Comparative Programming Languages

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Comparative Programming Languages

• Language Comparison Scheme, Smalltalk, Python,
  Ruby, Perl, Prolog, ML, C/STL, Java

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Introduction to Scheme

• A mid-1970s dialect of LISP, designed to be a
  cleaner, more modern, and simpler version than
  the contemporary dialects of LISP
• Uses only static scoping
• Functions are first-class entities
• They can be the values of expressions and
  elements of lists
• They can be assigned to variables and passed as
  parameters

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Scheme

• (define (gcd u v)
• (if ( v 0)
• u
• (gcd v (remainder u v))
• )
• )
• Once defined in the interpreter
• (gcd 25 10)
• 5

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Scheme atoms

• Constants
• numbers, strings, T True,
• Identifier names
• Function/operator names
• pre-defined user defined

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Scheme Expression vs. C

• In Scheme
• ( 3 ( 4 5 ))
• (and ( a b)(not ( a 0)))
• (gcd 10 35)

• In C
• 3 4 5
• (a b) (a ! 0)
• gcd(10,35)

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Smalltalk Expressions vs. C

• In Smalltalk
• 3 4 5
• ( 35, 4 mess. sent to 3, then 5 mess.)
• (a b) (a 0)
• 10 gcd 35
• gcd 35 mess. sent to 10

• In C
• 3 4 5
• (a b) (a ! 0)
• gcd(10,35)

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Ruby/Python Expressions vs. C(very similar)

• In Ruby/Python
• 3 4 5
• (a b) (a ! 0)
• gcd(10,35)

• In C
• 3 4 5
• (a b) (a ! 0)
• gcd(10,35)

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Ruby Expressions vs. CClass version

• In Ruby
• 3.plus(4).times(5)
• ( 35)
• (a b) (a ! 0)
• 10.gcd(35)

• In C
• 3 4 5
• (a b) (a ! 0)
• gcd(10,35)

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Evaluation Rules for Scheme Expressions

1. Constant atoms (numbers, strings) evaluate to
   themselves
2. Identifiers are looked up in the current
   environment and replaced by the value found there
   (using dynamically maintained symbol table)
3. A list is evaluated by recursively evaluating
   each element in the list as an expression the
   first expression must evaluate to a function.
   The function is applied to the evaluated values
   of the rest of the list.

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Smalltalk Conditionals

• If statement
• (v 0) ifTrue u.
• v gcd u \\ v)
• (a 0) ifTrue 0.
• 1/a
• (or use ifFalse for the
• else portion above)

• Cond statement
• None in Smalltalk(?)
• http//c2.com/cgi/wiki?CaseStatementsConsideredHar
  mful
• Case statements considered
• harmful??

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Ruby Conditionals

• If statement
• if v 0
• return u
• else
• return
• gcd(v, u v)
• end
• if a 0
• return 0
• else return 1/a
• end
• Use elsif for else if

• Cond statement
• case a
• when 0
• 0
• when 1
• 1
• else
• 1/a
• end

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Python Conditionals

• If statement
• if v 0
• return u
• else
• return gcd(v, u v)
• if a 0
• return 0
• else
• return 1/a
• Blocks are indented.
• Use elif for else if

• Cond statement
• Use
• if
• elif
• elif
• ...

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Example of COND

• (DEFINE (compare x y)
• (COND
• ((gt x y) (DISPLAY x is greater than y))
• ((lt x y) (DISPLAY y is greater than x))
• (ELSE (DISPLAY x and y are equal))
• )
• )

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Example of COND

• compare y
• (self gt y) ifTrue
• Transcript show x is greater than y
• printString.
• Transcript show cr.
• ifFalse
• (self lt y) ifTrue
• Transcript show y is greater than x
• printString. Transcript show cr.
• ifFalse
• Transcript show x and y are equal
• printString. Transcript show cr..

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Example of COND Ruby class vers.

• class Integer
• def compare(y)
• if self gt y
• print x is greater than y\n
• elsif self lt y
• print y is greater than x\n
• else
• print x and y are equal\n
• end
• Syntax 4 compare(5)

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Example of COND Ruby non-class vers.

• def compare(x, y)
• if x gt y
• print x is greater than y\n
• elsif x lt y
• print y is greater than x\n
• else
• print x and y are equal\n
• end
• Syntax compare(4,5)

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Example of COND Python

• def compare(x, y)
• if x gt y
• print x is greater than y
• elif x lt y
• print y is greater than x
• else
• print x and y are equal\n
• Syntax compare(4,5)
• (Also blocks are indented)

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Predicate Functions

• 1. EQ? takes two symbolic parameters it returns
  T if both parameters are atoms and the two are
  the same
• e.g., (EQ? 'A 'A) yields T
• (EQ? 'A '(A B)) yields ()
• Note that if EQ? is called with list parameters,
  the result is not reliable
• EQ? does not work for numeric atoms (use )

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Predicate Functions

• 2. LIST? takes one parameter it returns T if
  the parameter is a list otherwise()
• 3. NULL? takes one parameter it returns T if
  the parameter is the empty list otherwise()
• Note that NULL? returns T if the
  parameter is()
• 4. Numeric Predicate Functions
• , ltgt, gt, lt, gt, lt, EVEN?, ODD?, ZERO?,
  NEGATIVE?

