Control structures ii

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Java is a computer programming language that is concurrent, class-based, object-oriented, and specifically designed to have as few implementation dependencies as possible.

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Control structures ii

  1. 1. Chapter 5: Control StructuresChapter 5: Control Structures IIII JJavaava PProgramming:rogramming: From Problem Analysis to Program Design,From Problem Analysis to Program Design, Second EditionSecond Edition
  2. 2. 2 Chapter Objectives  Learn about repetition (looping) control structures.  Explore how to construct and use count-controlled, sentinel-controlled, flag-controlled, and EOF- controlled repetition structures.  Examine break and continue statements.  Discover how to form and use nested control structures.
  3. 3. 3 Why Is Repetition Needed?  There are many situations in which the same statements need to be executed several times.  Example:  Formulas used to find average grades for students in a class.
  4. 4. 4 The while Looping (Repetition) Structure  Syntax: while (expression) statement  Expression is always true in an infinite loop.  Statements must change value of expression to false.
  5. 5. 5 The while Looping (Repetition) Structure Example 5-1 i = 0; //Line 1 while (i <= 20) //Line 2 { System.out.print(i + " "); //Line 3 i = i + 5; //Line 4 } System.out.println(); //Line 5 Output 0 5 10 15 20
  6. 6. 6 The while Looping (Repetition) Structure Typically, while loops are written in the following form: //initialize the loop control variable(s) while (expression) //expression tests the LCV { . . . //update the loop control variable(s) . . . }
  7. 7. 7 Counter-Controlled while Loop  Used when exact number of data or entry pieces is known.  General form: int N = //value input by user or specified //in program int counter = 0; while (counter < N) { . . . counter++; . . . }
  8. 8. 8 Sentinel-Controlled while Loop  Used when exact number of entry pieces is unknown, but last entry (special/sentinel value) is known.  General form: Input the first data item into variable; while (variable != sentinel) { . . . input a data item into variable; . . . }
  9. 9. 9 Flag-Controlled while Loop  Boolean value used to control loop.  General form: boolean found = false; while (!found) { . . . if (expression) found = true; . . . }
  10. 10. 10 EOF(End of File)-Controlled while Loop  Used when input is from files.  Sentinel value is not always appropriate.  In an EOF-controlled while loop that uses the Scanner object console to input data, console acts at the loop control variable.  The method hasNext, of the class Scanner, returns true if there is an input in the input stream; otherwise, it returns false.  The expression console.hasNext() acts as the loop condition.  Expressions such as console.nextInt() update the value of the loop condition.
  11. 11. 11 EOF-Controlled while Loop  A general form of the EOF-controlled while loop that uses the Scanner object console to input data is: while (console.hasNext()) { //Get the next input and store in an //appropriate variable //Process data }
  12. 12. 12 EOF-Controlled while Loop  Suppose that inFile is a Scanner object initialized to the input file. In this case, the EOF- controlled while loop takes the following form: while (inFile.hasNext()) { //Get the next input and store in an //appropriate variable //Process data }
  13. 13. 13 Programming Example: Checking Account Balance  Input file: Customer’s account number, account balance at beginning of month, transaction type (withdrawal, deposit, interest), transaction amount.  Output: Account number, beginning balance, ending balance, total interest paid, total amount deposited, number of deposits, total amount withdrawn, number of withdrawals.
  14. 14. 14 Programming Example: Checking Account Balance  Solution:  Read data.  EOF-controlled loop.  switch structure of transaction types.  Determine action (add to balance or subtract from balance depending on transaction type).
  15. 15. 15 Programming Example: Fibonacci Number  Fibonacci formula for any Fibonacci sequence: an = an-1 + an-2  Input: First two Fibonacci numbers in sequence, position in sequence of desired Fibonacci number (n).  int previous1 = Fibonacci number 1  int previous2 = Fibonacci number 2  int nthFibonacci = Position of nth Fibonacci number  Output: nth Fibonacci number.
  16. 16. 16 Programming Example: Fibonacci Number (Solution) if (nthFibonacci == 1) current = previous1; else if (nthFibonacci == 2) current = previous2; else { counter = 3; while (counter <= nthFibonacci) { current = previous2 + previous1; previous1 = previous2; previous2 = current; counter++; } }
  17. 17. 17 The for Looping (Repetition) Structure  Specialized form of while loop.  Simplifies the writing of count-controlled loops.  Syntax: for (initial statement; loop condition; update statement) statement
  18. 18. 18 The for Looping (Repetition) Structure  Execution:  Initial statement executes.  Loop condition is evaluated.  If loop condition evaluates to true, execute for loop statement and execute update statement.  Repeat until loop condition is false.
  19. 19. 19 The for Looping (Repetition) Structure Example 5-8 The following for loop prints the first 10 nonnegative integers: for (i = 0; i < 10; i++) System.out.print(i + " "); System.out.println();
  20. 20. 20 The for Looping (Repetition) Structure Example 5-9 1. The following for loop outputs the word Hello and a star (on separate lines) five times: for (i = 1; i <= 5; i++) { System.out.println("Hello"); System.out.println("*"); } 2. The following for loop outputs the word Hello five times and the star only once: for (i = 1; i <= 5; i++) System.out.println("Hello"); System.out.println("*");
  21. 21. 21 The for Looping (Repetition) Structure  Does not execute if initial condition is false.  Update expression changes value of loop control variable, eventually making it false.  If loop condition is always true, result is an infinite loop.  Infinite loop can be specified by omitting all three control statements.  If loop condition is omitted, it is assumed to be true.  for statement ending in semicolon is empty.
  22. 22. 22 Programming Example: Classify Numbers  Input: N integers (positive, negative, and zeros). int N = 20; //N easily modified  Output: Number of 0s, number of even integers, number of odd integers.
  23. 23. 23 Programming Example: Classify Numbers (Solution) for (counter = 1; counter <= N; counter++) { number = console.nextInt(); System.out.print(number + " "); switch (number % 2) { case 0: evens++; if (number == 0) zeros++; break; case 1: case -1: odds++; } //end switch } //end for loop
  24. 24. 24 The do…while Loop (Repetition) Structure  Syntax: do statement while (expression);  Statements are executed first and then expression is evaluated.  Statements are executed at least once and then continued if expression is true.
  25. 25. 25 do…while Loop (Post-Test Loop)
  26. 26. 26 break Statements  Used to exit early from a loop.  Used to skip remainder of switch structure.  Can be placed within if statement of a loop.  If condition is met, loop is exited immediately.
  27. 27. 27 continue Statements  Used in while, for, and do...while structures.  When executed in a loop, the remaining statements in the loop are skipped; proceeds with the next iteration of the loop.  When executed in a while/do…while structure, expression is evaluated immediately after continue statement.  In a for structure, the update statement is executed after the continue statement; the loop condition then executes.
  28. 28. 28 Nested Control Structures  Provides new power, subtlety, and complexity.  if, if…else, and switch structures can be placed within while loops.  for loops can be found within other for loops.
  29. 29. 29 Nested Control Structures (Example) for (int i = 1; i <= 5; i++) { for (int j = 1; j <= i; j++) System.out.print(" *"); System.out.println(); } Output: * ** *** **** *****
  30. 30. 30 Chapter Summary  Looping mechanisms:  Counter-controlled while loop  Sentinel-controlled while loop  Flag-controlled while loop  EOF-controlled while loop  for loop  do…while loop  break statements  continue statements  Nested control structures

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