Iterative Constructs in Java

do-while

Reason — do-while loop is an exit controlled loop, i.e., it checks the condition at the exit point. Therefore, it always executes at least once.

do-while

Reason — do-while is an exit-controlled loop.

repeat

Reason — repeat is not a Java keyword. To repeat a certain action in Java, we use for, while and do-while loops.

jump

Reason — return, continue and break are jump statements in Java.

1

Reason — The for loop has a semicolon ; at the end. Thus, it becomes an empty loop and the program just waits till the loop finishes. Then the program executes the next statement System.out.println("Hello"); and prints Hello on the output screen.

5

Reason — The values of i and sum as per the execution of the for loop are as follows:

Thus, for loop terminates when the value of sum is 5. Hence the output is 5.

Infinite loop

Reason — The loop will run for infinite times as i is initialised as 10 and the update expression i++ increments i after each iteration. Hence, the test expression i > 5 will always remain true.

Loop executes 9 times

Reason — There is a misprint in the book. The loop executes 9 times. Value of i is printed 4 times inside the loop. The condition if (i++ % 2 == 0) will be false when i is odd and true when i is even. Value of i will be incremented after if check is performed due to post increment opertor i++. Execution of the loop is summarized in the table below:

As value of i is now 10, the test condition of while loop becomes false hence, 10^th iteration of the loop is not executed.

A loop is a set of instructions that is continually repeated until a certain condition is met. Looping control structures refer to certain looping constructs which execute a block of code repeatedly until a certain condition remains true. For example, for loop, while loop, do - while loop etc.

The essential parts of a looping control structure are as follows:

1. Initialisation — This segment initialises the loop control variable before starting the loop. It is executed only once at the beginning of the loop.
   For example, int counter = 1;
2. Test-condition — The test-condition is the expression that is evaluated at the beginning of each iteration. Its value determines whether the body of the loop is to be executed (test condition is true) or the loop is to be terminated (test condition is false).
   For example, counter <= 10
3. Update — This is the increment or decrement operation of the control variable. This operation is performed at the end of each iteration.
   For example, counter++ ;

(i) The break statement terminates the current loop or switch statement. The execution then continues from the statement immediately following the current loop or switch statement.

(ii) The continue statement tells the computer to skip the rest of the current iteration of the loop. However, instead of jumping out of the loop completely like break statement, it jumps back to the beginning of the loop and continues with the next iteration. This includes the evaluation of the loop controlling condition to check whether any further iterations are required.

(iii) Unlike break and continue statements which are used to control the flow of execution, System.exit(0) command terminates the execution of the program by stopping the Java Virtual Machine which is executing the program. It is generally used when due to some reason it is not possible to continue with the execution of the program.

The test expression i > 0 will always remain true and hence, the for loop will become an infinite loop and keep on iterating.

The test expression z < 1 && z > 100 will never be true as the conditions z < 1 and z > 100 cannot be true at the same time. Thus, the && operator will always result in false.

The test expression x == y will always remain true as in each iteration x = y is executed, which will store the value of y in x. Thus, an infinite loop will be generated.

Empty statement consists only of a semicolon ;. It is useful when we want to make a loop an empty loop.

To make a for loop an empty loop, we write the following code:

for (int i = 1 ; i <=10 ; i ++);

while loop

int sum = 0, i = 0;
while (i <= 100)    {
    sum = sum + i;
    i++;
}

do-while loop

int sum = 0, i = 0;
do {
    sum = sum + i;
    i++;
} while(i <= 100);

The loop repeats for infinite times.

The given code gives no output.

The value of x is 1. The test condition of while loop — x < 10 is true but the test condition of if — x % 2 == 0 is false. In the absence of update expression, the while loop continues infinitely.

The loop repeats for infinite times.

The given code gives no output.

The value of y is 1. The test condition of while loop — y < 10 is true but the test condition of if — y % 2 == 0 is false. Thus, the control does not enter the if statement and the value of y remains 1 infinitely. Thus, the while loop continues infinitely.

The loop executes 9 times.

3   
5   
7   
9   

The loop executes 9 times. Value of z is printed 4 times inside the loop. The condition if((z++) % 2 == 0) will be false when z is odd and true when z is even. Value of z will be incremented after if check is performed due to post increment operator z++. Execution of the loop is summarized in the table below:

As value of z is now 10, the test condition of while loop becomes false hence, 10^th iteration of the loop is not executed.

import java.util.Scanner;

public class KboatIntegers
{
    public static void main(String args[]) {
        Scanner in = new Scanner(System.in);
        int pSum = 0, pCount = 0, nCount = 0;
        System.out.print("Enter the number of integers: ");
        int num = in.nextInt();
        