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Predicate Functions - Python

• 1. returns True if both parameters are the
  same
• e.g., 'A' 'A' yields True
• 'A' 'A B' False
• 2. ! returns True if both parameters are not the
  same.
• e.g., 'A' ! 'A' yields False
• 'A' ! 'A B' True

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Predicate Functions - Python

• 3. type() returns the type of an object.
• type(1,2,3) list returns True
• 4. len() returns True if the parameter is the
  empty list otherwise the length of the list
  
• 5. Numeric Predicate Functions
• , !, gt, lt, gt, lt

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let function

• Allows values to be given temporary names within
  an expression
• (let ((a 2 ) (b 3)) ( a b))
• 5
• Semantics Evaluate all expressions, then bind
  the values to the names evaluate the body

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List Comprehensions - Python

• provide a concise way to create lists without
  resorting to use of map(), filter() and/or
  lambda.
• gtgtgt freshfruit ' banana', ' loganberry ',
  'passion fruit '
• gtgtgt weapon.strip() for weapon in freshfruit
• 'banana', 'loganberry', 'passion fruit'

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List Comprehensions - Python

• gtgtgt vec 2, 4, 6
• gtgtgt 3x for x in vec
• 6, 12, 18
• gtgtgt 3x for x in vec if x gt 3
• 12, 18
• gtgtgt 3x for x in vec if x lt 2
• gtgtgt x,x2 for x in vec
• 2, 4, 4, 16, 6, 36

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for statements - Python

• gtgtgt Measure some strings
• ... a 'cat', 'window', 'defenestrate'
• gtgtgt for x in a
• ... print x, len(x)
• cat 3
• window 6
• defenestrate 12

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for statements - Python

• gtgtgt Measure some strings
• ... a 'cat', 'window', 'defenestrate'
• gtgtgt for x in a
• ... print x, len(x)
• cat 3
• window 6
• defenestrate 12

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for statements - Python

• gtgtgt for x in a make a slice copy of the
  entire list
• ... if len(x) gt 6 a.insert(0, x)
• ...
• gtgtgt a
• 'defenestrate', 'cat', 'window', 'defenestrate'

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range() function - Python

• gtgtgt range(10)
• 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
• gtgtgt range(5, 10)
• 5, 6, 7, 8, 9
• gtgtgt range(0, 10, 3)
• 0, 3, 6, 9
• gtgtgt range(-10, -100, -30)
• -10, -40, -70

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range() function - Python

• gtgtgt a 'Mary', 'had', 'a', 'little', 'lamb'
• gtgtgt for i in range(len(a))
• ... print i, ai
• 0 Mary
• 1 had
• 2 a
• 3 little
• 4 lamb

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More looping - Python

• gtgtgt knights 'gallahad' 'the pure', 'robin'
  'the brave'
• gtgtgt for k, v in knights.iteritems()
• ... print k, v
• ...
• gallahad the pure
• robin the brave

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More looping - Python

• gtgtgt for i, v in enumerate('tic', 'tac', 'toe')
• ... print i, v
• ...
• 0 tic
• 1 tac
• 2 toe

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More looping - Python

• gtgtgt for i in reversed(xrange(1,10,2))
• ... print i
• ...
• 9
• 7
• 5
• 3
• 1

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More looping - Python

• gtgtgt basket 'apple', 'orange', 'apple', 'pear',
  'orange', 'banana'
• gtgtgt for f in sorted(set(basket))
• ... print f
• apple
• banana
• orange
• pear

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Quote () function - Lisp/Scheme

• A list that is preceeded by QUOTE or a quote mark
  () is NOT evaluated.
• QUOTE is required because the Scheme interpreter,
  named EVAL, always evaluates parameters to
  function applications before applying the
  function. QUOTE is used to avoid parameter
  evaluation when it is not appropriate
• QUOTE can be abbreviated with the apostrophe
  prefix operator
• Can be used to provide function arguments
• (myfunct (a b) (c d))

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Output functions - Lisp/Scheme

• Output Utility Functions
• (DISPLAY expression)
• (NEWLINE)

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Output functions - Python

• print
• print "Hello, World!"
• print "(33 2) / 5 11.5 ",(33 2) / 5
  11.5
• print "Halt!"
• s raw_input("Who Goes there? ")
• print "You may pass,", s

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define function - Scheme

• Form 1 Bind a symbol to a expression
• (define a 2)
• (define emptylist ( ))
• (define pi 3.141593)

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assignment and def functions - Python

• Form 1 Bind a symbol to a expression
• a 2
• emptylist
• pi 3.141593

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define function - Smalltalk

• Form 1 Bind a symbol to a expression
• a 2
• mylist OrderedCollection new 0
• mylist addAll (1 2 3 a b)
• pi 3.141593

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define function - Ruby/Python

• Form 1 Bind a symbol to a expression
• a 2
• emptylist
• mylist 1,2,3,'a'
• pi 3.141593

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