Alice in Action with Java

Alice in Action with Java Chapter 10 Flow Control in Java

Flow Control In Java • Purpose of using selective and repetitive execution • Implement methods that produce complex behaviors • Selective flow control statements: if and switch • Repetitive flow control statements: while, for, do Alice in Action with Java

Introductory Example: pH • Background for the use of pH values • pH: measures the acidity of a solution • pH scale of values: 0 – 14 • Labels for ranges in pH scale: acidic, neutral, alkaline • Main elements in a user story built around pH values • pH values of water samples have been recorded • Given pH values of samples, provide correct labels • Divide the problem into two subproblems • Build a class to model pH values and operations • Write a program that uses the class to label samples Alice in Action with Java

Subproblem 1: A PH Class • The PHclass needs a private instance variable • doubletype variable is named myValue • myValuewill be initialized by a constructor • PH constructor • Takes an argument that sets myValue • If pH argument is invalid, an error message displays • Otherwise, pH argument is assigned to myValue • The label()method • Returns error message if value is outside range • Otherwise, returns one of three labels Alice in Action with Java

Subproblem 1: A PH Class (continued) Alice in Action with Java

Subproblem 2: Processing Multiple pH Values • Objective: display labels for sequence of pH values • Pseudocode for PhLabeler program • Set the booleanvariable named “done” to false • While NOT done: • Prompt the user to enter a pH value (-1 to quit) • Read ph from the keyboard • If ph < the minimum pH value: set done to true; • Else: display phLabel(ph) • Observation: if statement nested in a while loop Alice in Action with Java

Subproblem 2: Processing Multiple pH Values (continued) Alice in Action with Java

Selective Execution • Directing flow based on the value of a condition • Two statements that provide selective execution • if statement: general-purpose selection structure • switch statement: multi-branch selection structure Alice in Action with Java

Java’s if Statement • General-purpose selection structure • Selects from one or more groups of statements • The else portion of the if statement is optional • One-branch if: envisioned as branching flow • Two-branch if: flow follows one of two branches Alice in Action with Java

Java’s if Statement (continued) Alice in Action with Java

Java’s if Statement (continued) • Pattern for Java’s if statement: if(Condition)Statement1[else Statement2] • Condition: any boolean expression • Statementi: set of Java statements within { } • Brackets are optional if only one statement is in path • Multi-branch ifstatements • Flow can move along multiple paths • Nest additional if statements in the else clause Alice in Action with Java

Java’s if Statement (continued) Alice in Action with Java

Java’s switch Statement • Objective: create a PetLicense class • Instance variables used in PetLicense class • chartype namedmyCode stores license code • doubletype namedmyFee stores license fee • Constructor for a PetLicenseobject • Takes a single chartype argument • Uses multi-branch if to select appropriate fee • If data is valid, instance variables are initialized • If data is invalid, an error message is displayed • switch: concise alternative to the multi-branch if Alice in Action with Java

Java’s switch Statement (continued) • Pattern for Java’s switch statement switch(IntegerCompatibleExpression){ CaseList1 StatementList1 ... CaseListN StatementListN default: StatementListN+1 } • The condition is an integer-compatible expression • Each case corresponds to a literal value • The use of default statement optional • Use of breakstatement below a case is recommended • PetLicense class meets criteria for use of switch Alice in Action with Java

Java’s switch Statement (continued) Alice in Action with Java

Java’s switch Statement (continued) • Oneswitch statement can be nested within another • A nested switch statement is used in TShirt class • TShirt constructor • Takes on String argument named size • myPricefor standard sizes set in cases ‘S’, ‘M’, ‘L’ • myPricefor ‘XS’and ‘XL’ are set in case ’X’ • Case ’X’ includes a nested switch statement • Other methods in TShirt class • A method to return mySize and one to return myPrice • toString()returns String value of data members Alice in Action with Java

Java’s switch Statement (continued) Alice in Action with Java

Repetitive Execution • Program execution flows sequentially by default • if and switch perform statements selectively • Repetitive execution: control flows in loops • Three repetition statements: while, for, and do Alice in Action with Java

