chapter03.java

// Read a character from the keyboard.
class KbIn {
  public static void main(String[] args)
    throws java.io.IOException {

    char ch;

    System.out.print("Press a key followed by ENTER: ");

    ch = (char) System.in.read(); // get a char

    System.out.println("Your key is: " + ch);
  }
}

// -----------------------------------------

// Guess the letter game.
class Guess {
  public static void main(String[] args)
    throws java.io.IOException {

    char ch, answer = 'K';

    System.out.println("I'm thinking of a letter between A and Z.");
    System.out.print("Can you guess it: ");

    ch = (char) System.in.read(); // read a char from the keyboard

    if(ch == answer) System.out.println("** Right **");
  }
}

// -----------------------------------------

// Guess the letter game, 2nd version.
class Guess2 {
  public static void main(String[] args)
    throws java.io.IOException {

    char ch, answer = 'K';

    System.out.println("I'm thinking of a letter between A and Z.");
    System.out.print("Can you guess it: ");

    ch = (char) System.in.read(); // get a char

    if(ch == answer) System.out.println("** Right **");
    else System.out.println("...Sorry, you're wrong.");
  }
}

// -----------------------------------------

// Guess the letter game, 3rd version.
class Guess3 {
  public static void main(String[] args)
    throws java.io.IOException {

    char ch, answer = 'K';

    System.out.println("I'm thinking of a letter between A and Z.");
    System.out.print("Can you guess it: ");

    ch = (char) System.in.read(); // get a char

    if(ch == answer) System.out.println("** Right **");
    else {
      System.out.print("...Sorry, you're ");

      // a nested if
      if(ch < answer) System.out.println("too low");
      else System.out.println("too high");
    }
  }
}

// -----------------------------------------

// Demonstrate an if-else-if ladder.
class Ladder {
  public static void main(String[] args) {
    int x;

    for(x=0; x<6; x++) {
      if(x==1)
        System.out.println("x is one");
      else if(x==2)
        System.out.println("x is two");
      else if(x==3)
        System.out.println("x is three");
      else if(x==4)
        System.out.println("x is four");
      else
        System.out.println("x is not between 1 and 4");
    }
  }
}

// -----------------------------------------

// Demonstrate the switch.
class SwitchDemo {
  public static void main(String[] args) {
    int i;

    for(i=0; i<10; i++)
      switch(i) {
        case 0:
          System.out.println("i is zero");
          break;
        case 1:
          System.out.println("i is one");
          break;
        case 2:
          System.out.println("i is two");
          break;
        case 3:
          System.out.println("i is three");
          break;
        case 4:
          System.out.println("i is four");
          break;
        default:
          System.out.println("i is five or more");
      }
  }
}

// -----------------------------------------

// Demonstrate the switch without break statements.
class NoBreak {
  public static void main(String[] args) {
    int i;

    for(i=0; i<=5; i++) {
      switch(i) {
        case 0:
          System.out.println("i is less than one");
        case 1:
          System.out.println("i is less than two");
        case 2:
          System.out.println("i is less than three");
        case 3:
          System.out.println("i is less than four");
        case 4:
          System.out.println("i is less than five");
      }
      System.out.println();
    }
  }
}

// -----------------------------------------

/*
    Try This 3-1

    A simple help system.
*/
class Help {
  public static void main(String[] args)
    throws java.io.IOException {
    char choice;

    System.out.println("Help on:");
    System.out.println("  1. if");
    System.out.println("  2. switch");
    System.out.print("Choose one: ");
    choice = (char) System.in.read();

    System.out.println("\n");

    switch(choice) {
      case '1':
        System.out.println("The if:\n");
        System.out.println("if(condition) statement;");
        System.out.println("else statement;");
        break;
      case '2':
        System.out.println("The switch:\n");
        System.out.println("switch(expression) {");
        System.out.println("  case constant:");
        System.out.println("    statement sequence");
        System.out.println("    break;");
        System.out.println("  // ...");
        System.out.println("}");
        break;
      default:
        System.out.print("Selection not found.");
    }
  }
}

