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C Programming. Program Quality Some of the material in these slide is adapted from chapters 5 and 6 of “Code Complete” (see course resource page). A high-quality project. Ease of understanding Readable by someone who did not write the code Ease of implementation

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C programming

C Programming

Program Quality

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

A high quality project
A high-quality project

  • Ease of understanding

    • Readable by someone who did not write the code

  • Ease of implementation

    • Small cohesive pieces are easy to code

  • Ease of testing

    • Each operation can be tested separately

  • Ease of modification

    • What to change is easy to identify

  • Correct translation from requirements

    • Easy to identify the code that implements each requirement

Top down design
Top-Down Design

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

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

  • Real life examples

    • Bake a cake

    • Mow the lawn

  • Each sub-problem becomes a component

Program components
Program Components

  • In languages like C, component refers to a routine or module

    • 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…

    • 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
High-Quality Functions

  • A high-qualtiy function is like a black box - you know the inputs and outputs, but not what happens inside

  • A 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
Cohesion and Coupling

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

  • Cohesion refers to how closely the operations in a function 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 are connected to each other

    • Strive for a low degree of coupling

  • Coupling and cohesion are complementary

Coincidental cohesion
Coincidental Cohesion

  • A 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 operation

Logical cohesion
Logical Cohesion

  • A function that performs one of several operations 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 operation

Procedural cohesion
Procedural Cohesion

  • A function that performs operations in a specific order. Unlike sequential cohesion, these operations 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 design

Temporal cohesion
Temporal Cohesion

  • A function that performs several operations which are related only because they must be performed 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 have them call separate functions for each operation.

Communicational cohesion
Communicational Cohesion

  • A function that performs operations on the same data 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 acceptable if necessary for practical reasons

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

Sequential cohesion
Sequential Cohesion

  • A function with operations that must be performed in a specific order, with the output of one 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 acceptable

Functional cohesion
Functional Cohesion

  • A 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 possible.

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

Function cohesion sumary
Function Cohesion Sumary

  • There are several generally accepted levels of cohesion 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 are considered to be acceptable levels of cohesion

Module cohesion
Module Cohesion

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

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

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

Function coupling
Function Coupling

  • There are several levels of function coupling listed below from 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

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

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

Cohesion coupling example
Cohesion/Coupling Example

  • A module that simulates an clock

  • This module is not the best design since it supports only a single clock, but it demonstrates the concepts of cohesion and coupling

    link to clock.h

    link to clock.c

Information hiding
Information Hiding

  • Information hiding has indisputably proven its value in practice.

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

  • Most prevalent with respect to modules

  • The key is to keep “secrets”

Module secrets
Module Secrets

  • Hide 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 through

    • The use of static data

    • The use of static functions

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

Defensive programming
Defensive Programming

  • Analogous 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 function won’t be hurt.

Note necessarily gigo
Note necessarily GIGO

  • Garbage In does not mean garbage out

  • Rather

    • Garbage in, nothing out

    • Garbage in, error message out

    • No gargbage in

  • Check the values of all input data

  • Check the values of all function parameters

  • Decide 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 function, catch it with assert( ).

  • assert( ) is a function which takes a single boolean 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

  • Examples

    assert( age > 0);

    assert ( strlen(filename) < 20);