Software Development Phases

Software Development Phases

● Specification of the task

● Design of a solution

● Implementation of solution

● Analysis of solution

● Testing and debugging

● Maintenance and evolution of the system

● Obsolescence

Specification

● Precise description of problem

● May be one of the most difficult phases

Design

● Algorithm – set of instructions for solving problem

● Not originally done in programming language

– pseudocode

– Formal modeling language (UML)

● Decompose problem into smaller – more manageable subtasks – map to subprograms

– Need to be as independent of each other as possible

● Reuse

● Modification

– All domain information should be located only 1 place in the code

● Each subprogram should only do 1 task

– Each subprogram should be treated as a black box – specification not implementation – called procedural abstraction

● An important topic: preconditionsAn important topic: preconditions and and postconditions

postconditions..

● They are a method of specifying what a function They are a method of specifying what a function accomplishes.

accomplishes.

Preconditions and Postconditions

Preconditions and Postconditions

Frequently a programmer must communicate Frequently a programmer must communicate

precisely

precisely whatwhat a function accomplishes, without any a function accomplishes, without any indication of

indication of howhow the function does its work. the function does its work.

Example

● You are the head of a You are the head of a programming team and programming team and

you want one of your you want one of your

programmers to write a programmers to write a

function for part of a function for part of a

project.

project.

HERE ARE

THE REQUIREMENTS FOR A FUNCTION THAT I

WANT YOU TO WRITE.

I DON'T CARE WHAT METHOD THE

FUNCTION USES, AS LONG AS THESE

REQUIREMENTS ARE MET.

What are Preconditions and Postconditions?

● One way to specify such requirements is with a One way to specify such requirements is with a pair of statements about the function.

pair of statements about the function.

● The preconditionThe precondition statement indicates what must be statement indicates what must be true before the function is called.

true before the function is called.

● The postconditionThe postcondition statement indicates what will be statement indicates what will be true when the function finishes its work.

true when the function finishes its work.

Example

void write_sqrt(double x) // Precondition: x >= 0.

// Postcondition: The square root of x has // been written to the standard output.

...

Example

void write_sqrt( double x) // Precondition: x >= 0.

// Postcondition: The square root of x has // been written to the standard output.

...

}

● The precondition and postcondition appear as comments in your

program.

Example

void write_sqrt( double x) // Precondition: x >= 0.

// Postcondition: The square root of x has // been written to the standard output.

...

}

● In this example, the precondition In this example, the precondition requires that

requires that

x >= 0x >= 0

be true whenever the function is be true whenever the function is called.

called.

Example

write_sqrt( -10 );

write_sqrt( 0 );

write_sqrt( 5.6 );

Which of these function calls Which of these function calls

meet the precondition ?

meet the precondition ?

Example

Which of these function calls Which of these function calls

meet the precondition ? meet the precondition ?

write_sqrt( -10 );

write_sqrt( 0 );

write_sqrt( 5.6 );

The second and third calls are fine, since the argument is greater than or equal to zero.

Example

Which of these function calls Which of these function calls

meet the precondition ? meet the precondition ?

write_sqrt( -10 );

write_sqrt( 0 );

write_sqrt( 5.6 );

But the first call violates the

precondition, since the argument is less than zero.

Example

void write_sqrt( double x) // Precondition: x >= 0.

// Postcondition: The square root of x has // been written to the standard output.

...

}

● The postcondition always indicates The postcondition always indicates what work the function has

what work the function has

accomplished. In this case, when the accomplished. In this case, when the

function returns the square root of function returns the square root of xx

has been written.

has been written.

Another Example

bool is_vowel( char letter )

// Precondition: letter is an uppercase or

// lowercase letter (in the range 'A' ... 'Z' or 'a' ... 'z') . // Postcondition: The value returned by the

// function is true if Letter is a vowel;

// otherwise the value returned by the function is // false.

...

Another Example

is_vowel( 'A' );

is_vowel(' Z' );

is_vowel( '?' );

What values will be returned What values will be returned

by these function calls ?

by these function calls ?

Another Example

is_vowel( 'A' );

is_vowel(' Z' );

is_vowel( '?' );

What values will be returned What values will be returned

by these function calls ?

by these function calls ?

false false

Nobody knows, because the Nobody knows, because the

precondition has been violated.

precondition has been violated.

Another Example

is_vowel( '?' );

What values will be returned What values will be returned

by these function calls ? by these function calls ?

Violating the precondition Violating the precondition

might even crash the computer.

might even crash the computer.

