Programming Languages

1
Programming Languages

• COMP 321
• Spring 2012
• Practicum 1

2
Practicum 1

• expressions
• lists (car, cdr, cons, list)
• let expressions
• lambda expressions
• proper list, single variable, improper list
• top level definitions (define)
• conditional expressions

3
Practicum 1

• reference
• The Scheme Programming Language, 3rd ed., R. Kent
  Dybvig, MIT Press, chapter 2, sections 1-7
• How to Design Programs, online book
  http//www.htdp.org/2003-09-26/Book/ chaps 1 2

4
Practicum 1

• Expressions prefix
• ( 3 5)
• ( ( (/ 4 2) ( 3 7) 3)

5
Practicum 1

• Lists
• The principle data structure in Scheme
• car and cdr operators to take apart lists
• cons and append operators to put together lists
• Lists always contain an empty pair () as their
  last item (like the \0 char in a C string)

6
Practicum 1

• Lists
• (a 4 z)
• ( 3 4)
• (car (a b c))
• (cdr (a b c))
• (car ((1 2)(3 4) 5))
• (cdr ((1 2)(3 4) 5))

7
Practicum 1

• lists
• car and cdr can be combined
• (cadr (a b c))
• (caddr (a b c))
• (cdar ((1 2)(3 4) 5))
• (cdadr ((1 2)(3 4) 5))

8
Practicum 1

• lists
• (cons a ())
• (cons a (b c))
• (cons a (cons b (cons c ())))
• (cons (a b) (c d))

9
Practicum 1

• lists
• (car (cons a (b c)))
• (cdr (cons a (b c)))
• (cons (car (a b c))
• (cdr (d e f)))
• (cons (car (a b c))
• (cdr (a b c)))

10
Practicum 1

• inproper lists (does not end in an empty pair)
• (cons a b)
• (cdr (a . b))
• (cons a (b . c))
• (a . b . c)

11
Practicum 1

• Lists creating lists
• (list a b c)
• (list a)
• (list)

12
Practicum 1

• Lists combining lists
• (append (a b) (d c))
• (append a (b c)) what does this do?
• (append (a b) c) why do you get
• this answer?

13
Practicum 1

• variables and let expressions
• Creating local environments
• Binding names to values
• (let ((x 2))
• ( x 3))
• (let ((x 2) (y 3))
• ( x y))

red parenthesis define the scope of the local
environment
blue parenthesis define the binding pairs
14
Practicum 1

• variables and let expressions
• (let ((a ( 4 4)))
• ( a a))
• Now we start to see the power of Scheme!
• (let ( (f ) (x 2)(y 3) )
• (f x y))

Scheme has complete orthagonality!
15
Practicum 1

• Nesting environments
• (let ((x 1))
• (let ((new-x ( x 1)))
• ( new-x new-x)) )

16
Practicum 1

• Nesting environments
• Local environment create scope!
• (let ((x 1))
• (let ((x ( x 1)))
• ( x x)
• )
• )