cs 330 programming languages 11 / 21 / 2006 instructor: michael eckmann

CS 330Programming Languages

11 / 21 / 2006

Instructor: Michael Eckmann

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Today’s Topics• Questions / comments?

• Scheme

– Note: slides and text book show all Scheme reserved words in caps, but they need to be lowercase (at least in the environment we'll be using)

– search the internet for an online book called "Teach yourself Scheme in Fixnum days" and focus on the following (very short) chapters:

• Chapter 2: Data types

• including: booleans, numbers, symbols, strings, lists

• Chapter 3: Forms

• Chapter 4: conditionals

• especially if, cond, and, or

• Chapter 5

• especially let and set!

• Chapter 6: recursion

• Chapter 10: alists and tables

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Today’s Topics• I recommend downloading and installing DrScheme on your own machines.

DrScheme is available on the Windows machines.

• It is available for Linux but I never asked Lisa to install it on the machines in the linux lab so it's not available there.

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Scheme• Scheme

– Static scoping exclusively– Small size– Functions are first-class citizens

• Can be values of expressions

• Can be elements of lists

• Can be assigned to variables

• Can be passed as parameters

– “simple” syntax --- more correct to say consistent?

– “simple” semantics

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Scheme• Scheme

– Expressions are evaluated by the function EVAL

– Literals evaluate to themselves– Function calls are evaluated by

• First evaluate all the parameter expressions

• Then evaluate the function after the values of the parameters are known

• The value of the last expression in the body is the value of the function (sound familiar?)

• All but the last should be familiar to imperative programmers.

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Scheme• Scheme

– Primitive functions

Arithmetic: +, -, *, /, ABS, SQRT, REMAINDER, MIN, MAX

e.g., (+ 5 2) yields 7

what would (- 24 (* 5 3)) yield?

• If * is given no parameters, it returns 1 (multiplicative identity.)

• If + is given no parameters, it returns 0 (additive identity.)

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Scheme• Scheme

– Primitive functions• If – is given more than two parameters, it acts as if

the second through the last are summed and this sum is subtracted from the first.

• If / is given more than two parameters, it acts as if the second through the last are multiplied together and this product is divided into the first.

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Scheme• Primitive functions

QUOTE -takes one parameter; returns the parameter without evaluation

• QUOTE is required because the Scheme interpreter, named EVAL, always evaluates parameters to function applications before applying the function, but QUOTE is used to avoid parameter evaluation when it is not appropriate

• QUOTE can be abbreviated with the apostrophe prefix operator

e.g., '(A B) is equivalent to (QUOTE (A B))

In this case, EVAL will not try to "call" function A

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Scheme• Scheme

– If you wanted a list with the symobls / 8 4 in that order and you did this:

(/ 8 4)

• Scheme would evaluate that as a function and the result will be 2

– So, to have the list not be evaluated, use the QUOTE:

'(/ 8 4)

• Scheme gives us the list with those three elements; no evaluation occurs.

• alternatively (QUOTE (/ 8 4))

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Scheme• Scheme

– (DEFINE hello 5.6)– If you wanted the symbol hello instead of the value in

some constant named hello

'hello

vs.

hello

– The first one gives us hello

– The second one evaluates to 5.6

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Scheme• Scheme

CAR takes a list parameter; returns the first element of that list

e.g., (CAR '(A B C)) yields A

(CAR '((A B) C D)) yields (A B)

CDR takes a list parameter; returns the list after removing its first element

e.g., (CDR '(A B C)) yields (B C)

(CDR '((A B) C D)) yields (C D)

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Scheme• Scheme

CONS takes two parameters, the first of which can be either an atom or a list and the second of which is a list; returns a new list that includes the first parameter as its first element and the second parameter as the remainder of its result

e.g., (CONS 'A '(B C)) returns (A B C)

(CONS '(A B) '(B C)) returns ((A B) B C)

LIST - takes any number of parameters; returns a list with the parameters as elements

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Scheme• Scheme

– DEFINE – used to create programmer-defined functions / or bind names to values of expressions

– When names are bound to values of expressions they are NOT variables, instead they are named constants.

– Either two atoms as parms, or two lists.

– Example when two atoms are given (creates a named constant):

(DEFINE games_in_season 162)

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Scheme• Scheme

– DEFINE – used to create programmer-defined functions / or bind names to values of expressions

– The form when two lists are given

• binds the expressions collectively as a function to a function name and its parameters:

(DEFINE (func_name parameters)

expression {expression}

)

– Examples on next slide

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Scheme• Scheme

– Examples:(DEFINE (square x) (* x x))

(DEFINE (hypotenuse side1 side1)

(SQRT (+ (square side1) (square side2))) )

What would these look like as imperative language functions?

