression>• Examples:

– expr 5+3 will evaluate to 8– expr 5*(7+3) will evaluate to 50

• General Rule of Thumb– Anything in <expression> is exactly the same as it would

appear in C. If an expression is valid in C, then it is valid after expr

– The only thing to remember is replacing all variable names with $<varname>, but this is generally not an issue

20

Setting results of Evaluation

• [<expression>] denotes the value after evaluation of an expression

• Example:– set a [expr 5+6] will set the “result” of the expr to a

– set b [SomeFunction 6] will set the “return value” of “SomeFunction” to b

• Careful!– [set a [expr 5+6]] will result in a syntax error, because 11

is not a function!

21

Keyboard Input

• gets is a built-in Tcl command which allows for keyboard input by user

• Example:puts “Please enter your name: “

set userName [gets stdin]

puts “Your name is $userName”

• Example:puts “Enter a number: “

set input [gets stdin]

set output [Add2 $input]

puts “$input plus 2 is $output”

22

Programming Assignment # 1

• Write a program that asks the user for two numbers and prints the average of the two numbers.

• Sample Output:

23

Solution to PA # 1

24

Conditionals - if

• Syntax: if { condition } { action1 } else { action2 }• Example:

if { $a > 2 } {

puts “$a is greater than 2”

} else {

puts “$a is not greater than 2”

}• There is no option as far as bracket style is

concerned – it is enforced like above

25

Conditionals – on the condition

• A condition is exactly as it would appear in C• Any nonzero number is taken to be true• Examples:

if { $a > 5 }

if { $a > $b }

if { ($a > 5) || ($b < 3) || ($c != $d) }

if { $a }

If { !$done }

• C rule of thumb

– The condition works exactly as in C

26

Conditionals - elseif

• Used for chaining if statements• Example:

if { $a > 0 } {

puts “$a is a positive number”

} elseif { $a < 0 } {

puts “$a is a negative number”

} else {

puts “$a is zero”

}

27

Incr – Incrementing/Decrementing

• incr is an overloaded version of C’s ++,--,+=,-=• Syntax: incr varName ?offset=1• Examples:

incr i ;# Will increase i by one, the default

incr i -1 ;# Will reduce i by one

incr i 3 ;# Will increase i by three

incr i 0 ;# Will not change I• Common mistake:

– incr $i 2 will cause a syntax error. We need to the variable name to incr, not the value of the variable!

28

Loops - while

• The while loop is exactly the same as it is in C!• Syntax: while { condition } { action }• Example:

set i 1

while { $i <= 10 } {

puts “On iteration $i of 10”

incr i

}

29

Loops - for

• The for loop is exactly the same as it is in C!• Syntax: for { initCond } { condition } { update }• Example:

for { set i 1 } { $i <= 10 } { incr i } {

puts “On iteration $i of 10”

}

30


• Write a program that keeps asking user for numbers till the blank string is seen, and prints the average of the numbers:

• Sample Output:

31

Solution to PA # 2

32

Procedures

• By example:proc Add2 { number } {

set a [expr $number+2]return $a

}• “proc” is a reserved word, and it denotes the start

of a procedure• { number } this is a list of variables• return $a will return the value stored in a• A procedure MUST be defined before being called.

– No prototypes! – Except in Itcl, more later

33

Procedures: Optional Parameters

• Optional Parameters are very easy to set up in Tcl• By Example:

Proc Add { number { by 1 } } {

return [expr $number+$by]

}

• The variable by takes on a default of 1 unless specified otherwise in the function callAdd 1 ;# Will return 2

Add 1 2 ;# Will return 3

Add 1 –1 ;# Will return 0

34

Global Scope

• Scope works the same as in C except for one thing: accessing global variables inside procedures– Global variables have to be explicitly declared as such inside a

procedure• Example:

set errorCount 0

…

proc TriggerError { errorString } {

global errorCount

puts “Error: $errorString”

incr errorCount

}

35

Lists in Tcl

• A list is declared using the list built-in function:• Example:

set a [list 1 2 3 4]• The same can also be written as:

set a { 1 2 3 4 }• Retrieving an element within a list: lindex• lindex is 0-based, just like C

set b [lindex $a 3] ;# b has the value 4

set b [lindex $a end] ;# b will have value 4

set b [lindex $b 0] ;# b will have the value 1

36

List Operations

• concat : Concatenate two lists

set b [concat [list 1 2 3 4] [list 5 6 7]]

b will be { 1 2 3 4 5 6 7 }• lappend : Append value at the end of list – By Name

set a [list 1 2 3]

lappend a 4– Now a will be { 1 2 3 4 }– Note: lappend takes a list Name, not a list

