Chapter 6 Iteration Lecture Slides to Accompany An Introduction to Computer Science Using Java (2nd Edition) by S.N. Kamin, D. Mickunas, E. Reingold

Chapter Preview In this chapter we will: discuss the use of repetition (iteration) in programming algorithms describe the Java while, for, and do-while statements demonstrate the use of loop invariants to verify the correctness of loops show the use of loops in data entry and computer animation

while Loop Repeated executes body of the loop which can be a single or compound statement A while loop will execute as long as its condition is true An infinite loop will occur if the condition never becomes false Example: count = 1; while (count <= 10) count = count + 1;

while Loop From Temperature Conversion Program OutputBox out = new OutputBox( ); out.println(“\tDEGREES C\tDEGREES F”); // initialize the loop variable cent = LOW_TEMP; // test the loop variable while (cent <= HIGH_TEMP) { // convert C to F fahr = (9.0 / 5.0) * cent ; out.println(“\t” + cent “\t\t” fahr); // increment loop variable cent = cent + 1.0; }

for Statement Used to implement pre-test loops that are controlled by incrementing a variable each time the loop is executed Loop variable computations for initialization, testing, and incrementing appear in a single statement Example: for(count=1; count <= 10; count = count + 1);

Comparing for and while Done using for for ( statement1; condition; statement2 ) statement 3; Equivalent while loop statement1; while (condition) { statement3; statement2; }

Temperature Conversion Program Loop Implemented Using for OutputBox out = new OutputBox( ); out.println(“\tDEGREES C\tDEGREES F”); for (cent = LOW_TEMP; cent <= HIGH_TEMP; cent = cent + 1.0;) { // convert C to F fahr = (9.0 / 5.0) * cent ; out.println(“\t” + cent “\t\t” fahr); }

do-while Statement Used to implement post-test loops that always execute the loop body at least one time A do-while loop will continue execute as long as its condition is true An infinite loop will occur if the condition never becomes false Example: count = 1; do { count = count + 1; } while (count <= 10);

Comparing do-while and while Done using do-while do statement; while (condition); Equivalent while loop statement; while (condition) statement;

Comparing do-while and while Done using do-while numberOfDigits = 0; rest = number; do { rest = rest / 10; numberOfDigits++; } while (rest != 0); Equivalent while loop numberOfDigits = 0; rest = number; if (number = 0) numberOfDigits = 1; else while (rest > 0) { rest = rest / 10; numberOfDigits++; };

Reading Input Using a Loop Done using do-while read score if ( not end of input) do { process score read score } while ( not end of input ); Done using while read score while ( not end of input ) { process score read score };

Sentinel Controlled Loop int i = 0; InputBox in = new InputBox(); In.setPrompt(“Enter data or –1 to terminate”); // read first score score = in.readInt(); while ( score != -1 ) { i++; // insert code to process score // read next score score = in.readInt(); }

Reading to End of Input int i = 0; InputBox in = new InputBox(); In.setPrompt(“Enter score (empty input ends data”); // read first score score = in.readInt(); while ( !in.eoi() ) { i++; // insert code to process score // read next score score = in.readInt(); }

Loop-and-a-Half Done using while read score while ( !in.eoi( ) ) { process score read score }; Done using do-while do { read score if ( !in.eoi( ) ) process score } while ( !in.eoi( ) );

Infinite Loop with Escape Done using do-while do { read if ( in.eoi( ) ) return process } while ( true ); Done using while while (true) { read if ( in.eoi( ) ) return process score };

Drawing in Java // code to read and display GIF image from file import java.awt.*; Import CSLib.*; … DrawingBox g = new DrawingBox(); … Toolkit tools = Toolkit.getDefaultToolkit(): Image mouse = tools.getImage(“C:/KMR/images/mouse.gif”); g.drawImage(mouse, x, y);

Nested Loops int advance = 0; // Draw clock and read input repeatedly do { for (int i = 1; i <= advance; i++) { // Advance time c.setMinute(c.getMinute()+1); // Update clock display d.clear(); c.display(d, SIZE/2, SIZE/2, SIZE/2); } in.setPrompt(“Advance how many minutes?”); advance = in.readInt(); } while (!in.eoi());