Java’s while Statement • Used for processing a series of values • Input loops: read and process a series of values • Sentinel-controlled input loops • Utilize a sentinel (invalid value) to falsify a condition • Problem: extract the initials in a name • Members implemented in the Initialsclass • Instance variables called myName and myInitials • Constructor to initialize the instance variables • Methods to return myName and myInitials Alice in Action with Java

Java’s while Statement (continued) • StringTokenizerclass:used to splitStrings • Overview of the Initials class constructor • String argument (name) is passed to constructor • Instance variables are initialized • StringTokenizer object called names is initialized • names and while loop are used to extract initials • General pattern for Java’s while statement while(Condition)Statement • Statementcomprises one or more statements • Curly braces ({ }) required with multiple statements Alice in Action with Java

Java’s while Statement (continued) Alice in Action with Java

Java’s for Statement • Repetition structure for solving counting problems • Counting input loop • Simpler design than the sentinel-controlled input loop • Provides repetition when number of inputs is fixed • Illustration: computing the city’s air pollution index • A for loop counts from 1 to NUM_READINGS • Each iteration gets a reading and adds it to sum • After loop terminates, index is computed and output • Java’s for loop is very flexible • Example: the Java for loop can count down Alice in Action with Java

Java’s for Statement (continued) Alice in Action with Java

Java’s for Statement (continued) • Overview ofNinetyNineBottlesSongprogram • for loop in constructor counts down from 99 to 0 • Each iteration of the loop adds a verse to myLyrics • main()method constructs and displays the song • Plotting a function of the form y = f(x) • for loop iterates through x values and computes f(x) • SineWaves class for plotting a sine function • SineWaves should be compiled with Plotter class • Two methods use for loops to plot the sine waves • Range of values and increment amounts differ Alice in Action with Java

Java’s for Statement (continued) • TextGraphics class illustrates nested for loops • Understanding drawBox()in TextGraphicsclass • Method takes two arguments for height and width • Outer forloop counts the rows (builds the height) • Inner forloop prints asterisk symbol through width • General pattern for Java’s for loop for (InitialExpr; Condition; ChangeExpr) Statement • Curly braces ({ }) required with multiple statements • Scope of loop control variable goes to end of loop only Alice in Action with Java

Java’s do Statement • Pattern for Java’s do statement: do Statement while(Condition); • Loop provides one-trip behavior with posttest condition • You must type a semicolon after the condition • Overview of the GuessingGame class • Generates pseudo-random number between 1 and 100 • Retrieves input from user until the number is guessed • Critical logic: multi-branch if nested in a do loop Alice in Action with Java

Java’s do Statement (continued) Alice in Action with Java

Java’s do Statement (continued) • Query-controlled input loop • A loop that terminates when user enters specific value • Overview of the GameController class • Utilizes GuessingGameobject • Repeats game using query-controlled do loop • Default constructor • Constructor provided by Java • Used when a class does not include instance variables • GameController utilizes a default constructor Alice in Action with Java

Java’s do Statement (continued) Alice in Action with Java

Choosing the Right Loop • Solving a problem with fixed counting • Recommendation: use the for loop • Solving a problem without fixed counting • If one-trip behavior is needed, use a do loop • If zero-trip behavior is needed, use a while loop • Using guidelines to choose loop for Initials • Cannot formulate problem as a counting type • Number of words in a name not known in advance • Zero-trip behavior is needed, as name might be null • Result of analysis: while loop is selected Alice in Action with Java

Choosing the Right Loop (continued) Alice in Action with Java

Example: The Chaos Game • Overview of the ChaosGame class • Uses the algorithm for generating a Sierpinski Triangle • drawVertices()plots vertices of outer triangle • drawPoints()plots the vertices of the inner triangles • switchstatement is nested in a forloop • New point generated and plotted with each iteration • More points to interpolate leads to better resolution • Observations • Structure emerges from seemingly random behavior • Computers quickly help users visualize patterns in data Alice in Action with Java

Example: The Chaos Game (continued) Alice in Action with Java

Alice in Action with Java