// -----------------------------------------

// Show square roots of 1 to 99.
class SqrRoot {
  public static void main(String[] args) {
    double num, sroot;

    for(num = 1.0; num < 100.0; num++) {
      sroot = Math.sqrt(num);
      System.out.println("Square root of " + num +
                         " is " + sroot);
    }
  }
}

// -----------------------------------------

// A negatively running for loop.
class DecrFor {
  public static void main(String[] args) {
    int x;

    for(x = 100; x > -100; x -= 5)
      System.out.println(x);
  }
}

// -----------------------------------------

// Use multiple loop control variables in a for.
class MultipleLoopVars {
  public static void main(String[] args) {
    int i, j;

    for(i=0, j=10; i < j; i++, j--)
      System.out.println("i and j: " + i + " " + j);
  }
}

// -----------------------------------------

// Loop until an S is typed.
class ForTest {
  public static void main(String[] args)
    throws java.io.IOException {

    int i;

    System.out.println("Press S to stop.");

    for(i = 0; (char) System.in.read() != 'S'; i++)
      System.out.println("Pass #" + i);
  }
}

// -----------------------------------------

// Parts of the for can be empty.
class Empty {
  public static void main(String[] args) {
    int i;

    for(i = 0; i < 10; ) {
      System.out.println("Pass #" + i);
      i++; // increment loop control var
    }
  }
}

// -----------------------------------------

// Move more out of the for loop.
class Empty2 {
  public static void main(String[] args) {
    int i;

    i = 0; // move initialization out of loop
    for(; i < 10; ) {
      System.out.println("Pass #" + i);
      i++; // increment loop control var
    }
  }
}

// -----------------------------------------

// The body of a loop can be empty.
class Empty3 {
  public static void main(String[] args) {
    int i;
    int sum = 0;

    // sum the numbers through 5
    for(i = 1; i <= 5; sum += i++) ;

    System.out.println("Sum is " + sum);
  }
}

// -----------------------------------------

// Declare loop control variable inside the for.
class ForVar {
  public static void main(String[] args) {
    int sum = 0;
    int product = 1;

    // compute the sum and product of the numbers 1 through 5
    for(int i = 1; i <= 5; i++) {
      sum += i; // i is known throughout the loop
      product *= i;
    }

    // but, i is not known here

    System.out.println("Sum is " + sum);
    System.out.println("Product is " + product);
  }
}

// -----------------------------------------

// Demonstrate the while loop.
class WhileDemo {
  public static void main(String[] args) {
    char ch;

    // print the alphabet using a while loop
    ch = 'a';
    while(ch <= 'z') {
      System.out.print(ch);
      ch++;
    }
  }
}

// -----------------------------------------

// Compute integer powers of 2.
class Power {
  public static void main(String[] args) {
    int e;
    int result;

    for(int i=0; i < 10; i++) {
      result = 1;
      e = i;
      while(e > 0) {
        result *= 2;
        e--;
      }

      System.out.println("2 to the " + i +
                         " power is " + result);
    }
  }
}

// -----------------------------------------

// Demonstrate the do-while loop.
class DWDemo {
  public static void main(String[] args)
    throws java.io.IOException {

    char ch;

    do {
      System.out.print("Press a key followed by ENTER: ");
      ch = (char) System.in.read(); // get a char
    } while(ch != 'q');
  }
}

// -----------------------------------------

// Guess the letter game, 4th version.
class Guess4 {
  public static void main(String[] args)
    throws java.io.IOException {

    char ch, ignore, answer = 'K';

    do {
      System.out.println("I'm thinking of a letter between A and Z.");
      System.out.print("Can you guess it: ");

      // read a character
      ch = (char) System.in.read();