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Functional Programming• Scheme

– Examples:(DEFINE (mystery m) (CAR (CDR m)))

What does this function do?

How is it called?

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Functional Programming• Scheme

– Examples:(DEFINE (mystery m) (CAR (CDR m)))

What does this function do?

CDR returns the list remaining after first element removed

CAR returns the first element of a list

so, a CAR of a CDR is the second element

How is it called?

(mystery '(W X Y Z))

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Functional Programming• Scheme

– Numeric “predicate” functions return a Boolean

– #T or #F

– =, <>, >, <, >=, <=, EVEN?, ODD?, ZERO?

– Empty list () is considered #F

– Any non-empty list is considered #T

– Symbolic Atoms and Lists “predicate” functions

– EQ?, LIST?, NULL?, EQUAL?

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Functional Programming• Scheme

– There is a difference between

• =

• eq?

• equal?

– = is used for numeric comparison

– eq? is used for symbol/atom comparison

– equal? is used for symbol/atom or list comparison

• There's more to it than what I say above ---- I'll photocopy some examples and their results for eq?, equal? and eqv? for your edification.

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Functional Programming• Scheme

– =, <>, >, <, >=, <=, EVEN?, ODD?, ZERO?

– EQ?, LIST?, NULL?, EQUAL?

– Examples:

(= n 0)

(NULL? somelist)

(EQ? 'A (CAR somelist))

(LIST? '() )

(EQUAL? '(a b z) '(a b z))

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Functional Programming• Scheme

– and, or, not

– Examples:

(or (= n 0) (= n 1))

(not (= n 0))

(and (eq? 'a (car lis)) (= y 1))

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Functional Programming• Scheme

– input / output

– (read)

– (display "Hello World")

– (write "Hello World")

– (define x 5)

– (write x)

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Functional Programming• Scheme

– IF takes three parms

(IF predicate then_expression else_expression )

– COND – is like switch / case statements

(COND

( predicate1 expression { expression } )

( predicate2 expression { expression } )

...

( predicaten expression { expression } )

( ELSE expression { expression } )

)

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Functional Programming• COND

– Predicates are evaluated in order until one is #T

– Then the expressions that follow that first #T predicate are evaluated and the value of the last one evaluated is the returned value.

– If no predicate is #T, then COND returns (), the empty list.

– ELSE at the end acts like a default if none of the other predicates are true it will evaluate that one.

– Can you see any differences or similarities to switch statements in Java and C++?

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Functional Programming• COND

– Can you see any differences or similarities to switch statements in Java and C++?

– In COND, there are implied “breaks” after each predicate (case.)

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Scheme• Recursion

– Scheme programmers typically use recursion in Scheme whenever possible

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Functional Programming• Example of a function containing an IF

(DEFINE (factorial n)

(IF (= n 0)

1

(* n (factorial (- n 1)))

)

)

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Functional Programming• Example of a function containing a COND

(DEFINE (compare x y)

(COND

( (> x y) ( DISPLAY “x is > y” ))

( (< x y) ( DISPLAY “x is < y” ))

( ELSE ( DISPLAY “x is = y” )

)

)

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Functional Programming• Want to try to write a function that determines whether an

atom is in a particular list? e.g. (member 'B '(A B C)) should return #T and (member 'X '(A B C)) should return () (which is #F.)

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Functional Programming• Want to try to write a function that determines whether an

atom is in a particular list? e.g. (member 'B '(A B C)) should return #T and (member 'X '(A B C)) should return () (which is #F.)

(DEFINE (member atm lis)

(COND

((NULL? lis) '())

((EQ? atm (CAR lis)) #T)

(ELSE ( member atm (CDR lis)))

)

)

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Schemewhen writing functions dealing with a list lis, if you want to go

through the elements one at a time, use (car lis) to get the first

and recurse on (cdr lis) also, you'll need to tell your function

when to stop, that is, when the list is null, so (null? lis) should

be the first thing you check.

In general recursive functions need to

1st check the base case (the case that will be true when the recursion should stop)

and in the recursive step(s) make sure that the function is working towards the base case (What do I mean here?)

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Scheme• Let's run DrScheme and try some things out. While we're

coding, as I introduce any new built-in functions or anything we haven't seen before I'll explain what they are.

• I'll then save the code and post it.