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Programming Logic and Design Seventh Edition Chapter 2 Elements of High-Quality Programs

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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

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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

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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

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Working with Variables (continued) Figure 2-1 Flowchart and pseudocode for the number-doubling program

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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

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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

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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

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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

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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

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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

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Performing Arithmetic Operations Standard arithmetic operators: + (plus sign)—addition − (minus sign)—subtraction * (asterisk)—multiplication / (slash)—division

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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

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Performing Arithmetic Operations (continued) Left-to-right associativity Operations with the same precedence take place from left to right

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Performing Arithmetic Operations (continued) Table 2-1 Precedence and associativity of five common operators

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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

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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

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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

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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

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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

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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

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Figure 2-3 Program that produces a bill using only main program Figure 2-3 Program that produces a bill using only main program

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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

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Figure 2-5 The billing program with constants declared within the module Figure 2-5 The billing program with constants declared within the module

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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

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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

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Understanding the Most Common Configuration for Mainline Logic Mainline logic of almost every procedural computer program follows a general structure Declarations for global variables and constants Housekeeping tasks Detail loop tasks End-of-job tasks

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Understanding the Most Common Configuration for Mainline Logic (cont’d) Figure 2-6 Flowchart and pseudocode of mainline logic for a typical procedural program

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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

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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

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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 within another flowchart symbol

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Using Program Comments (continued)

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Figure 2-13 Flowchart that includes annotation symbols Figure 2-13 Flowchart that includes annotation symbols

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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

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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

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Designing Clear Statements Avoid confusing line breaks Use temporary variables to clarify long statements

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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

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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

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Using Temporary Variables to Clarify Long Statements (continued) Figure 2-14 Two ways of achieving the same salespersonCommission result

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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

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Writing Clear Prompts and Echoing Input (continued) Figure 2-15 Beginning of a program that accepts a name and balance as input

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Figure 2-16 Beginning of a program that accepts a Figure 2-16 Beginning of a program that accepts a name and balance as input and uses a separate prompt for each item

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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

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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: Need for good planning and design increases