        System.out.println("Enter " + num + " integers: ");
        for (int i = 1; i <= num; i++) {
            int n = in.nextInt();
            if (n >= 0) {
                pSum += n;
                pCount++;
            }
            else 
                nCount++;
        }
        
        System.out.println("Positive numbers = " + pCount);
        System.out.println("Negative numbers = " + nCount);
        System.out.println("Sum of positive numbers  = " + pSum);
    }
}

public class KboatSumOdd
{
    public static void main(String args[]) {
        long sumOdd = 0;
        int i = 1;
        do {
            sumOdd += i;
            i += 2;
        } while(i <= 50);
                    
        System.out.println("Sum of 50 odd positive numbers = " + sumOdd);        
    }
}

import java.util.Scanner;

public class KboatTribonacci
{
    public static void main(String args[]) {
        Scanner in = new Scanner(System.in);
        System.out.print("Enter no. of terms : ");
        int n = in.nextInt();
        
        if(n < 3)
            System.out.print("Enter a number greater than 2");
        else    {
            int a = 0, b = 0, c = 1;
            System.out.print(a + " " + b + " " + c);
        
            for (int i = 4; i <= n; i++) {
                int term = a + b + c;
                System.out.print(" " + term);
                a = b;
                b = c;
                c = term;
            }
        }
    }
}

public class KboatValOfPi
{
        public static void main(String args[]) {
            double pi = 0.0d, j = 1.0d;
            int i = 1;
            while(i <= 100000)  {
                if(i % 2 == 0)
                    pi -= 4/j;
                else
                    pi += 4/j;
                j += 2;
                i++;
            }
            System.out.println("Pi = " + pi);
        
    }
}

import java.util.Scanner;

public class KboatMenuPattern
{
    public static void main(String args[]) {
        Scanner in = new Scanner(System.in);
        System.out.println("Enter 1 for pattern 1");
        System.out.println("Enter 2 for Pattern 2");
        System.out.print("Enter your choice: ");
        int choice = in.nextInt();
        switch (choice) {
            case 1:
            for (int i = 69; i >= 65; i--) {
                for (int j = 65; j <= i; j++) {
                    System.out.print((char)j);
                }
                System.out.println();
            }
            break;

            case 2:
            String word = "BLUE";
            int len = word.length();
            for(int i = 0; i < len; i++) {
                for(int j = 0; j <= i; j++) {
                    System.out.print(word.charAt(i));
                }
                System.out.println();
            }
            break;

            default:
            System.out.println("Incorrect choice");
            break;

        }
    }
}

import java.util.Scanner;

public class KboatPronicNumber
{
    public static void main(String args[]) {
        Scanner in = new Scanner(System.in);
        System.out.print("Enter the number to check: ");
        int num = in.nextInt();
        
        boolean isPronic = false;
        
        for (int i = 1; i <= num - 1; i++) {
            if (i * (i + 1) == num) {
                isPronic = true;
                break;
            }
        }
        
        if (isPronic)
            System.out.println(num + " is a pronic number");
        else
            System.out.println(num + " is not a pronic number");
    }
}

import java.util.Scanner;

public class KboatSpyNumber
{
    public static void main(String args[]) {
        
        Scanner in = new Scanner(System.in);
        
        System.out.print("Enter Number: ");
        int num = in.nextInt();
        
        int digit, sum = 0;
        int orgNum = num;
        int prod = 1;
        
        while (num > 0) {
            digit = num % 10;
            
            sum += digit;
            prod *= digit;
            num /= 10;
        }
        
        if (sum == prod)
            System.out.println(orgNum + " is Spy Number");
        else
            System.out.println(orgNum + " is not Spy Number");
        
    }
}

import java.util.Scanner;

public class KboatNivenNumber
{
    public static void main(String args[]) {
        Scanner in = new Scanner(System.in);
        System.out.print("Enter number: ");
        int num = in.nextInt();
        int orgNum = num;
        
        int digitSum = 0;
        
        while (num != 0) {
            int digit = num % 10;
            num /= 10;
            digitSum += digit;
        }
        
        /*
         * digitSum != 0 check prevents
         * division by zero error for the
         * case when users gives the number
         * 0 as input
         */
        if (digitSum != 0 && orgNum % digitSum == 0)
            System.out.println(orgNum + " is a Niven number");
        else
            System.out.println(orgNum + " is not a Niven number");
    }
}

import java.util.Scanner;

public class KboatFibonacciNDigitSum
{
    public static void main(String args[]) {
        Scanner in = new Scanner(System.in);
        System.out.println("1. Fibonacci Series");
        System.out.println("2. Sum of digits");
        System.out.print("Enter your choice: ");
        int ch = in.nextInt();

