cs 330 programming languages 10 / 31 / 2006 boo! instructor: michael eckmann

CS 330Programming Languages

10 / 31 / 2006

Boo!

Instructor: Michael Eckmann

Michael Eckmann - Skidmore College - CS 330 - Fall 2006

Today’s Topics• Questions / comments?• Midterm Exam grade explanation• Continue with Chapter 5

– Binding– Type Checking– Scope– Lifetime– Named Constants

• Chapter 6– Data types


0-4.0 scale grade coversion• Your exam was graded on a curve.• The total points: 111

• The mean: 83.63

• The standard deviation: 18.54

• Lowest: 61

• Highest: 108

• Your RawScore is the number of points you earned out of 111 (the numerator on the front of your test.)

(RawScore - 83.63) EC * 4.0 2.8 + ----------------------- + ------------ 18.54 111

examples for a raw score of 105 + 4 EC and of 71 + 0 EC


0-4.0 scale grade coversion

These are the minimum numbers for each letter grade.

e.g.

a 2.9 is a B

a 3.53 is an A-

0.51 D -

0.84 D

1.17 D +

1.51 C-

1.84 C

2.17 C +

2.51 B-

2.84 B

3.17 B+

3.51 A-

3.84 A

4.17 A+


Announcement• Ken Appel talk at 5:30 on Tuesday (today!) in Bolton 281.

• The title is:

• Mathematicians' Attitudes Towards Computer Proofs

• Also Wednesday at 10:10-11:05 a.m. in Linux Lab Ken Appel will give a talk on the Four Color Theorem.

Binding


• Binding attributes to variables

– Static vs. Dynamic binding

– Explicit vs. Implicit variable declaration

• Can someone explain the difference between Static and Dynamic binding of attributes to variables?

• Can someone else explain the difference between Explicit and Implicit variable declarations?

Binding


• Binding attributes to variable• Static binding – first occurs before run time and stays same

throughout program execution

• note: load-time is occurs before run-time•Dynamic binding

•Either it first occurs during run time•OR it can change during execution it is dynamic

• Variable declaration

– Explicit (what we're used to -- declare a variable by giving it a name and a type.)

– Implicit – associate a type based on naming convention and declaration happens on first appearance

An aside about memory


• How is memory divided up and categorized for executing programs? A typical

setup is as follows. I'll draw a diagram on the board.

– executable code

– static data area

• used for statically declared objects like globals and constants

– stack (grows one way)

• used for local variable allocation during “procedure / method / subroutine / function” calls. Note: I will constantly use these four words interchangeably.

– heap (grows the other way)

• used for dynamic objects

– objects that are allocated at runtime, may change and size not known until run time

• e.g. linked lists with unknown number of nodes, trees with unknown number of nodes, etc.

Binding• Storage bindings and lifetime

• Static variables (lifetime is the total time of execution)

• e.g. globals, static variables

• allocated in the static data area

• Stack-dynamic variables (what's the lifetime of these?)

• e.g. Local variables to methods

• allocated on the stack. When are they allocated and deallocated?

• Explicit heap-dynamic variables

• e.g. Variables used for data structures that shrink and grow during execution, or those only referenced through pointers or references. Can anyone give examples?

• allocated / deallocated on the heap

• Implicit heap-dynamic variables

• All attributes (type, size, etc.) of these are bound when value is assigned. e.g. The JavaScript example of list a few slides ago.

• allocated / deallocated on the heap

Binding


• Evaluation of these kinds of variables. Think in terms of memory space, cost

of execution, reliability, efficiency etc.

• Static variables

• What are some advantages and disadvantages?

Binding




• Static variables


+ ability to have history sensitive variables inside a method/function

+ efficient - addressing is direct

+ allocation is done before run-time so there is no run-time overhead of allocation and deallocation

- reduced flexibility

with only static variables you couldn't write recursive routines -why?

- could waste memory

can't share storage with other variables that do not have to overlap existence. Example?

Binding




• Stack-dynamic variables


Binding




• Stack-dynamic variables


+ allows recursion

+ share memory space with other stack-dynamic variables

- slower (than static) because bindings to storage/memory occur at runtime

Binding




• Explicit heap-dynamic variables (e.g. Java references to objects)


Binding




• Explicit heap-dynamic variables (e.g. Java references to objects)


+ flexibility / expressivity

+ passing references/pointers to methods/functions is more efficient than passing by value --- why?

- pointers/references could be difficult to use correctly

- slower at runtime b/c indirect addressing (what is indirect addressing?)

- complex implementation of how these variables are stored and accessed in memory

- complicated and costly heap management (e.g. Java's garbage collection)

Binding




• Implicit heap-dynamic variables (everything bound at assignment)


Binding




• Implicit heap-dynamic variables (everything bound at assignment)


+ extremely flexible, can easily write very generic code (same code works for many types) -- how is this possible?

