CS 330 Programming Languages 11 / 13 / 2007 Instructor: Michael Eckmann.

CS 330Programming Languages

11 / 13 / 2007

Instructor: Michael Eckmann

Michael Eckmann - Skidmore College - CS 330 - Fall 2007

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

– Data types• Records• Pointers

• Functional programming – Scheme


Records• There are some similarities and differences between records

and arrays.

– Records are used when the data is of different types

– Arrays are used when the data is all the same type

– Record elements are referenced with static field names as “subscripts.”

– Array elements are referenced with dynamic subscripts.

• Implementation

– In the descriptor, the address of the record is stored as well as the name, type and offset address of each field.

Pointers• Pointer types

– Store memory addresses or nil (no address.)

• For indirect addressing

• For dynamic storage management

– Can access data at a particular memory address in the heap

• Provide a way to have data structures that shrink and grow during execution.

• For pass-by-reference into a function – instead of a copy, it passes just a reference/address– allows the variable's value to change inside the function

• For generic programming pointer to a void allows casting to any other kind of pointer


Pointers• Using pointers

– Pointer reference (the address)

– Indirect reference (aka dereference) gives us what data is in the memory address stored in the pointer

• Dereferencing is usually explicit (as in C, C++ with the * operator) but it is sometimes implicit (ala Ada)

• Dereferencing example

– variable ptr is a pointer and contains the memory address 7080

– Memory address 7080 contains the integer value 206

– j is a variable of type integer.


Pointers int j;

int *ptr;// code missing

// to assign val to ptr

j = *ptr;// line above assigns

// value in address

// that ptr points to


Pointers• Accessing fields of records via pointers to records

– C++ (pointer to a struct)

– (*ptr).field_name (* dereferences, . accesses field )

– ptr -> field_name ( -> does both )• Because this is a common operation, there's a

shorthand

– Ada

– ptr.field_name • (because of implicit dereferencing acts just like -> in

C++)


Pointers• Problems

– Dangling pointer (or dangling reference in those languages that don't have pointers)

– Memory leakage

– Did anyone read about these in the text or know what they are from prior knowledge?



– Dangling pointer (or dangling reference in those languages that don't have pointers)

• When a pointer contains the address of a variable that was deallocated already

• example:– pointer p1 points to some heap dynamic variable– pointer p2 = p1 // make p2 also point there– deallocate p1's heap dynamic variable– // now, p2 is a dangling pointer

– If programmer cannot explicitly deallocate heap dynamic variables then there will be no possibility of dangling pointers.



– Memory leakage

• When a pointer's value changes (that is a different address is stored in it) and the address it was pointing to didn't have it's data deallocated

• Garbage collection (frees deallocated memory for use)– C++ (done explicitly by programmer)– Java (done automagically)


Pointers• Let's look at these problems in Ada, C/C++, Fortran 95, Java

• Ada

– Pointers are of type access

– Handles the dangling pointer problem by design

• Implicit deallocation of memory at end of scope is done

• Also allows explicit deallocation by the programmer via the deallocator: Unchecked_Deallocation

(which can cause the dangling pointer problem)

– Memory leakage

• Nothing by design to prevent this



• C/C++

– Both the dangling pointer problem and memory leakage exist in these languages

– Can do pointer arithmetic

– Can have pointers to any type and pointers to functions

– One use of pointers is to pass variables by reference (so they may be changed within the function) --- contrast with pass by value, where the variable's value is copied to new temporary space in the function.

– & is used to get the address of a variable

– Examples of this stuff ...


Pointers• C/C++

int *ptr;

int count=30, init=70;

ptr = &init;

count = *ptr;

What does this code do?


Pointers• C/C++

int *ptr; // declare a pointer to an int

int count=30, init=70;

ptr = &init; // store the address of init in ptr

count = *ptr;

// dereference ptr and store what's in the address

// that ptr is pointing to

// so what value does count have?


Pointers• C/C++ (pointer arithmetic)

int *ptr; // declare a pointer to an int

int grades[ 100 ];

ptr = grades; // grades is a “pointer” to grades[0] and now

// so is ptr

*(ptr + 1)

// here the + is pointer addition, so it actually adds

// the size of one int to ptr so that it dereferences

// (ptr + 1) to grades[1]

ptr[index]; // can use ptr like an array.

// What is the difference between ptr and grades?



• Fortran95

– Has dangling pointer problem because allows explicit Deallocation command on a pointer

• so how does that make dangling pointers possible?

– Like Ada, pointers are implicitly dereferenced, but Fortran95 also provides a way to explicitly not dereference a pointer

• What does this mean again?

• Why might this be?



• Java

– Anybody know whether we have dangling pointers/references or memory leakage problems in Java?


Pointers• Reference types in C/C++ and Java and C#

– C/C++ reference type is a special kind of pointer type

• Used when declaring parameters to a function so that it is passed by reference

• Formal parameter is specified with an &

• But inside the function, it is implicitly dereferenced.– Makes code more readable and safer

• Can do same thing with regular pointers but code is less readable (b/c explicit dereferencing required) and less safe



– C/C++ reference type is a special kind of pointer type

• Would there be any use for passing by reference using a constant reference parameter (that is, one that disallows its contents to be changed)?

– Java

• References replace C++'s pointers– Why?

• Java references refer to class instances (objects), so arithmetic with them doesn't make sense.

• No dangling references b/c implicit deallocation



– C#

• Has both Java-like references and C++-like pointers.

• Best (or worst) of both worlds?

• Pointers are discouraged --- methods that use pointers need to be modified with the unsafe keyword.


Pointers• Implementation of pointers

– Pointers hold an address

• Solutions to dangling pointer problem– Tombstones

• A tombstone points to (holds the address of) where the data is (a heap-dynamic variable.)

• Pointers can only point to a tombstone (which in turn points to the actual data.)

• What does this solve?

• When deallocate, the tombstone is set to nil.

• Why aren't they used do you think? --- Know of any languages that use them?


Pointers• Solutions to dangling pointer problem (continued)

– locks-and-keys

• Pointers and variables need different implementation for this method

• Pointers are ordered pairs of an integer key and an address.

• Heap-dynamic variables – include a header cell that stores a lock value – and storage cell(s) for the variable itself

• During allocation a lock value is calculated and placed in the key portion of the pointer AND the header cell of the variable.

• Key and header cell are compared when the pointer is dereferenced and if they're the same it's a legal reference otherwise it is an illegal reference (which causes run-time error.)



– Locks-and-keys (continued)

• Multiple pointers may point to the variable but they all must have the same key.

• When variable is deallocated (explicitly), the variable's header cell is changed to an illegal lock value (so no key will match it ever.)

– Any other solutions you can think of to handle the dangling pointer problem?



– Any other solutions you can think of to handle the dangling pointer problem?

• Don't allow programmer to explicitly deallocate heap-dynamic variables. Like Java and LISP. Also like C#'s references (but not like C#'s pointers.)


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” 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


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 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))


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?


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))

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CS 330 Programming Languages 11 / 13 / 2007 Instructor: Michael Eckmann.

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