        switch (ch) {
            case 1:
            int a = 0, b = 1;
            System.out.print(a + " " + b);
            /*
             * i is starting from 3 below
             * instead of 1 because we have 
             * already printed 2 terms of
             * the series. The for loop will 
             * print the series from third
             * term onwards.
             */
            for (int i = 3; i <= 10; i++) {
                int term = a + b;
                System.out.print(" " + term);
                a = b;
                b = term;
            }
            break;

            case 2:
            System.out.print("Enter number: ");
            int num = in.nextInt();
            int sum = 0;
            while (num != 0) {
                sum += num % 10;
                num /= 10;
            }
            System.out.println("Sum of Digits " + " = " + sum);
            break;

            default:
            System.out.println("Incorrect choice");
            break;
        }
    }
}

import java.util.Scanner;

public class KboatSpecialNum
{
    public static void main(String args[]) {
        Scanner in = new Scanner(System.in);
        System.out.print("Enter number: ");
        int num = in.nextInt();

        int t = num;
        int sum = 0, fact;
        while (t != 0) {
            int d = t % 10;
            fact = 1;
            for (int i = 1; i <= d; i++) 
                fact *= i;
            sum += fact;
            t /= 10;
        }

        if (sum == num)
            System.out.println(num + " is a special number");
        else
            System.out.println(num + " is not a special number");

    }
}

import java.util.Scanner;

public class KboatPrimeAutomorphic
{
    public static void main(String args[]) {
        Scanner in = new Scanner(System.in);
        System.out.println("1. Prime number");
        System.out.println("2. Automorphic number");
        System.out.print("Enter your choice: ");
        int choice = in.nextInt();
        System.out.print("Enter number: ");
        int num = in.nextInt();

        switch (choice) {
            case 1:
            int c = 0;
            for (int i = 1; i <= num; i++) {
                if (num % i == 0) {
                    c++;
                }
            }
            if (c == 2) 
                System.out.println(num + " is Prime");
            else
                System.out.println(num + " is not Prime");
            break;

            case 2:
            int numCopy = num;
            int sq = num * num;
            int d = 0;

            /*
             * Count the number of 
             * digits in num
             */
            while(num > 0) {
                d++;
                num /= 10;
            }

            /*
             * Extract the last d digits
             * from square of num
             */
            int ld = (int)(sq % Math.pow(10, d));

            if (ld == numCopy)
                System.out.println(numCopy + " is automorphic");
            else
                System.out.println(numCopy + " is not automorphic");
            break;

            default:
            System.out.println("Incorrect Choice");
            break;
        }
    }
}

import java.util.Scanner;

public class KboatBuzzGCD
{
    public static void main(String args[]) {
        Scanner in = new Scanner(System.in);
        System.out.println("1. Buzz number");
        System.out.println("2. Calculate GCD");
        System.out.print("Enter your choice: ");
        int choice = in.nextInt();

        switch (choice) {
            case 1:
                System.out.print("Enter a number: ");
                int num = in.nextInt();
                if (num % 10 == 7 || num % 7 == 0)
                    System.out.println(num + " is a Buzz Number");
                else
                    System.out.println(num + " is not a Buzz Number");
            break;

            case 2:
                System.out.print("Enter first number: ");
                int a = in.nextInt();
                System.out.print("Enter second number: ");
                int b = in.nextInt();
                while (b != 0) {
                    int t = b;
                    b = a % b;
                    a = t;
                }
                System.out.println("GCD = " + a);
                break;

            default:
            System.out.println("Incorrect Choice");
            break;
        }
    }
}

import java.util.Scanner;

public class KboatSeries
{
    public static void main(String args[]) {
        Scanner in = new Scanner(System.in);
        System.out.print("Enter n: ");
        int n = in.nextInt();
        int x = 3;
        double sum = 0;
        
        for (int i = 1; i <= n; i++) {
                double term = Math.pow(x, i);
                if (i % 2 == 0)
                    sum -= term;
                else
                    sum += term;
            }
            System.out.println("Sum = " + sum);
    }
}

import java.util.Scanner;

public class KboatSeries
{
    public static void main(String args[]) {

        Scanner in = new Scanner(System.in);
        System.out.print("Enter x: ");
        int x = in.nextInt();
        System.out.print("Enter n: ");
        int n = in.nextInt();
        double sum = 0;

        for (int i = 1; i <= n; i++) {
            sum += (i / Math.pow(x, i));
        }
        
        System.out.println("Sum=" + sum);
    }
}

import java.util.Scanner;

public class KboatSeries
{
    public static void main(String args[]) {
        double sum = 0;
        for (int i = 1; i <= 49; i++)
            sum += i / (double)(i + 1);
        System.out.println("Sum = " + sum);
    }
}

public class KboatSeries
{
    public static void main(String args[]) {    
        double sum = 0;
        for (int i = 1; i <= 10; i++)
            sum += i / Math.sqrt(i);
        System.out.println("Sum = " + sum);
    }
}