Transcript CS 211 Computer Programming II 1

CS 211
Computer Programming II
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Staff
 Instructor
 Vana Doufexi
 [email protected]
 Ford Building, 2133 Sheridan, #2-229
 Teaching Assistant
 TBA
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Class resources
 Class webpage
 http://www.cs.northwestern.edu/academics/courses/211
 Use it to:
 download class notes, handouts and assignments
 look up your grades
 look up class policies
 Class newsgroup
 newsgroup name: cs.211
 nntp server: news.cs.northwestern.edu
 Use it to
 check announcements
 discuss class material
 discuss assignments (but NEVER post solutions)
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Goals
 Learn how to design a project (using OOD)
 Learn C++
 Learn how to write clear, well-designed code
 Learn how to debug efficiently
 Get familiar with Linux
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Object-oriented design
 Main idea:
 Model the problem as a collection of objects that have certain
attributes and interact with one another and the world.
 Example:
 Simulating a transportation company that owns trucks and
trains.
 objects (nouns) : truck, train
 attributes : load, capacity, speed, destination, etc.
 operations (verbs) : unload, fill up, travel, etc.
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Concept: abstraction
 Main idea:
 Focus on the essential details about an entity and consequently
on the common elements between entities.
 Example:
 Our simulator may consist of a large class of objects called
Vehicle. All vehicles share some attributes (e.g. load,
capacity) and specific operations may be applied to them
(e.g. unload, move_to_destination).
 There may be several levels of abstraction, each with more
detail that the previous level: the Vehicle class consists of
two subclasses of objects, the Truck and the Train. Each
subclass has its own attributes and operations, specific to it,
in addition to the common ones (e.g. number_of_wheels
for trucks, number of cars for trains).
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Concept: abstraction
 Big advantages:
 It enables us to present a general interface, hiding
implementation details.
 It makes it easier to add new types of objects (e.g. Ship as a
subclass of Vehicle, Tanker and Car-ferry as subclasses of Ship)
Vehicle
Three levels of abstraction
Ship
Tanker
Truck
Train
Car ferry
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Concept: information hiding
 Main idea:
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Hide information about a module's structure and implementation
from other modules. Allow other modules to access information
only through a well-defined interface.
 Example:
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A simulation of a banking system contains a Client module and a
BankAccount module.
 An object of type Client may only use the Deposit operation of
a BankAccount to deposit money.
 The way the balance will be updated by Deposit is controlled
by the BankAccount.
 The Client object should not be allowed to see how the
BankAccount object keeps track of the balance and it should
not be allowed to change the balance except through the
Deposit function.
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Concept: information hiding
 Big advantages:
 Other modules are not concerned with the internal design.
 Example: You don't need to know how the engine works
to drive a car.
 We may modify the implementation without having to
modify the way the module interacts with other modules
 Example: If the "implementation" of the engine changes
(e.g. to a diesel engine), the interface (steering wheel,
gas pedal, gear box, etc) remains the same.
 The integrity of the design is maintained.
 Example: In the BankAccount/Client example, the Client
is not allowed to change the internal structure of the
BankAccount.
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Concept: software reuse
 Main idea:
 Use previously designed software in a new application
 Big advantages:
 Less development time (and as a result, less cost)
 More reliable product (the software you are reusing has
been tested extensively)
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Concept: maintainability
 Main idea:
 Make your program easy to modify in order to correct faults,
or adapt it to a changed environments, or improve
performance.
 Abstraction, information hiding, modularity enhance
the maintainability of your code.
 In addition, you should always write clear, readable
and easily understandable code.
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Concept: readability
 Main issues:
 Code formatting (the physical layout of your code)
 Horizontal and vertical spacing, indentation, line length
 Naming of identifiers (variables, function names)
 Use clear, descriptive names.
 Use nouns for variables, verbs for functions.
 Avoid shorthand.
 Comments
 Describe the intent of the programmer
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good example: convert dollars to euros
bad example: multiply by 1.2
BE CONSISTENT!
 Do not mix up naming and formatting styles.
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The compilation process
 Stage 1: Lexical analysis
 The compiler identifies individual "tokens" such as identifier
names, strings, plus signs, numbers, comments, etc.
 Errors discovered: unterminated comments/strings,
unrecognized symbols.
 Stage 2: Syntax analysis
 The compiler identifies the structure of the program and
checks whether it satisfies the syntactic rules of the
language.
 Errors discovered: syntax errors, such as missing semicolons.
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The compilation process
 Stage 3: Semantic analysis
 The compiler identifies the meaning of the program, collects
information such as types of variables and performs tests
such as type checks.
 Errors discovered: type errors (e.g. trying to assign a
number to a string variable).
 Stage 4: Code generation
 The compiler generates object code.
 (We have skipped a couple of stages that are not important
to this discussion)
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The compilation process
 Stage 5: Linking
 The linker combines one or more files containing object code
from separately compiled program modules into a single file
containing executable code.
 Errors discovered: missing libraries
 Finally: Execution
 The program can now be executed.
 Errors discovered: run-time errors are caused when your
program tries an operation that is not allowed (e.g. when it
tries to access a part of memory that it shouldn't). These are
not caught by the compiler and are the most difficult to
identify and solve. This is where good debugging practices
are needed.
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Debugging
 Often the most time consuming stage of the
developing process.
 It must be performed in a systematic way.
 BAD! : Let's try to change this line, to see if it fixes the
problem.
GOOD! : Let's use the debugger.
 The debugger can show you what happens inside
your program as the latter executes. It allows you to
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execute your program line by line,
make your program stop at a specific point or when a
certain condition is satisfied,
check the value of an expression at some point.
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Elements of Programming Languages
 Variables and types
 Expressions
 Assignments
 Control flow constructs
 Statements
 Functions
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Don't forget
 Make certain your net-id works
 Drop by the Tech PC classroom to check it out
 Check your email for information on your CS account
and make sure it works.
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