      // discard any other characters in the input buffer
      do {
        ignore = (char) System.in.read();
      } while(ignore != '\n');

      if(ch == answer) System.out.println("** Right **");
      else {
        System.out.print("...Sorry, you're ");
        if(ch < answer) System.out.println("too low");
        else System.out.println("too high");
        System.out.println("Try again!\n");
      }
    } while(answer != ch);
  }
}

// -----------------------------------------

/*
    Try This 3-2

    An improved Help system that uses a
    do-while to process a menu selection.
*/
class Help2 {
  public static void main(String[] args)
    throws java.io.IOException {
    char choice, ignore;

    do {
      System.out.println("Help on:");
      System.out.println("  1. if");
      System.out.println("  2. switch");
      System.out.println("  3. for");
      System.out.println("  4. while");
      System.out.println("  5. do-while\n");
      System.out.print("Choose one: ");

      choice = (char) System.in.read();

      do {
        ignore = (char) System.in.read();
      } while(ignore != '\n');
    } while( choice < '1' | choice > '5');

    System.out.println("\n");

    switch(choice) {
      case '1':
        System.out.println("The if:\n");
        System.out.println("if(condition) statement;");
        System.out.println("else statement;");
        break;
      case '2':
        System.out.println("The switch:\n");
        System.out.println("switch(expression) {");
        System.out.println("  case constant:");
        System.out.println("    statement sequence");
        System.out.println("    break;");
        System.out.println("  // ...");
        System.out.println("}");
        break;
      case '3':
        System.out.println("The for:\n");
        System.out.print("for(init; condition; iteration)");
        System.out.println(" statement;");
        break;
      case '4':
        System.out.println("The while:\n");
        System.out.println("while(condition) statement;");
        break;
      case '5':
        System.out.println("The do-while:\n");
        System.out.println("do {");
        System.out.println("  statement;");
        System.out.println("} while (condition);");
        break;
    }
  }
}

// -----------------------------------------

// Using break to exit a loop.
class BreakDemo {
  public static void main(String[] args) {
    int num;

    num = 100;

    // loop while i-squared is less than num
    for(int i=0; i < num; i++) {
      if(i*i >= num) break; // terminate loop if i*i >= 100
      System.out.print(i + " ");
    }
    System.out.println("Loop complete.");
  }
}

// -----------------------------------------

// Read input until a q is received.
class Break2 {
  public static void main(String[] args)
    throws java.io.IOException {

    char ch;

    for( ; ; ) {
      ch = (char) System.in.read(); // get a char
      if(ch == 'q') break;
    }
    System.out.println("You pressed q!");
  }
}

// -----------------------------------------

// Using break with nested loops.
class Break3 {
  public static void main(String[] args) {

    for(int i=0; i<3; i++) {
      System.out.println("Outer loop count: " + i);
      System.out.print("    Inner loop count: ");

      int t = 0;
      while(t < 100) {
        if(t == 10) break; // terminate loop if t is 10
        System.out.print(t + " ");
        t++;
      }
      System.out.println();
    }
    System.out.println("Loops complete.");
  }
}

// -----------------------------------------

// Using break with a label.
class Break4 {
  public static void main(String[] args) {
    int i;

    for(i=1; i<4; i++) {
one:  {
two:    {
three:    {
            System.out.println("\ni is " + i);
            if(i==1) break one;
            if(i==2) break two;
            if(i==3) break three;

            // this is never reached
            System.out.println("won't print");
          }
          System.out.println("After block three.");
        }
        System.out.println("After block two.");
      }
      System.out.println("After block one.");
    }
    System.out.println("After for.");
  }
}

// -----------------------------------------

// Another example of using break with a label.
class Break5 {
  public static void main(String[] args) {

done:
    for(int i=0; i<10; i++) {
      for(int j=0; j<10; j++) {
        for(int k=0; k<10; k++) {
          System.out.println(k + " ");
          if(k == 5) break done; // jump to done
        }
        System.out.println("After k loop"); // won't execute
      }
      System.out.println("After j loop"); // won't execute
    }
    System.out.println("After i loop");
  }
}