• llength : Return the length of a list– llength [list 1 2 3] will be 3– llength [list] will be 0

• Many others… Read any Tcl book

37

Looping over lists - foreach

• Because looping over lists is a common operation, there is a built-in construct to make this easier

• By Example:

proc PrintList { inputList } {

foreach element $inputList {

puts $element

}

}• element takes on the value of each of the members

of the list*

*In Perl, element would be a reference to the member of the list. Any update to element would change the list. Not so in Tcl…

38


• Write a procedure reverseList that reverses a given list, and returns the reversed list.

39

Solution to PA # 3

40

Arrays

• All Arrays in Tcl are one-dimensional and associative – called a hash in other PLs

• We will see later how more dimensions can be “induced” over single dimensional arrays

• By example:

set myArray(0) 1

set myArray(1) 2

set myArray(2) 3

puts $myArray(0)

puts $myArray(1)

41

Arrays, contd.

• The “keys” to arrays need not be natural numbers• Arrays are used mainly to achieve key/data pairs• Example:

set address(“sunil”) “cupertino”

set address(“dan”) “berkeley”

set address(“praveen”) “san jose”

puts $address(“sunil”)

puts $address(“dan”)

…

42

Arrays – Multiple Dimensions

• Since arrays are associative, we can form a “composite key” by choosing a delimiter like ,

• Example:

set addresses(“sunil”,”street”) “meteor”

set addresses(“sunil”,”city”) “cupertino”

set addresses(“dan”,”street”) “hearst”

set addresses(“dan”,”city”) “berkeley”

puts $addresses(“dan”,”street”)

…

43

Arrays – Retrieving all keys

• array names arrayName will return a list of all keys associated with the array

• By Example:set myArray(0) 1

set myArray(“chocolate”) “chip”

set keyList [array names myArray] ;# keyList will be { 0 chocolate }

• Common iteration technique:foreach key [array names myArray] {

set value $myArray($key)

# Now $key and $value might be used for any action necessary…

}

44

Arrays – One Major Drawback

• Arrays cannot be passed by value to and from other procedures!

• Why?– Internally, each element of an array is just another

variable. Arrays happen to be a special case where all the variables start with the same letters, and contain a “(“ after the first name

• Give up?– Don’t! There is pass-by-reference; we will get to it later…

45

Regular Expressions

• Regular Expressions are used to match patterns• Invocation: regexp <regexp> $var• Returns 1 if $var matches <regexp>, 0 otherwise• Simple Sub-string Examples:

regexp {mcd} “mcdata” ;# 1

regexp {cda} “mcdata” ;# 1

regexp {mcdata} “mcdata” ;# 1

regexp {macdata} “mcdata” ;# 0

regexp {mc data} “mcdata” ;# 0

46

Regular Expressions, contd.

• Grouping– Subsections of regular expressions are grouped using

parentheses ()

– Ex:• regexp {(mcdata)(colorado)} mcdatacolorado ;# 1

• Multiple matching choice: Enclose in [ ]– regexp {[0123456789]} $var

– Will match any one of 0-9 in $var• regexp {[0123456789]} mcdata7 ;# 1• regexp {[0123456789]} mcdata ;# 0

47


• Range Specification– Within [ ], ranges can be used instead of verbatim

– Ex: regexp {[A-Zu-z3-9]} …

• Quick Class Exercise– Write a regular expression to match standard California

Vehicle Number Plates

– The first digit is a nonzero number, followed by 3 upper-case letters, followed by 3 more numbers

regexp {[1-9][A-Z][A-Z][A-Z][0-9][0-9][0-9]}

48


• Matching beginning and end of input– Robust regular expressions are written to take everything

into account, from beginning to the end of input– Not all situations call for such strict regular expressions,

but it useful to know how to write them nevertheless• ^ matches beginning of string• $ matches end of string• {^mcdata$} will ONLY match “mcdata”• {^[0-9]$} will ONLY match a string containing

exactly 1 digit• {^[0-9]} will ONLY match a string starting with a

digit

49


• Matching “any” character– “any” character is matched with “.”

