cs 330 programming languages 11 / 15 / 2007 instructor: michael eckmann

CS 330Programming Languages

11 / 15 / 2007

Instructor: Michael Eckmann

Michael Eckmann - Skidmore College - CS 330 - Fall 2007

Today’s Topics• Questions / comments?• Chapter 6

– Pointers• solutions to memory leaks

• Functional programming – Scheme


Pointers• Solutions to memory leakage problem

– Reference counters• Store a reference counter for each memory cell whose value is

the number of pointers currently pointing to it.• When pointers change value or are destroyed, the counter is

decremented. It is also checked to see if it is zero. If it is, then the memory can be reclaimed.

– Any drawbacks to this method?


Pointers• Solutions to memory leakage problem

– Reference counters (continued)• Any drawbacks to this method?

– Space– Time– Circular references


Pointers• Solutions to memory leakage problem (continued)

– Garbage collection• Every heap cell needs a bit or possibly a field as a flag to

indicate whether or not it contains garbage.• When want to reclaim memory the garbage collector will

– 1. Set all the flags as “containing garbage”– 2. Go through the program and determine all the memory

that is being pointed to --- and changes the flags for those cells as “not containing garbage”

– 3. All the ones still marked as “containing garbage” are reclaimed.

– All preceding discussion assumed that the heap cells were all the same size. Major difficulties arise when non-uniform size cells are used. Why?


Functional Programming• Read the paper “Why Functional Programming Matters” (1984)

by John Hughes found here:• http://www.md.chalmers.se/~rjmh/Papers/whyfp.html• (contains a link to a pdf of the paper.)• Please read before Tuesday the 20th so we can discuss it together

as a class along with the first 5 sections of chapter 15.• Suggestions on reading this paper (or any technical paper)

– If you don't understand something after a little thought, that's o.k. continue on to get the gist of the whole thing then, come back to what wasn't understood.

http://www.md.chalmers.se/~rjmh/Papers/whyfp.html


Functional Programming• Let's look at a webpage that contains some good bullet points

regarding functional programming in general.


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

– small, simple syntax --- consistent syntax– simple semantics


Scheme• Note that the textbook uses capital letters

for the keywords in the language and we will do the same on the slides, but when we code using DrScheme this is not necessary.


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.


Scheme• Scheme

– Primitive functionsArithmetic: +, -, *, /, 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.)


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.


Scheme• Scheme

– Primitive functionsQUOTE -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. 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))


Scheme• Scheme

– If you wanted a list with the symbols / 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))


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


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)


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


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)


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


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?


Functional Programming• Scheme

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

What does this function do?

How is it called?



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

What does this function do?CDR returns the list remaining after first element

removedCAR returns the first element of a listso, a CAR of a CDR is the second element

How is it called?(mystery '(W X Y Z))



– Numeric “predicate” functions return a Boolean – #T or #F– =, <>, >, <, >=, <=, EVEN?, ODD?, ZERO?– our textbook says that empty list () is considered #F

and any non-empty list is considered #T (this differs from other Scheme documentation which says #F is false, and all other objects are treated as true (e.g. (), “false”, 3, nil are all considered true)

– Symbolic Atoms and Lists “predicate” functions– EQ?, LIST?, NULL?, EQUAL?



– 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.



– =, <>, >, <, >=, <=, EVEN?, ODD?, ZERO?– EQ?, LIST?, NULL?, EQUAL?

– Examples:(= n 0)(NULL? somelist)(EQ? 'A (CAR somelist))(LIST? '() )(EQUAL? '(a b z) '(a b z))



– and, or, not

– Examples:(or (= n 0) (= n 1))(not (= n 0))(and (eq? 'a (car lis)) (= y 1))



– input / output– (read)– (display "Hello World")– (write "Hello World")– (define x 5)– (write x)



– 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 } )

)


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++?


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.)


Scheme• Recursion

– Scheme programmers typically use recursion in Scheme whenever possible


Functional Programming• Example of a function containing an IF

(DEFINE (factorial n)(IF (= n 0)

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

))


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” )

))


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 #F.


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) #F)((EQ? atm (CAR lis)) #T)(ELSE ( member atm (CDR lis))))

)


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?)


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.

cs 330 programming languages 11 / 15 / 2007 instructor: michael eckmann

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