Task

In this task, you will display “every other” number until 20 (see Figure 22-1).

Figure 22-1 Displaying every other number

Solution

Here’s the code:

static void Main(string[] args)
{
    // Output
    for (int number = 2; number <= 20; number += 2)
    {
        Console.WriteLine(number);
    }

    // Waiting for Enter
    Console.ReadLine();
}

Discussion

The most difficult point of the exercise is to realize how to write an iterator of the for loop. Since you want to augment the number variable by 2, the corresponding statement will be number += 2.

Alternative Solution

It is interesting to solve the exercise in another way. You can have an ordinary loop from 1 to 10 stepping by 1 and display twice the amount of its control variable rather than the variable itself.

static void Main(string[] args)
{
    // Output
    for (int line = 1; line <= 10; line++)
    {
        int displayedNumber = 2 * line;
        Console.WriteLine(displayedNumber);
    }

    // Waiting for Enter
    Console.ReadLine();
}

Task

In this task, you will display numbers going down from 10 to 1 (see Figure 22-2).

Figure 22-2 Numbers going down

Solution

Here’s the code:

static void Main(string[] args)
{
    // Output
    for (int number = 10; number >= 1; number--)
    {
        Console.WriteLine(number);
    }

    // Waiting for Enter
    Console.ReadLine();
}

Discussion

Note the following:

• The loop’s initializeris possibly the simplest. You just start at ten.

• The iterator is not difficult either; the numbers go down, which is why you just subtract 1 at the end of each turn.

• The most difficult is the loop condition. You have to formulate it in such a way that it holds as long as you want the loop to go on and that it ceases holding at the moment you want to quit. The correct test is whether the number variable is greater than or equal to 1.

Task

In this task, you will generate a series from 9 to 0 with the numbers decreasing by 0.9 in every step (see Figure 22-3).

Figure 22-3 Decreasing by 0.9

Seemingly Correct Solution

Using the style of the previous exercise, you could write the following:

static void Main(string[] args)
{
    // Output
    for (double number = 9; number >= 0; number -= 0.9)
    {
        Console.WriteLine(number.ToString("N1"));
    }

    // Waiting for Enter
    Console.ReadLine();
}

Testing

However, testing discloses the missing last member of the series: 0 (see Figure 22-4).

Figure 22-4 Missing last number

How can that be?

The Cause of the Error

The exercise shows how working with decimal numbers can be tricky; you need to be careful because decimal arithmetic can be imprecise!

You can sense the cause when you omit the formatting on a single decimal place (.ToString("N1")). Try it (see Figure 22-5).

Figure 22-5 Omitting the formatting

You can see that the expected second-to-last series member is slightly less than it is supposed to be. Further subtraction of 0.9 gets you slightly below zero, which is why the expected last zero is not displayed.

Correct Solution

Working with decimal numbers , you need to specify a loop’s terminal value with a slight free play.

The correct solution of the exercise thus looks like this:

static void Main(string[] args)
{
    // Output
    for (double number = 9; number >= -0.0001; number -= 0.9)
    {
        Console.WriteLine(number.ToString("N1"));
    }

    // Waiting for Enter
    Console.ReadLine();
}

Check the result!

Task

In addition to numbers in a 1 to 10 series, you can display the corresponding second power in every line of output (see Figure 22-6).

Figure 22-6 Displaying the second power

Solution

Here’s the code:

static void Main(string[] args)
{
    // Output
    for (int number = 1; number <= 10; number++)
    {
        int secondPower = number * number;
        Console.WriteLine(number.ToString() + " " + secondPower.ToString());
    }

    // Waiting for Enter
    Console.ReadLine();
}

Task

In this task, you will generate a 1 to 20 series with a couple of numbers in every line of output (see Figure 22-7).

Figure 22-7 Displaying more than one number on a line

Solution

This exercise closely resembles the task of the alternating loop from the previous chapter. It can be solved in a number of ways, too. I will choose one of them: you will add a space after an odd number and a line break after an even number .

Here’s the code:

static void Main(string[] args)
{
    // Output
    for (int number = 1; number <= 20; number++)
    {
        Console.Write(number);

        // What goes after the number depends on even/odd test
        if (number % 2 != 0)
        {
            // Odd number, displaying space
            Console.Write(" ");
        }
        else
        {
            // Even number, new line
            Console.WriteLine();
        }
    }

    // Waiting for Enter
    Console.ReadLine();
}

Task

You will have two, a bit arbitrary, number series with a different count of members. The first one is descending by 2 in every step (111, 109, …, 97), and the second one is ascending by 3 in every step (237, 240, …, 270).

The program will display both a number from the first series and a number from the second series in every row (see Figure 22-8).

Figure 22-8 More complex alternating

Solution

Here’s the code:

static void Main(string[] args)
{
    // Preparation
    int first = 111;

    // Output
    for (int second = 237; second <= 270; second += 3)
    {
        // Preparing first text
        string firstText = first >= 97 ?
            first.ToString().PadLeft(3) : "   ";

        // Actual output
        Console.WriteLine(firstText + " " + second.ToString());

        // Changing x
        first -= 2;
    }

    // Waiting for Enter
    Console.ReadLine();
}

Discussion

Note the following:

• One of the series (the longer one) is displayed using the loop’s control variable. The other one uses another independent variable .

• In every step, you check whether the shorter series still goes on.

• To achieve some nice formatting, you use the PadLeft method call, which adds spaces to the left of its parameter to reach the specified total number of characters.