• {^.[0-9]} will match any string with a digit as the second character

• How to match . or [ or ^ or $, etc.– All regular expression characters can be escaped using \

– {^\.\[$} will match ONLY the string “.[“

– A backslash may itself be escaped

– {\\} will match any string that contains a \

50


• Group Modifiers• *

– Denotes at least 0 of• Ex: {[0-9]*} means match if at least 0 digits• Note: will always also match empty string!

• +– Denotes at least 1 of

• Ex: {[0-9]+} means match if at least 1 digit• Will never match empty string

• ?– Denotes either exactly 0 or exactly 1 of (or “optionally”)

• Ex: {^[0-9]?} means match if first digit is present or not

51


• Update PA#2 to validate input.• I will try to break your program!

My Solution:

{^-?([0-9])+(\.([0-9])+)?$}

52

Passing by Reference

• Tcl has a very powerful call-by-reference model– More appropriately called: call-by-name

• Illustrate by example: Suppose I want to write incrproc Incr { varName { by 1 } } {

upvar $varName varset var [expr $var+$by]return $var

}set i 1Incr i ;# Will have the desired effect

• Notice that we are passing the variable to the procedure “by name”

53

Passing by Reference, contd.

• upvar aliases a variable in the current scope to a variable in another scope, usually the calling scope.

• upvar ?level=1 varName localVar• When localVar is updated, the variable with the

name “varName” is updated in the scope specified.• upvar is a very difficult concept to grasp completely

the first time around, so if you plan on passing by reference, read Welch’s book.

Practice Part 2 Introduction to ITcl

55

Itcl = [incr Tcl]

• Itcl is the Object-oriented version of Tcl• Itcl does for Tcl what C++ does for C• Provides manageability for large-scale projects• Supports multiple-inheritance, except in a

degenerate case• Itcl manifests itself as a package that can be

brought into the standard Tcl environment– Simply insert:

package require Itclnamespace import -force itcl::*

56

Objects

• Objects = Data + Procedures*• Traditionally, data structures were separate from

the procedures that acted upon them• The Object-Oriented Paradigm (OOP) is the drive to

combine the data and procedures into one package.

*We owe this succinct definition to Grady Booch in his book: Object Oriented design with applications

Data Procedures

Object

57

Defining Objects - Classes

class Car {

variable year

variable color

variable doors

variable cylinders

method Drive { } {

puts “I am driving!!”

}

}

The object “type” is Car

year is an “attribute”

Keep track of color

Same with doors

Number of cylinders

A Car can “do” this: It can Drive

58

Declaring Objects - constructor

• In order to be able to declare objects, we need to define a special method within the class that is called when we “create” or “construct” the objectclass Car {

constructor { inYear inColor inDoors inCylinders } {set year $inYearset color $inColorset doors $inDoorsset cylinders $inCylinders

} …}

• Here, we populate the variables inside the class with what we get during “creation”

59

Declaring Objects - Instantiation

• We can proceed to declare a Car by:Car sunilCar 1997 Biege 4 4

• We can also perform the Drive operation on the object just declared by:sunilCar Drive

60

Inheritance

• What happens when we realize that we have more attributes that are specific to say, a Toyota?

• Declare a new class and rewrite all the basic attributes for that class over again?

• Yes and No!!• Write a new class, but “use” the attributes already

given to us by the already existing class.– We can do this because a Toyota is but a special case of

a Car. – “A Toyota IS A Car”

– This IS A relationship drives inheritance!

61

Base/Member Initialization

Class Toyota {

inherit Car ;

variable exhaustType

constructor {inYear inColor inDoors inCylinders inExType } {

Car::constructor $inYear $inColor $inDoors $inCylinders } {

set exhaustType $inExType

}

}

Toyota myCar 1997 Beige 4 4 RearSingle

myCar Drive

• Can a Toyota Drive? We did not teach it!

•Sure it can! It inherited Car

62

Questions ?

• The dumbest question is the one not asked…- Aristotle

Thanks !

Presented By: Sunil Ramesh Tcl Programming: Theory & Practice.

Documents

Transcript of Presented By: Sunil Ramesh Tcl Programming: Theory & Practice.