- slower at runtime b/c of overhead asssoc. with maintaining all the dynamic attributes (they can change remember)

- reduced error detection, therefore reduced safety

Type Checking / Strong Typing


• Type checking --- checks that the operands of an operation are of compatible types

– what does compatible type mean?• If all bindings of variables to types are static then type

checking can be done when?• If any bindings of variables to types are dynamic then type

checking must be done when?



• Type checking --- checks that the operands of an operation are of compatible types

– what does compatible type mean?• If all bindings of variables to types are static then type

checking can be done before run-time.• If any bindings of variables to types are dynamic then type

checking is required at run-time. This is Dynamic Type Checking.



• Strong Typing – defined by the text as --- a language is strongly typed if type errors are always detected (whether at compile-time or run-time).

• According to this definition, what would you say about Java --- is it strongly typed?



• Strong Typing – defined by the text as --- a language is strongly typed if type errors are always detected (whether at compile-time or run-time).

• According to this definition, what would you say about Java --- is it strongly typed?– Yes nearly. Only casting (which is explicitly done by

the programmer) could cause an error to go undetected.

– Also, type coercion reduces the usefulness of strong typing to some extent. What is type coercion and why does it reduce the usefullness of strong typing?

Scope


• Scope is the range of statements in which a variable is visible (which means where it can be referenced.)

• Static scope• Static – scope of variables can be determined prior to

run-time. It is a spatial relationship.– Nesting of

• functions/methods (p. 229-230) • Blocks (p. 231)

– Answers the question – When we reference a name of a variable which variable is it?

Scope


• Dynamic scope• Scope of variables are determined at run-time. It is a temporal relationship, based on calling sequence.– Advantage: convenience– Disadvantage: poor readability– Also answers the question – When we reference a

name of a variable which variable is it? -- but we may get a different answer vs. if the language used static scoping.

Scope


• Dynamic scope example from Perl:$var = 10;

sub1(); # will print Yikes

sub2(); # will print 10

sub sub1

{

local $var = “Yikes”;

sub2();

}

sub sub2

{

print $var . “\n”;

}

Scope


• I'll let you read up on the evaluation of Static and Dynamic scope in the text.

Scope vs. lifetime


• Scope is typically spatial, lifetime is always temporal.• They are related in many cases, but two examples where they

are clearly different concepts:

• static variables can be declared in a function in C++ & Java. These variables are used to retain values between subsequent calls. e.g. If you wanted to know how many times you called the constructor for some class (i.e. how many instances of that class were created) during the program you could use a static variable that has one added to it each time in that constructor.

• What's the scope of a variable like this? What's its lifetime?

Scope vs. lifetime


• Many languages do not allow you to reference variables of a function that calls a particular function.

• e.g.Function1()

{ // do some stuff here

}

Function2()

{

int x;

Function1();

}

• What's the scope of x? What's its lifetime?

Constants and initialization


• What good are named constants?– Enhances readability. Anything else?

• Initialization – Binds a value to a variable when that variable is

bound to storage (i.e. when memory is allocated.)– Occurs once for static variables– Occurs each time code is executed for dynamic

variables (either stack-dynamic or heap-dynamic.)

Ch. 6 - Data Types


• A Data type

– defines a set of allowable values – defines the operations on those values

Ch. 6 - Data Types


• How well do the types match real-world problems

• User defined types --- example anyone?

• Abstract data types --- example anyone?

• Structured data types

– Arrays, records

• Data types and operations support in hardware vs. software

• Descriptor

– collection of attributes about a var stored in memory.

– Static (during compilation process) vs. dynamic (during run time)

– Used for type checking and allocation & deallocation

Primitive Data Types


• Not defined in terms of other types

• Used with type constructors to provide structured types

• Integer types– Signed vs. unsigned– Sign-magnitude vs. Two's complement vs. one's

complement representation of integers.

• Floating point are approximations to decimal numbers. Why only approximations? Let's convert 0.1 to binary.– Fully continuous? – Precision and range.



• Floating point are approximations to decimal numbers.

– Fully continuous? – Precision and range.

• single precision IEEE format:– 1 sign bit, 8 bits for exponent, 23 bits for

fraction• double precision IEEE format:

– 1 sign bit, 11 bits for exponent, 52 bits for fraction



• How to “fix” problems with floating point?

• Intervals– Interval arithmetic could guarantee that result of all

operations are within the interval– Have a min and max bound on results.



• Decimal types

– Most languages we use don't have a decimal type.– Cobol does, so does C#.– BCD– 1 or 2 digits per byte– Why have decimal types if they waste storage?