// -----------------------------------------

// Where you put a label is important.
class Break6 {
  public static void main(String[] args) {
    int x=0, y=0;

// here, put label before for statement.
stop1: for(x=0; x < 5; x++) {
         for(y = 0; y < 5; y++) {
           if(y == 2) break stop1;
           System.out.println("x and y: " + x + " " + y);
         }
       }

       System.out.println();

// now, put label immediately before {
      for(x=0; x < 5; x++)
stop2: {
         for(y = 0; y < 5; y++) {
           if(y == 2) break stop2;
           System.out.println("x and y: " + x + " " + y);
         }
       }
  }
}

// -----------------------------------------

// This program contains an error.
class BreakErr {
  public static void main(String[] args) {

    one: for(int i=0; i<3; i++) {
      System.out.print("Pass " + i + ": ");
    }

    for(int j=0; j<100; j++) {
      if(j == 10) break one; // WRONG
      System.out.print(j + " ");
    }
  }
}

// -----------------------------------------

// Use continue.
class ContDemo {
  public static void main(String[] args) {
    int i;

    // print even numbers between 0 and 100
    for(i = 0; i<=100; i++) {
      if((i%2) != 0) continue; // iterate
      System.out.println(i);
    }
  }
}

// -----------------------------------------

// Use continue with a label.
class ContToLabel {
  public static void main(String[] args) {

outerloop:
    for(int i=1; i < 10; i++) {
      System.out.print("\nOuter loop pass " + i +
                       ", Inner loop: ");
      for(int j = 1; j < 10; j++) {
        if(j == 5) continue outerloop; // continue outer loop
        System.out.print(j);
      }
    }
  }
}

// -----------------------------------------

/*
   Try This 3-3

   The finished Java statement Help system
   that processes multiple requests.
*/
class Help3 {
  public static void main(String[] args)
    throws java.io.IOException {
    char choice, ignore;

    for(;;) {
      do {
        System.out.println("Help on:");
        System.out.println("  1. if");
        System.out.println("  2. switch");
        System.out.println("  3. for");
        System.out.println("  4. while");
        System.out.println("  5. do-while");
        System.out.println("  6. break");
        System.out.println("  7. continue\n");
        System.out.print("Choose one (q to quit): ");

        choice = (char) System.in.read();

        do {
          ignore = (char) System.in.read();
        } while(ignore != '\n');
      } while( choice < '1' | choice > '7' & choice != 'q');

      if(choice == 'q') break;

      System.out.println("\n");

      switch(choice) {
        case '1':
          System.out.println("The if:\n");
          System.out.println("if(condition) statement;");
          System.out.println("else statement;");
          break;
        case '2':
          System.out.println("The switch:\n");
          System.out.println("switch(expression) {");
          System.out.println("  case constant:");
          System.out.println("    statement sequence");
          System.out.println("    break;");
          System.out.println("  // ...");
          System.out.println("}");
          break;
        case '3':
          System.out.println("The for:\n");
          System.out.print("for(init; condition; iteration)");
          System.out.println(" statement;");
          break;
        case '4':
          System.out.println("The while:\n");
          System.out.println("while(condition) statement;");
          break;
        case '5':
          System.out.println("The do-while:\n");
          System.out.println("do {");
          System.out.println("  statement;");
          System.out.println("} while (condition);");
          break;
        case '6':
          System.out.println("The break:\n");
          System.out.println("break; or break label;");
          break;
        case '7':
          System.out.println("The continue:\n");
          System.out.println("continue; or continue label;");
          break;
      }
      System.out.println();
    }
  }
}

// -----------------------------------------

/*
   Use nested loops to find factors of numbers
   between 2 and 100.
*/
class FindFac {
  public static void main(String[] args) {

    for(int i=2; i <= 100; i++) {
      System.out.print("Factors of " + i + ": ");
      for(int j = 2; j < i; j++)
        if((i%j) == 0) System.out.print(j + " ");
      System.out.println();
    }
  }
}