Java Do While Loop
Java do-while loop is an Exit control loop. Unlike for or while loop, a do-while check for the condition after executing the statements of the loop body.
Example:
public class GFG {
public static void main(String[] args) {
int c = 1;
// Using do-while loop
do {
System.out.println("GeeksforGeeks: " + c);
c++;
} while (c <= 5);
}
}
Output
GeeksforGeeks: 1 GeeksforGeeks: 2 GeeksforGeeks: 3 GeeksforGeeks: 4 GeeksforGeeks: 5
Practical Application of Do While Loop
Suppose you are implementing a game where you show some options to the user, press 1 to do this .., press 2 to do this .. etc and press 'Q' to quit the game. So here you want to show the game menu to the user at least once, so you write the code for the game menu inside the do-while loop.
Syntax:
do
{
// Loop Body
Update_expression
}
// Condition check
while (test_expression);
Note: The test_expression for the do-while loop must return a boolean value , else we would get compile-time error.
Example:
class GFG {
public static void main(String[] args)
{
// initial counter variable
int c = 0;
do {
// Body of loop that will execute minimum 1 time for sure no matter what
System.out.println("Print Statement");
c++;
}
// Checking condition
// Note: It is being checked after inimum 1 iteration
while (c < 0);
}
}
Output
Print Statement
Explanation: In the above code, we figured out that the condition is checked later as the body inside do will get executed one time without fail as the condition is checked later onwards.
Hence whenever we want to display the menu and later on proceed command on the terminal, we always use do-while loop.
Components of do-while Loop
A. Test Expression: In this expression, we have to test the condition. If the condition evaluates to true then we will execute the body of the loop and go to update expression. Otherwise, we will exit from the while loop. For example:
i <= 10
B. Update Expression: After executing the loop body, this expression increments/decrements the loop variable by some value. For example:
i++;
Execution of do-While loop
1. Control falls into the do-while loop.
2. The statements inside the body of the loop get executed.
3. Updation takes place.
4. The flow jumps to Condition
5. Condition is tested.
- If Condition yields true, go to Step 6.
- If Condition yields false, the flow goes outside the loop
6. The flow goes back to Step 2.
Flowchart do-while loop:
Example 1: This program will try to print "Hello World" 5 times.
class GFG {
public static void main(String args[])
{
// Declaring and initialization expression
int c = 1;
// Do-while loop
do {
// Body of do-while loop
// Print statement
System.out.println("Hello World");
// Update expression
c++;
}
// Test expression
while (c < 6);
}
}
Output
Hello World Hello World Hello World Hello World Hello World
Explanation:
- int c = 1; → Start counter at 1.
- do-while loop → Runs at least once.
- System.out.println("Hello World"); → Prints message.
- c++; → Increase counter by 1.
- Loop runs while c < 6.
- Prints "Hello World" 5 times.
Example 2: do-while loop without curly braces {}
import java.io.*;
class GFG {
public static void main(String[] args) {
int c = 1;
do {
// Only single statement in do block
System.out.println("Hello GFG!");
}
// This condition is false, so the loop will execute only once
while (c >= 3);
}
}
Output
Hello GFG!
Related Articles:
Do While Loop
