C Programming

Program QualityProgram Quality

Some of the material in these slide is adapted from chapters 5 and 6 Some of the material in these slide is adapted from chapters 5 and 6 of “Code Complete” (see course resource page)of “Code Complete” (see course resource page)

A high-quality project

• Ease of understandingEase of understanding– Readable by someone who did not write the code

• Ease of implementationEase of implementation– Small cohesive pieces are easy to code

• Ease of testingEase of testing– Each operation can be tested separately

• Ease of modificationEase of modification– What to change is easy to identify

• Correct translation from requirementsCorrect translation from requirements– Easy to identify the code that implements each requirement

Top-Down Design

• A central part of programming in a language like C is taking A central part of programming in a language like C is taking a large and complex problem and decomposing it into a large and complex problem and decomposing it into smaller problems that can more easily be solved.smaller problems that can more easily be solved.

• This is not a one step process -- we further decompose the This is not a one step process -- we further decompose the sub problems into sub-sub problems until the problems are sub problems into sub-sub problems until the problems are eacg easy to solveeacg easy to solve

• Real life examplesReal life examples– Bake a cake– Mow the lawn

• Each sub-problem becomes a componentEach sub-problem becomes a component

Program Components

• In languages like C, In languages like C, componentcomponent refers to a refers to a routineroutine or or modulemodule– A routine is a function or main program– A module is a collection of routines, data, variables, etc.

• Typically one C source file and associated header file

• Components…Components…– hide internal details and processing from one another– hide design decisions

• When a hidden design decision changes, only that component must be changed.

– isolate complex parts of problem, where "decisions" need to be made.

– provide layers of abstraction and information hiding

High-Quality Functions

• A high-qualtiy function is like a black box - you know the inputs A high-qualtiy function is like a black box - you know the inputs

and outputs, but not what happens insideand outputs, but not what happens inside

• A high-quality function has a well-chosen nameA high-quality function has a well-chosen name– A verb followed by a noun. The name should reflect what the function does

• printGreeting( )

• getPositiveInteger( )

– A descriptive name of what’s returned

• sin( )

• nextRandomInt( )

– Avoid vague names

• outputReport( )

• performUserInput( )

– Follow naming conventions

Cohesion and Coupling

• One measure of the quality of a function or module is often One measure of the quality of a function or module is often described in terms of described in terms of cohesioncohesion and and couplingcoupling..

• Cohesion refers to how closely the operations in a function Cohesion refers to how closely the operations in a function or module are related.or module are related.– Strive for a high degree of cohesion– String cohesion results in code with high reliability

• Coupling refers to how closely two functions or modules Coupling refers to how closely two functions or modules are connected to each otherare connected to each other– Strive for a low degree of coupling

• Coupling and cohesion are complementaryCoupling and cohesion are complementary

Coincidental Cohesion

• A function that performs unrelated operationsA function that performs unrelated operations– A function that

• Checks for new email messages

• Prints a list of students in a course

• Changes my Unix password

• Fix this by making a separate function for each Fix this by making a separate function for each operationoperation

Logical Cohesion

• A function that performs one of several A function that performs one of several operations based upon a control flag parameteroperations based upon a control flag parameter– A function that

• Prints student name OR• Prints student age OR• Prints student email address OR• Prints a class roster OR• Prints a list of faculty members

• Fix this by writing a separate function for each Fix this by writing a separate function for each operationoperation

Procedural Cohesion

• A function that performs operations in a specific A function that performs operations in a specific order. Unlike sequential cohesion, these order. Unlike sequential cohesion, these operations do not share the same dataoperations do not share the same data– A function that

• Prints a revenue report

• Prints a report of expenses

• Prints a list of employee names and office numbers

• Fix this by rethinking your designFix this by rethinking your design

Temporal Cohesion

• A function that performs several operations A function that performs several operations which are related only because they must be which are related only because they must be performed at the same timeperformed at the same time– A function named initialize( ) that

• Initializes an array• Initializes some variables• Seeds the random number generator• Prints instructions

• These functions are considered acceptable, but These functions are considered acceptable, but have them call separate functions for each have them call separate functions for each operation.operation.

Communicational Cohesion

• A function that performs operations on the same A function that performs operations on the same data but are not otherwise relateddata but are not otherwise related– A function named getNameAndPhoneNumber( ) that returns

both the name and phone number from a structure of student information

– A function named verifyAgeAndID( ) that verifies that a student’s age and UMBC ID are both valid from a structure of student information

• Communicational cohesion is considered Communicational cohesion is considered acceptable if necessary for practical reasonsacceptable if necessary for practical reasons

• Note how the names of these functions are a clue Note how the names of these functions are a clue to their communicational cohesionto their communicational cohesion

Sequential Cohesion

• A function with operations that must be A function with operations that must be performed in a specific order, with the output of performed in a specific order, with the output of one operation being the input to the nextone operation being the input to the next– A function that

• Inputs the number of students• Inputs the final exam grade for each student• Calculates the min, max and average final exam grade• Displays the student grades• Displays the min, max, and average grade

• Sequential cohesion is acceptableSequential cohesion is acceptable

Functional Cohesion

• A function that performs only one operationA function that performs only one operation– calculateAge( ) -- Calculates a person’s age, given their birthday– getFileName( ) -- Gets the name of a file from the user– displayStudenNames( ) -- Displays student names to the screen– appendValue( ) - addes a new value to the end of an array

• Strive to achieve functional cohesion whenever Strive to achieve functional cohesion whenever possible.possible.

