Chapter 2 Elements of High-Quality Programs

Programming Logic and Design Chapter 2 Elements of High-Quality Programs Objectives In this chapter, you will learn about: • Declaring and using variables and constants • Performing arithmetic operations • The advantages of modularization • Modularizing a program • Hierarchy charts • Features of good program design 2 • Declaring and Using Variables and Constants Data types – Numeric consists of numbers – String is anything not used in math • Different forms – Integers and floating-point numbers – Literal and string constants – Unnamed constants 3 Working with Variables • Named memory locations • Contents can vary or differ over time • Declaration – Statement that provides a data type and an identifier for a variable • Identifier – Variable’s name 4 Working with Variables (continued) Figure 2-1 Flowchart and pseudocode for the numberdoubling program 5 Working with Variables (continued) • Data type – Classification that describes: • What values can be held by the item • How the item is stored in computer memory • What operations can be performed on the data item • Initializing the variable – Declare a starting value for any variable • Garbage – Variable’s unknown value before initialization 6 Figure 2-2 Flowchart and pseudocode of number-doubling program with variable declarations 7 Naming Variables • Programmer chooses reasonable and descriptive names for variables • Programming languages have rules for creating identifiers – Most languages allow letters and digits – Some languages allow hyphens – Reserved keywords are not allowed • Variable names are case sensitive 8 Naming Variables (continued) • Camel casing – Variable names such as hourlyWage have a “hump” in the middle • Be descriptive – Must be one word – Must start with a letter – Should have some appropriate meaning 9 Assigning Values to Variables • Assignment statement – set myAnswer = myNumber * 2 • Assignment operator – Equal sign – Always operates from right to left • Valid – set someNumber = 2 – set someOtherNumber = someNumber • Not valid – set 2 + 4 = someNumber 10 Understanding the Data Types of Variables • Numeric variable – Holds digits – Can perform mathematical operations on it • String variable – Can hold text – Letters of the alphabet – Special characters such as punctuation marks • Type-safety – Prevents assigning values of an incorrect data type 11 Declaring Named Constants • Named constant – Similar to a variable – Can be assigned a value only once – Assign a useful name to a value that will never be changed during a program’s execution • Magic number – Unnamed constant – Use taxAmount = price * SALES_TAX_AMOUNT instead of taxAmount = price * .06 12 Performing Arithmetic Operations • Standard arithmetic operators: + (plus sign)—addition − (minus sign)—subtraction * (asterisk)—multiplication / (slash)—division 13 Performing Arithmetic Operations (continued) • Rules of precedence – Also called the order of operations – Dictate the order in which operations in the same statement are carried out – Expressions within parentheses are evaluated first – Multiplication and division are evaluated next • From left to right – Addition and subtraction are evaluated next • From left to right 14 Performing Arithmetic Operations (continued) • Left-to-right associativity – Operations with the same precedence take place from left to right 15 Performing Arithmetic Operations (continued) Table 2-1 Precedence and associativity of five common operators 16 • Understanding the Advantages of Modularization Modules – Subunit of programming problem – Also called subroutines, procedures, functions, or methods • Modularization – Breaking down a large program into modules – Reasons • Abstraction • Allows multiple programmers to work on a problem • Reuse your work more easily 17 Modularization Provides Abstraction • Abstraction – Paying attention to important properties while ignoring nonessential details – Selective ignorance • Newer high-level programming languages – Use English-like vocabulary – One broad statement corresponds to dozens of machine instructions • Modules provide another way to achieve abstraction 18 Modularization Allows Multiple Programmers to Work on a Problem • Easier to divide the task among various people • Rarely does a single programmer write a commercial program – Professional software developers can write new programs quickly by dividing large programs into modules – Assign each module to an individual programmer or team 19 Modularization Allows You to • Reuse Work Reusability – Feature of modular programs – Allows individual modules to be used in a variety of applications – Many real-world examples of reusability • Reliability – Assures that a module has been tested and proven to function correctly 20 Modularizing a Program • Main program – Basic steps (mainline logic) of the program • Include in a module – Module header – Module body – Module return statement • Naming a module – Similar to naming a variable – Module names are followed by a set of parentheses 21 Modularizing a Program (continued) • When a main program wants to use a module – “Calls” the module’s name • Flowchart – Symbol used to call a module is a rectangle with a bar across the top – Place the name of the module you are