[course03] 02 While Loops

The Basic While Loop

A while loop is used to repeat a given statement over and over. Of course, it's not likely that you would want to keep repeating it forever. That would be an infinite loop, which is generally a bad thing

while (boolean-expression)
     statement
     
// with block

while (boolean-expression) {
    statements
}

When the computer comes to a while statement, it evaluates the boolean-expression, which yields either true or false as its value. If the value is false, the computer skips over the rest of the while loop and proceeds to the next command in the program. If the value of the expression is true, the computer executes the statement or block of statements inside the loop.

example : a while loop that simply prints out the numbers 1, 2, 3, 4, 5

int number;   // The number to be printed.
number = 1;   // Start with 1.
while ( number < 6 ) {  // Keep going as long as number is < 6.
    System.out.println(number);
    number = number + 1;  // Go on to the next number.
}
System.out.println("Done!");

The do...while Statement

do
    statement
while ( boolean-expression );

//with block
do {
    statements
} while ( boolean-expression );

example:

boolean wantsToContinue;  // True if user wants to play again.
do {
   Checkers.playGame();
   System.out.print("Do you want to play again? ");
   Scanner keyboard = new Scanner(System.in);
   wantsToContinue = keyboard.nextBoolean();
} while (wantsToContinue == true);

do while and while do

Use a do-while if you must execute the body of the loop at least once.

do {
    doSomething
} while ( boolean-expression );

//has exactly the same effect as

doSomething
while ( boolean-expression ) {
    doSomething
}

while ( boolean-expression ) {
    doSomething
} 

// can be replaced by

if ( boolean-expression ) {
   do {
       doSomething
   } while ( boolean-expression );
}

break and continue

When the computer executes a break statement in a loop, it will immediately jump out of the loop.

while (true) {  // looks like it will run forever!
   System.out.print("Enter a positive number: ");
   N = TextIO.getlnInt();
   if (N > 0)   // the input value is OK, so jump out of loop
      break;
   System.out.println("Your answer must be > 0.");
}
// continue here after break

The continue statement is related to break, but less commonly used. A continue statement tells the computer to skip the rest of the current iteration of the loop. However, instead of jumping out of the loop altogether, it jumps back to the beginning of the loop and continues with the next iteration

example

// Demonstrates the while loop.

public class WhileTest {
  static boolean condition() {
    boolean result = Math.random() < 0.99;
    System.out.print(result + ", ");
    return result;
  }
  public static void main(String[] args) {
    while(condition())
      System.out.println("Inside 'while'");
    System.out.println("Exited 'while'");
  }
} /* (Execute to see output) *///:~

CalculateNumberSum

/**
 * 783 ~ 18
 */
public class CalculateNumberSum {
    public static int calculateSum(int n){
        int sum = 0;
        int cur_number = 0;
        while (n != 0){
            cur_number = n % 10;
            sum = sum + cur_number;
            n = n / 10;
        }
        return sum;
    }

    public static void main(String[] args) {
        System.out.println(calculateSum(785));
    }
}

Prime Number

public class PrimeNumber {
    /**
     * check positive n is a prime number
     *
     * @param n
     * @return
     */
    public static boolean isPrime(int n) {
        if (n == 2){
            return true;
        }
        if (n % 2 == 0){
            return false;
        }
        for (int i = 3; i <= n - 1; i = i+2){
            if (n % i == 0){
                return false;
            }
        }
        return true;
    }

    public static void main(String[] args) {
        boolean flag = isPrime(2);
        System.out.println(flag);
    }
}

Number 4 or 7

find the nth number that is a multiple of either 4 or 7

public class Number4Or7N {
    /**
     *
     * @param n
     * @return
     */
    public static boolean isNumber4Or7(int n){
        if (n % 4 == 0 || n % 7 == 0){
            return true;
        }else {
            return false;
        }
    }

    public static int number4Or7N(int countN){
        int guess = 3;
        int count = 0;
        while(count < countN){
            guess++;
            if (isNumber4Or7(guess)){
                count++;
            }
        }
        return guess;
    }

    public static void main(String[] args) {
        System.out.println(number4Or7N(3));
    }
}

find the nth number that is a

public class PrimeN {

    /**
     * check positive n is a prime number
     *
     * @param n
     * @return
     */
    public static boolean isPrime(int n) {
        if (n == 2){
            return true;
        }
        if (n % 2 == 0){
            return false;
        }
        for (int i = 3; i <= n - 1; i = i+2){
            if (n % i == 0){
                return false;
            }
        }
        return true;
    }

    public static int primeN(int countN){
        int guess = 1;
        int count = 0;
        while (count < countN){
            guess ++;
            if (isPrime(guess)){
                count++;
            }
        }
        return guess;
    }

    public static void main(String[] args) {
        System.out.println(primeN(100));
    }
}