• Decimal types

– Most languages we use don't have a decimal type.– Cobol does, so does C#.– BCD– 1 or 2 digits per byte– Why have decimal types if they waste storage?

• Because they precisely store decimal values which floating point types do not



• Boolean

– Could be represented by a single bit, but are often represented by a byte for more efficient memory access.

• Character– ASCII (8 bit)– ISO 8859-1 (8 bit)– Unicode (16 bit) --- Java & c# use this

• 1st 128 are ASCII

ASCII


Unicode


• http://www.unicode.org/standard/WhatIsUnicode.html

• Has characters in alphabets of many languages

http://www.unicode.org/standard/WhatIsUnicode.html

Character strings


• Implemented as

– Character array vs. primitive type– Static vs. dynamic length

• C & C++ strings are terminated with a null character. – Length is not maintained but can be determined. The

end of the string is determined by the null character.– All that needs to be stored is a pointer to the first

character in the string.– Limited dynamic length (limited because there is a

maximum length)

Character strings


• Java has two different types for Strings.

– String• Static length constant strings

– StringBuffer• Dynamic length and allows direct subscripting

• Fortran 95– String is a primitive type– Has typical operations for strings

• Issues– What to do when assigning strings of different

lengths? etc.

Character strings


• Pattern matching with character strings

– Perl, JavaScript, PHP built in pattern matching with regular expressions

– Included in class libraries of C++, Java and C#

Character strings


• Length

– Static length– Limited dynamic length– Dynamic length

• Requires dynamic storage allocation and deallocation

• Maximum flexibility

Character strings


• Implementation

– Most often software (as opposed to hardware) implementation of storage, retrieval and manipulation

• Descriptor– Type name– Length (for static strings)– Address of first character

Character stringsDescriptor for static strings


Character strings


• Descriptor for limited dynamic

– Type name (Limited dynamic string)– Maximum length– Current length– Address of first character

• For dynamic length strings– Could be stored in linked list– Could be stored in adjacent storage cells

• What about when length changes? How can this be done?

Character strings


• Linked list vs. adjacent evaluation

– Linked lists• string operations become complex (following

pointers around)• allocation and deallocation is simple and fast• but requires more storage (for the pointers or

references)– Adjacent

• faster string operations (because contiguous)• less storage• but allocation and deallocation is slower

Ordinal / Enumeration


• Ordinal types– Range of values are associated with positive integers

• Enumeration types– Named constants represent positive integers– Design issues

• Are enumeration type values coerced to integer?– Same operations and range of values of integers

• Are any types coerced to enumeration type values?– Could break the allowable values of the enum

type– Range of values intentionally limited.

Enumeration


• e.g. In C#

enum months {Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec};

• e.g. In C++


• Represented as 0 to 11 typically but could be given programmer specified values in the declaration.

• Look the same, but C# enum types not coerced to integer whereas C++'s are --- so, C# doesn't allow operations that don't make sense, while C++ does.

• In C++, we can add 1 to an enum

Enumeration


• e.g. In C++


// declares currentMonth of type months and assigns

// an initial value

months currentMonth = Jun;

• In C++, we can add 1 to an enum– e.g. currentMonth++;

• Any danger here?

Enumeration


• As of Java 1.5 (aka 5.0), Java does support enumerations. When used, they create full classes for the enum. See:

http://java.sun.com/j2se/1.5.0/docs/guide/language/enums.html

• What are advantages to enumeration types?

Enumeration


• What are advantages to enumeration types?– All forms of implementation of them offer better

readability– Reliability

• No arithmetic operations on the enum types in Ada and C# --- this prevents adding which might not make sense like in adding months together

• Ada and C# also restrict values within the range for the type.

• C treats them like integers so we don't get those advantages

Enumeration


– Reliability• C++ allows numeric values to be assigned to enum

types if they are explicitly cast.• C++ also allows values to be assigned to enum

types but the range is checked for validity.– This just checks that the integer is between the

lowest and highest enum value. Enum values need not be consecutive integers. Any problem here?

Subrange types


• Subrange types

• e.g. In Ada:

• type Days is (Mon, Tue, Wed, Thu, Fri, Sat, Sun);

• subtype Weekdays is Days range Mon..Fri;

• Day1 : Days;

• Day2 : Weekdays;

• Day2 := Day1;

• Only allowed if Day 1 isn't Sat or Sun.

Subrange types


• Subrange types

– When do you think they're type checked?

– When do you think they're range checked?

Subrange types


• Only current language that supports is Ada 95.

• Subrange types are implemented as their parent types but with additional range checks.

• Advantages / Disadvantages?

Subrange types


• Advantages / Disadvantages?

– Increased code size

– Increased execution time due to range checking

– Readability increases and reliability increases

cs 330 programming languages 10 / 31 / 2006 boo! instructor: michael eckmann

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