calling inside the rectangle – Draw each module separately with its own sentinel symbols 22 Figure 2-3 Program that produces a bill using only main program 23 Modularizing a Program (continued) • Statements taken out of a main program and put into a module have been encapsulated • Main program becomes shorter and easier to understand • Modules are reusable • When statements contribute to the same job, we get greater functional cohesion 24 Figure 2-5 The billing program with constants declared within the module 25 • Declaring Variables and Constants within Modules Place any statements within modules – Input, processing, and output statements – Variable and constant declarations • Variables and constants declared in a module are usable only within the module – Visible – In scope, also called local • Portable – Self-contained units that are easily transported 26 Declaring Variables and Constants within Modules (continued) • Global variables and constants – Declared at the program level – Visible to and usable in all the modules called by the program – Many programmers avoid global variables to minimize errors 27 • Understanding the Most Common Configuration for Mainline Mainline logic of Logic almost every procedural computer program follows a general structure – – – – Declarations for global variables and constants Housekeeping tasks Detail loop tasks End-of-job tasks 28 Understanding the Most Common Configuration for Mainline Logic (cont’d) Figure 2-6 Flowchart and pseudocode of mainline logic for a typical procedural program 29 Creating Hierarchy Charts • Hierarchy chart – Shows the overall picture of how modules are related to one another – Tells you which modules exist within a program and which modules call others – Specific module may be called from several locations within a program • Planning tool – Develop the overall relationship of program modules before you write them • Documentation tool 30 Features of Good Program Design • Use program comments where appropriate • Identifiers should be chosen carefully • Strive to design clear statements within your programs and modules • Write clear prompts and echo input • Continue to maintain good programming habits as you develop your programming skills 31 Using Program Comments • Program comments – Written explanations of programming statements – Not part of the program logic – Serve as documentation for readers of the program • Syntax used differs among programming languages • Flowchart – Use an annotation symbol to hold information that expands on what is stored 32 wchart symbol Using Program Comments (continued) Figure 2-12 Pseudocode that declares some variables and includes comments 33 Figure 2-13 Flowchart that includes annotation symbols 34 Choosing Identifiers • General guidelines – Give a variable or a constant a name that is a noun (because it represents a thing) – Give a module an identifier that is a verb (because it performs an action) – Use meaningful names • Self-documenting – Use pronounceable names – Be judicious in your use of abbreviations – Avoid digits in a name 35 Choosing Identifiers (continued) • General guidelines (continued) – Use the system your language allows to separate words in long, multiword variable names – Consider including a form of the verb to be – Name constants using all uppercase letters separated by underscores (_) • Programmers create a list of all variables – Data dictionary 36 Designing Clear Statements • Avoid confusing line breaks • Use temporary variables to clarify long statements 37 Avoiding Confusing Line Breaks • Most modern programming languages are free-form • Make sure your meaning is clear • Do not combine multiple statements on one line 38 Using Temporary Variables to Clarify Long Statements • Temporary variable – Work variable – Not used for input or output – Working variable that you use during a program’s execution • Consider using a series of temporary variables to hold intermediate results 39 Using Temporary Variables to Clarify Long Statements (continued) Figure 2-14 Two ways of achieving the same salespersonCommission result 40 Writing Clear Prompts and Echoing Input • Prompt – Message displayed on a monitor to ask the user for a response – Used both in command-line and GUI interactive programs • Echoing input – Repeating input back to a user either in a subsequent prompt or in output 41 Writing Clear Prompts and Echoing Input (continued) Figure 2-15 Beginning of a program that accepts a name and balance as input 42 Figure 2-16 Beginning of a program that accepts a name and balance as input and uses a separate prompt for each item 43 Maintaining Good Programming Habits • Every program you write will be better if you: – Plan before you code – Maintain the habit of first drawing flowcharts or writing pseudocode – Desk-check your program logic on paper – Think carefully about the variable and module names you use – Design your program statements to be easy to read and use 44 Summary • Programs contain literals, variables, and named constants • Arithmetic follows rules of precedence • Break down programming problems into modules – Include a header, a body, and a return statement • Hierarchy charts show relationship among modules • As programs become more complicated: anning and design increases 45