• Functions with functional cohesion are easy to Functions with functional cohesion are easy to name. If your function isn’t easy to name, that’s a name. If your function isn’t easy to name, that’s a hint it may not have functional cohesion.hint it may not have functional cohesion.

Function Cohesion Sumary

• There are several generally accepted levels of cohesion There are several generally accepted levels of cohesion listed here from worst to best.listed here from worst to best.– Coincidental - a function that performs several unrelated operations– Logical - a function that performs one of several operations selected by a

control flag passed to the function as a parameter– Procedural - operations performed in a specific order– Temporal - operations combined into one function because must all be

performed at the same time (e.g. initialization)– Communicational - operations make use of the same data but are not

related in any other way– Sequential - contain operations performed in a specific order, share data

from step to step, but don’t make up a complete function when done together

– Functional - a function that performs one and only one operation

• Only functional, sequential, communicational and temporal Only functional, sequential, communicational and temporal are considered to be acceptable levels of cohesionare considered to be acceptable levels of cohesion

Module Cohesion

• In C, a module equates of a .c source file which In C, a module equates of a .c source file which contains several functions and (possibly) data. contains several functions and (possibly) data.

• The cohesion of a module refers to how the data The cohesion of a module refers to how the data and functions within the module are related.and functions within the module are related.

• A cohesive module should contain data and a A cohesive module should contain data and a group of functions that clearly belong together.group of functions that clearly belong together.

Function Coupling• There are several levels of function coupling listed below There are several levels of function coupling listed below

from high/tight/strong to low/loose/weakfrom high/tight/strong to low/loose/weak– Content coupling - one function uses code inside another function (e.g. via

goto statement)• Never acceptable

– Common coupling - two functions use the same global data• Acceptable if the functions don’t change the global data

– Control coupling - one function passes a parameter to another to tell the second function what to do (see logical cohesion)

• Never acceptable– Stamp (data structure) coupling - a function passes a struct to another

function• Acceptable if the 2nd functions uses all or most members of the

struct– Data coupling - a function passes only the primitive data another function

needs• The most desirable level of coupling

Module Coupling

• Module coupling refers to how the module relates Module coupling refers to how the module relates to the rest of the program.to the rest of the program.

• A module should provide a complete set of A module should provide a complete set of operations that allow the module’s user to tailor operations that allow the module’s user to tailor the operations to his own needsthe operations to his own needs

•

Cohesion/Coupling Example

• A module that simulates an clockA module that simulates an clock• This module is not the best design since it This module is not the best design since it

supports only a single clock, but it demonstrates supports only a single clock, but it demonstrates the concepts of cohesion and couplingthe concepts of cohesion and coupling

link to clock.hlink to clock.h

link to clock.clink to clock.c

Information Hiding

• Information hiding has indisputably proven its Information hiding has indisputably proven its value in practice.value in practice.

• Integral part of structured programming (in Integral part of structured programming (in languages like C) and in OOPlanguages like C) and in OOP

• Most prevalent with respect to modulesMost prevalent with respect to modules• The key is to keep “secrets”The key is to keep “secrets”

Module Secrets

• Hide design and implementation decisionHide design and implementation decision– Is a “clock” a struct, or just a few independent variables?– Does the clock keep 24-hour time or 12-hour time?– What is the format of the input file?

• Secrets are kept throughSecrets are kept through– The use of static data– The use of static functions

• The interface (the contents of the .h file) should The interface (the contents of the .h file) should reveal as little as possible about how the module reveal as little as possible about how the module works. A module is like an iceberg.works. A module is like an iceberg.

Defensive Programming

• Analogous to defensive drivingAnalogous to defensive driving– Don’t assume that “the other guy” is going to do the right thing.

– Take responsibility for protecting yourself against drunken programmers

• If your function is passed “bad” data, your If your function is passed “bad” data, your function won’t be hurt.function won’t be hurt.

Note necessarily GIGO

• Garbage In does not mean garbage outGarbage In does not mean garbage out• RatherRather

– Garbage in, nothing out– Garbage in, error message out– No gargbage in

• Check the values of all input dataCheck the values of all input data• Check the values of all function parametersCheck the values of all function parameters• Decide how to handle bad parmeter valuesDecide how to handle bad parmeter values

– Return false, or other error code– Print an error message and continue– Exit your program

assert( )

• When a bad parameter value is fatal to your When a bad parameter value is fatal to your function, catch it with assert( ).function, catch it with assert( ).

• assert( ) is a function which takes a single assert( ) is a function which takes a single boolean expression as a parameterboolean expression as a parameter– If the boolean expression is true, nothing happens

– If the boolean expression is false, an error is displayed and your program terminates

• ExamplesExamplesassert( age > 0);

assert ( strlen(filename) < 20);