while Loop

The while loop is a control structure used to repeatedly execute a block of code as long as a specified condition is true. It’s commonly used when the number of iterations isn’t known in advance but depends on a condition that changes as the code runs.

Syntax of while Loop

while (condition) {
    // Code to execute as long as condition is true
}

• Condition: A boolean expression evaluated before each iteration.
• Execution: If the condition is true, the code inside the loop executes. Once the code block finishes, the condition is checked again. If still true, the code executes again. If false, the loop ends.

Example of a while Loop

int counter = 1;
while (counter <= 5) {
    System.out.println("Count is: " + counter);
    counter++;  // Update condition to eventually end the loop
}

In this example, the loop prints numbers from 1 to 5, updating counter each time until counter reaches 6, at which point the loop ends.

Key Points about while Loop

• Entry Condition: The while loop checks the condition before executing the loop body. If the condition is false initially, the loop will not run.
• Infinite Loops: If the condition never becomes false, the loop will run indefinitely. Make sure to update the condition within the loop.

do-while Loop – Ensuring at Least One Execution

The do-while loop is similar to while but guarantees that the code inside the loop executes at least once, even if the condition is initially false.

Syntax of do-while Loop

do {
    // Code to execute at least once
} while (condition);

• Execution: The loop body executes once unconditionally. After that, the condition is evaluated. If true, the code executes again. If false, the loop ends.

Example of a do-while Loop

int counter = 1;
do {
    System.out.println("Count is: " + counter);
    counter++;
} while (counter <= 5);

In this example, the loop prints numbers from 1 to 5, similar to the while loop example. However, even if counter were initialized to a value greater than 5, the loop would still print “Count is: 6” once before ending.

Key Points about do-while Loop

• Exit Condition: The do-while loop checks the condition after executing the loop body, so the code inside runs at least once.
• Use Cases: Useful when you need the loop to run at least one time, such as for menu options in a console application that must display initially.

Choosing Between while and do-while

• while: Use when you may not want the loop to run if the condition is false initially.
• do-while: Use when you need the loop to execute at least once, regardless of the initial condition.

Example Comparison of while and do-while

int number = 0;

// while loop
while (number > 0) {
    System.out.println("Number is greater than 0");
}

// do-while loop
do {
    System.out.println("This will print at least once.");
} while (number > 0);

In this example:

• The while loop does not execute because number > 0 is false.
• The do-while loop executes once, printing "This will print at least once.", and then exits because the condition is false.

Summary

Both while and do-while loops are essential for repeating tasks based on conditions:

• while for conditional, potentially zero-iteration loops.
• do-while for guaranteed one-time execution before condition checking.