C# Do While

Syntax:

do
{
   a block of statement(s);
}while(condition);

• In the statement, we'll run the statements first and then check the condition.
• If the condition returns true, we'll run the statements again.
• Then we'll check the condition again. The loop continues running until the condition returns false.
• If the condition is false, we'll run the following code of the do while statement.
• So comparing with while loop statement in the previous section, the statements in the do while loop run at least once.

Note
The semi-colon at the end of do while statement cannot be omitted.

To compare with while loop statement, we'll change the example 01-32-01 as follows.

using System;

namespace DoWhile1
{
    class Program
    {
        static void Main(string[] args)
        {
            const int MaxRun = 10;
            int i = 0;

            do{
                Console.Write("{0} ", i);
                i++;
            }while( i < MaxRun );

            Console.WriteLine();
            Console.Read();
        }
    }
}

Output

0 1 2 3 4 5 6 7 8 9 

• Line 9: Declare a constant MaxRun to hold the maximum number(exclude itself) to be printed out.
• Line 10: Declare an int variable i and initialize it with 0.
• Line 12: Do while loop statement starts here. We'll run the statements in the curly brackets.
• Line 13: Output i's value 0.
• Line 14: i increments by 1.
• Line 15: It is the first time we check the condition. It returns true. So we will run statements (line 13-14) again. The we'll check the condition again until the condition returns false. After running 10 times, i = 10 and i < MaxRun returns false. Then the control goes to the next statement line 17.
• Line 17: Output a new line. See the result, 0-9 has been outputted.

What will be happened if we remove line 14 and make the increment happened in the condition? Check the example below.

using System;

namespace WhileLoop1
{
    class Program
    {
        static void Main(string[] args)
        {
            const int MaxRun = 10;
            int i = 0;

            do
                Console.Write("{0} ", i);
            while( ++i < MaxRun );

            Console.WriteLine();
            Console.Read();
        }
    }
}

• Line 12-14: Here is the do while loop statement. The curly braces were omitted because there is only one statement running in the loop.
• Line 14: ++i instead of i++ were added because we need to increment first then compare.

The example below will show you how to output an inverted triangle of stars.

using System;

namespace WhileLoop2
{
    class Program
    {
        static void Main(string[] args)
        {
            const int Max = 9;
            int i = Max;

            do{
                int j = 0;
                do{
                    Console.Write("*");
                }while( ++j < i );
                Console.WriteLine();
                i--;
            }while( i > 0 );
            Console.Read();
        }
    }
}

Output

*********
********
*******
******
*****
****
***
**
*

• Line 9: Declare an int constant to hold the maximum lines to be printed.
• Line 10: Declare an int variable i to hold the amount of stars to be printed. The initial value is the constant Max.
• Line 12: Start the outer do while loop. In each loop, we output one line of stars.
• Line 13: Declare a variable j to calculate how many stars were already printed.
• Line 14: Start the nested do while loop to print the stars of each line.
• Line 15: Output one star.
• Line 16: If the condition returns true, we'll go back to line 14 to loop again. Otherwise we'll go to line 17.
• Line 17: Output a new line to make the current line printing over.
• Line 18: i decrements by 1 so at the next cycle we'll print less stars in the next line.
• Line 19: If i>0, the outer loop will be over otherwise we'll go to line 12 to start the next cycle.