Tcl Programming: Theory & Practice

Tcl Programming:Theory & Practice Presented By: Sunil Ramesh

Origins of Tcl ⋋-Calculus - 1930 LISP - 1950 TCL - 1990 Alonzo Church, Princeton John McCarthy, AI Lab, Stanford C - 1970 John Ousterhout, UC Berkeley Dennis Ritchie, Bell Labs

Theory Part 1Lambda Calculus Anything which can be computed can be computed using the lambda calculus - Church’s Thesis

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.

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

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.

Pairs • Pair – a construct with exactly 2 elements car cdr • Constructing a pair • (define a (cons 1 2)) will yield a 1 2 • Referencing “left” and “right” elements • (car a) will yield 1 • (cdr a) will yield 2

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))

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: a 2 3 4 1 • (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)

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… a 2 3 4 1

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

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

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

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

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

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.

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

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!

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”

Programming Assignment # 1 • Write a program that asks the user for two numbers and prints the average of the two numbers. • Sample Output:

Solution to PA # 1

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

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

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” }

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!

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 }

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” }

Programming Assignment # 2 • Write a program that keeps asking user for numbers till the blank string is seen, and prints the average of the numbers: • Sample Output:

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

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 call Add 1 ;# Will return 2 Add 1 2 ;# Will return 3 Add 1 –1 ;# Will return 0

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 }

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

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

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…

Programming Assignment # 3 • Write a procedure reverseList that reverses a given list, and returns the reversed list.

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)

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”) …

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”) …

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… }

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…

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

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

Regular Expressions, contd. • 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]}

Regular Expressions, contd. • 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

Regular Expressions, contd. • 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 \

Regular Expressions, contd. • 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

Tcl Programming: Theory & Practice