While Loop ​

The loop statement allows the computer to perform loop calculations based on conditions, continue the loop when the conditions are met, and exit the loop when the conditions are not met.

For example, to calculate the sum from 1 to 100:

1 + 2 + 3 + 4 + … + 100 = ?

n addition to using sequence formulas, you can let the computer do 100 loop accumulations. Because computers are characterized by very fast calculations, it takes less than 1 second to make the computer cycle 100 million times. Therefore, many calculation tasks cannot be calculated by humans, but computers can calculate them using a simple and crude method such as loops. Results can be obtained quickly.

Let’s first look at the while conditional loop provided by Java. Its basic usage is:

while (conditional-expression) {
Loop statement
}
// Continue to execute subsequent code

The while loop first determines whether the condition is true before each loop starts. If the calculation result is true , execute the statements in the loop body once. If the calculation result is false , jump directly to the end of the while loop and continue execution.

We use a while loop to accumulate from 1 to 100, which can be written like this:

public class Main {
    public static void main(String[] args) {
        int sum = 0; // Accumulated sum, initialized to 0
        int n = 1;
        while (n <= 100) { // The loop condition is n <= 100
            sum = sum + n; // Add n to sum
            n ++; // n adds 1 to itself
        }
        System.out.println(sum); // 5050
    }
}

Note that the while loop first determines the loop condition and then loops. Therefore, it is possible to not perform a loop at all.

For the judgment of loop conditions and the processing of auto-increasing variables, special attention should be paid to boundary conditions. Consider why the following code does not get the correct results:

public class Main {
    public static void main(String[] args) {
        int sum = 0;
        int n = 0;
        while (n <= 100) {
            n ++;
            sum = sum + n;
        }
        System.out.println(sum);
    }
}

If the loop condition is always satisfied, the loop becomes an infinite loop. An infinite loop will cause 100% CPU usage, and users will feel that the computer is running slowly, so avoid writing infinite loop code.

If there is a problem with the logic of the loop condition, it will also cause unexpected results:

public class Main {
    public static void main(String[] args) {
        int sum = 0;
        int n = 1;
        while (n > 0) {
            sum = sum + n;
            n ++;
        }
        System.out.println(n); // -2147483648
        System.out.println(sum);
    }
}

On the surface, the above while loop is an infinite loop. However, Java's int type has a maximum value. After reaching the maximum value, adding 1 will turn it into a negative number. As a result, the while loop is unexpectedly exited.

Practise ​

Use while to calculate the sum from m to n :

public class Main {
	public static void main(String[] args) {
		int sum = 0;
		int m = 20;
		int n = 100;
		// Use while to calculate M+...+N:
		while (false) {
		}
		System.out.println(sum);
	}
}

Summary ​

while loop first determines whether the loop condition is met, and then executes the loop statement;

The while loop may not be executed even once;

When writing loops, pay attention to loop conditions and avoid infinite loops.