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Determinate Loop Pattern, Java's for Statement, and Scanner objects

Determinate Loop Pattern, Java's for Statement, and Scanner objects 2 nd part of Chapter 6: Repetition. Algorithmic Pattern: The Determinate loop. We often need to perform some action a specific number of times: Produce 89 paychecks Count down to 0 (take 1 second of the clock).

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Determinate Loop Pattern, Java's for Statement, and Scanner objects

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  1. Determinate Loop Pattern, Java's forStatement, and Scanner objects 2nd part of Chapter 6: Repetition

  2. Algorithmic Pattern: The Determinate loop • We often need to perform some action a specific number of times: • Produce 89 paychecks • Count down to 0 (take 1 second of the clock). • Simulate playing a game of "Let's Make a Deal" 10,000 times • The determinate loop pattern repeats some action a specific number of times

  3. Determinate Loops • This template repeats a process n times • replace comments with appropriate statements • int n =/* how often we must repeat the process */ • for( int j = 1; j <= n; j = j + 1 ) { • // the process to be repeated • } • determinate loops must know the number of repetitions before they begin • know exactly how many employees, or students, or whatever that must be processed, for example

  4. General FormThe Java for loop for (initial statement;loop-test ;update-step) { repeated-part } • When a for loop is encountered, the initial-statement is executed (used here quite often as int j = 1). The loop-test evaluates. If loop-test is false, the for loop terminates. If loop-test is true, the repeated-part executes followed by the update-step.

  5. Flow chart view of a for loop Initial statement True False Loop test Iterative part update-step

  6. Example for loop that produces an average • Scanner keyboard = new Scanner(System.in); • double sum = 0.0; • System.out.print("How many do you want to average? "); • int n = keyboard.nextInt(); • // Do something n times • for (int j = 1; j <= n; j = j + 1) { • System.out.print("Enter number: "); // <- Repeat 3 • int number = keyboard.nextInt(); // <- statements • sum = sum + number; // <- n times • } • double average = sum / n; • System.out.print("Average = " + average);

  7. Code Demo: Use the debugger to trace this code • int n = 5; • for (int j = 1; j <= n; j = j + 1) { • System.out.println(j); • } • for (int k = 10; k >= 0; k = k - 2) { • System.out.println(k); • }

  8. Other Incrementing Operators • It is common to see determinate loops of this form where n is the number of repetitions • for( int j = 1; j <= n; j++ ) } • // ... } • The unary ++ and -- operators add 1 and subtract 1 from their operands, respectively. • int n = 0; • n++; // n is now 1 equivalent to n=n+1; or n+=1; • n++; // n is now 2 • n--; // n is now 1 again • The expression count++ is equivalent to the more verbose count = count + 1;

  9. Other Assignment Operators • Java has several assignment operators in addition to=(-=and+=) j -= 2;is the equivalent ofj = j - 2; sum += x;is the equivalent of sum = sum + x; • What is sum when a user enters 7 and 8? • int sum = 0; • int x = 0; • System.out.print("Enter a number: "); • x = keyboard.nextInt(); // user enters 7 • sum += x; • System.out.print("Enter a number: "); • x = keyboard.nextInt(); // user enters 8 • sum += x;

  10. Indeterminate loop with a Scanner • Sometimes a stream of input from the keyboard or a file needs to be read until there is no more data in the input stream • Consider a Scanner object constructed with a String argument • The string represents an input stream • You will need Scanner in project 2, methods 9 and 10: sumInScanner and maximumInScanner

  11. These assertions pass @Test publicvoidshowScanner() { Scanner scannerWithInts = newScanner("1 2 3"); assertEquals(1, scannerWithInts.nextInt()); assertEquals(2, scannerWithInts.nextInt()); assertEquals(3, scannerWithInts.nextInt()); Scanner scanner = new Scanner("There are five words here."); assertEquals("There", scanner.next()); assertEquals("are", scanner.next()); assertEquals("five", scanner.next()); assertEquals("words", scanner.next()); assertEquals("here.", scanner.next()); }

  12. A test method to test num100s @Test publicvoidtestNum100s() { ControlFuncf = newControlFun(); Scanner scanner0 = newScanner("1 2 3"); Scanner scanner1 = newScanner("4 100 2 5"); Scanner scanner3 = newScanner("100 100 2 -3 5 3 2 -100 100"); assertEquals(0, cf.num100s(scanner0)); assertEquals(1, cf.num100s(scanner1)); assertEquals(3, cf.num100s(scanner3)); }

  13. Answer publicintnum100s (Scanner scanner) { intresult = 0; while(scanner.hasNextInt()) { intnext = scanner.nextInt(); if(next == 100) result++; } returnresult; }

  14. Careful using next too often! • These assertions should pass with the code that follows on the next slide @Test publicvoidtestSumOfNegs() { ControlFun cf = newControlFun(); Scanner scanner0 = newScanner("1 2 3"); Scanner scannerA = newScanner("1 -2 3"); Scanner scannerB = newScanner("-4 1 -2 3"); assertEquals(0, cf.sumOfNegatives(scanner0)); assertEquals(-2, cf.sumOfNegatives(scannerA)); assertEquals(-6, cf.sumOfNegatives(scannerB)); }

  15. What's wrong with this method? publicintsumOfNegatives(Scanner scanner) { intresult = 0; while(scanner.hasNextInt()) { if(scanner.nextInt() < 0) { result += scanner.nextInt(); } } returnresult; }

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