Understanding While and Do-While Loops in Java

Introduction

The while and do-while loops are control flow statements in Java that allow repeated execution of a block of code as long as a specified condition is true. These loops are commonly used when the number of iterations is not known beforehand.

1. The While Loop

The while loop evaluates a condition before executing the loop body. If the condition is true, the loop body is executed. Otherwise, the loop terminates.

Syntax

while (condition) {
  // Code to be executed
}
      

Example

int i = 1;
while (i <= 5) {
  System.out.println("Value of i: " + i);
  i++;
}
      

Output:

Value of i: 1
Value of i: 2
Value of i: 3
Value of i: 4
Value of i: 5
      

2. The Do-While Loop

The do-while loop executes the loop body first and then evaluates the condition. This ensures that the loop body is executed at least once, even if the condition is initially false.

Syntax

do {
  // Code to be executed
} while (condition);
      

Example

int i = 1;
do {
  System.out.println("Value of i: " + i);
  i++;
} while (i <= 5);
      

Output:

Value of i: 1
Value of i: 2
Value of i: 3
Value of i: 4
Value of i: 5
      

Key Differences Between While and Do-While Loops

Examples of While and Do-While Loops

1. While Loop to Find the Sum of Numbers

int sum = 0;
int i = 1;
while (i <= 5) {
  sum += i;
  i++;
}
System.out.println("Sum: " + sum);
      

Output:

Sum: 15
      

2. Do-While Loop for User Input

Scanner scanner = new Scanner(System.in);
int number;
do {
  System.out.print("Enter a number (0 to stop): ");
  number = scanner.nextInt();
  System.out.println("You entered: " + number);
} while (number != 0);
      

Sample Output:

Enter a number (0 to stop): 5
You entered: 5
Enter a number (0 to stop): 3
You entered: 3
Enter a number (0 to stop): 0
You entered: 0
      

Unsolved Problems

  1. Write a program to print 1,2,3,4,5,6,7,8,9,10 using for loop.
  2. Write a program to find the sum of all even numbers between 1 and 100 using a for loop.
  3. Write a program to print 2,4,6,8,10 using for loop.
  4. Write a program to print 1,3,5,7,9 using for loop.
  5. Write a program to print 1,4,9,16,,,,81,100 using for loop.
  6. Write a program to print 2,6,12,20,,,,90 using for loop.
  7. Write a program to print 1/1,1/2,1/3,1/4, 1/10 using for loop.
  8. Write a program to print Print the Fibonacci Sequence using for loop.
  9. Write a program to generate a multiplication table for a given number using a for loop.
  10. Change a given number to its reverse form. 123 will become 321 using for loops.