Always make sure the precondition is valid . . .

● The programmer who calls the function is The programmer who calls the function is responsible for

responsible for ensuring that the precondition is ensuring that the precondition is valid

valid when the function is called.when the function is called.

AT THIS POINT, MY PROGRAM CALLS YOUR

FUNCTION, AND I MAKE SURE THAT THE PRECONDITION IS

VALID.

. . . so the postcondition becomes true at the function’s end.

● The programmer who The programmer who

writes the function counts writes the function counts on the precondition being on the precondition being

valid, and

valid, and ensures that ensures that the postcondition

the postcondition becomes true

becomes true at the at the function’s end.

function’s end.

THEN MY FUNCTION WILL EXECUTE, AND WHEN

IT IS DONE, THE

POSTCONDITION WILL BE TRUE.

I GUARANTEE IT.

A Quiz

Suppose that you call Suppose that you call

a function, and you a function, and you

neglect to make sure neglect to make sure that the precondition that the precondition

is valid.

is valid.

Who is responsible if Who is responsible if

this inadvertently this inadvertently

causes a 40-day flood causes a 40-day flood

or other disaster?

or other disaster?

 YouYou

 The programmer who The programmer who wrote that torrential wrote that torrential

function function

 NoahNoah

A Quiz

Suppose that you call Suppose that you call

a function, and you a function, and you

neglect to make sure neglect to make sure that the precondition that the precondition

is valid.

is valid.

Who is responsible if Who is responsible if

this inadvertently this inadvertently

causes a 40-day flood causes a 40-day flood

or other disaster?

or other disaster?

 YouYou

The programmer who The programmer who calls a function is

calls a function is

responsible for ensuring responsible for ensuring

that the precondition is that the precondition is

valid.

valid.

On the other hand, careful

programmers also follow these rules:

● When you write a function, you should make When you write a function, you should make every effort to detect when a precondition has every effort to detect when a precondition has

been violated.

been violated.

● If you detect that a precondition has been If you detect that a precondition has been

violated, then take appropriate action (repair or violated, then take appropriate action (repair or

terminate).

terminate).

On the other hand, careful

programmers also follow these rules:

● When you write a function, you should make When you write a function, you should make every effort to detect when a precondition has every effort to detect when a precondition has

been violated.

been violated.

● If you detect that a precondition has been If you detect that a precondition has been

violated, then take appropriate action and halt violated, then take appropriate action and halt

the program...

the program...

● ...rather than causing...rather than causing a disaster.a disaster.

Example

void write_sqrt( double x) // Precondition: x >= 0.

// Postcondition: The square root of x has // been written to the standard output.

{

assert(x >= 0);

...

detecting violations of a precondition.

precondition.

Advantages of Using Preconditions and Postconditions

● Succinctly describes the behavior of a Succinctly describes the behavior of a function...

function...

● ... without cluttering up your thinking with ... without cluttering up your thinking with details of how the function works.

details of how the function works.

● At a later point, you may reimplement the At a later point, you may reimplement the function in a new way ...

function in a new way ...

● ... but programs (which only depend on the ... but programs (which only depend on the

precondition/postcondition) will still work with precondition/postcondition) will still work with

no changes.

no changes.

Precondition Precondition

● The programmer who calls a The programmer who calls a function ensures that the

function ensures that the precondition is valid.

precondition is valid.

● The programmer who writes a The programmer who writes a function can bank on the

function can bank on the

precondition being true when precondition being true when the function begins execution.

the function begins execution.

Postcondition Postcondition

● The programmer The programmer who writes a

who writes a

function ensures function ensures

that the that the

postcondition is postcondition is

true when the true when the

function finishes function finishes

executing.

executing.

Summary

Standard Library & Standard Namespace

● #include directive

– Similar to import directive in other languages

– Much more primitive

● Namespaces – allows limiting visibility of “names”

● Standard namespace – standard C++ libraries

● 2 choices

– using namespace std:

– Fully qualified identfier

Exception Handling

● Similar to Java and C#

Running Time Analysis

● Machine independent method of measuring run time efficiency

● Determine basic operation for the algorithm

● Expression the number of operations as a function of the data size

● Interested in the order of magnitude of this function – expressed using Big-O notation

Running Time Analysis

● Worst case – maximum number of operations

● Best case – minimum number of operations

● Average case – depends upon the underlying probability distribution

Testing and Debugging

● Good test data

– Correct output known

– Include values most likely to causes errors

● Boundary values

– Fully exercise code

● Every line of code is executed by at least 1 test case