© The Author(s), under exclusive license to APress Media, LLC, part of Springer Nature 2021
R. VystavělC# Programming for Absolute Beginnershttps://doi.org/10.1007/978-1-4842-7147-6_20

20. First Loops

Radek Vystavěl1  
(1)
Ondřejov, Czech Republic
 

You are entering the most difficult chapters of this book. Loops are a mighty tool that all programmers need as much as the air they breathe. Understanding loops is not easy, which is why you will go through many exercises with loops.

Repeating the Same Text

A loop is a tool to efficiently write repetitions of the same or more often a similar activity. So that you can properly appreciate loops, you will solve some of the tasks twice, first without a loop and then with it. You will start with some repetition of the same activity, and after that you will move on to using loops to repeat similar activities.

Task

You will write a program that displays “I will start learning tomorrow.” ten times in a row (see Figure 20-1).
../images/458464_2_En_20_Chapter/458464_2_En_20_Fig1_HTML.jpg
Figure 20-1

Ten repetitions

Solution

Here is the code:
static void Main(string[] args)
{
    // Output
    Console.WriteLine("I will start learning tomorrow.");
    Console.WriteLine("I will start learning tomorrow.");
    Console.WriteLine("I will start learning tomorrow.");
    Console.WriteLine("I will start learning tomorrow.");
    Console.WriteLine("I will start learning tomorrow.");
    Console.WriteLine("I will start learning tomorrow.");
    Console.WriteLine("I will start learning tomorrow.");
    Console.WriteLine("I will start learning tomorrow.");
    Console.WriteLine("I will start learning tomorrow.");
    Console.WriteLine("I will start learning tomorrow.");
    // Waiting for Enter
    Console.ReadLine();
}

Solution Using a Loop

Think a bit about the previous exercise. Can you imagine that someone might want you to change the displayed sentence? Can you imagine repeating it a hundred times rather than ten times? Can you imagine the number of repetitions being entered by the user?

To solve these problems, you need a new tool: a loop.

Solution

Here is the code:
static void Main(string[] args)
{
    // Output
    for (int count = 0; count < 10; count++)
    {
        Console.WriteLine("I will start learning tomorrow.");
    }
    // Waiting for Enter
    Console.ReadLine();
}

How the for Loop Works

You use the for construction to indicate repetition. Its general syntax looks like this:
for (initializer; loopCondition; iterator)
{
    statement;
    statement;
    statement;
    ...
}
The for loop works like this:

  • initializer is performed once before entering the loop.

  • loopCondition is being evaluated before every turn of the loop. If it holds, the computer enters the loop and executes the statements inside its body.

  • The iterator statement is executed after every turn of the loop is completed. After that, loopCondition is evaluated again.

Figure 20-2 shows the program flow.
../images/458464_2_En_20_Chapter/458464_2_En_20_Fig2_HTML.jpg
Figure 20-2

The program flow

The Loop

In this case, the required number of repetitions is achieved by counting the loop turns performed so far. For that purpose, you use the count variable.

At the beginning (initializer), the variable is set to zero.

After completing every loop turn (iterator), the variable is incremented by one.

The loop body (the display of a line of text) is repeated as long as (loopCondition) the number of lines in the output has not reached ten. As soon as the count variable becomes ten, the condition (count < 10 or 10 < 10) will no longer be fulfilled, the loop will terminate, and the computer will continue executing the statements following the loop.

Explore It Yourself

You should take the time to explore the inner workings of loops to grasp them thoroughly. Use debugging tools you already know: stepping and inspecting the count variable .

Tip

Visual Studio can help you write a for loop without mistakes. Just enter for, press the Tab key twice, and edit the generated loop header.

Choosing the Number of Repetitions

The for loop allows you to solve cases when you do not know the number of repetitions in advance (at the time of code writing).

Task

You will modify the previous exercise to let the user specify the number of the sentence repetitions (see Figure 20-3).
../images/458464_2_En_20_Chapter/458464_2_En_20_Fig3_HTML.jpg
Figure 20-3

Letting the user specify the number of sentence repetitions

Solution

Here’s the code:
static void Main(string[] args)
{
    // Input
    Console.Write("Enter number of repetitions: ");
    string input = Console.ReadLine();
    int howManyTimes = Convert.ToInt32(input);
    // Output
    for (int count = 0; count < howManyTimes; count++)
    {
        Console.WriteLine("I will start learning tomorrow.");
    }
    // Waiting for Enter
    Console.ReadLine();
}

Discussion

Note the following:

  • Compared to the previous task, you replaced the fixed number of repetitions with a variable value entered by the user.

  • Carefully choose the name of the variable to store the required total number of repetitions; here it’s howManyTimes. Specifically, you should distinguish it from the count variable storing the current number of repetitions.

Throwing a Die Repeatedly

You will see one more example of repeating the same activity.

Task

You will write a program that throws a die 20 times (see Figure 20-4).
../images/458464_2_En_20_Chapter/458464_2_En_20_Fig4_HTML.jpg
Figure 20-4

Throwing a die 20 times

Solution

Here’s the code:
static void Main(string[] args)
{
    // Random number generator
    Random randomNumbers = new Random();
    // Output
    for (int count = 0; count < 20; count++)
    {
        int thrown = randomNumbers.Next(1, 6 + 1);
        Console.Write(thrown.ToString() + " ");
    }
    // Waiting for Enter
    Console.ReadLine();
}

Repeating Similar Lines

What if the repeated activity was not the same but just similar?

Task

You will output ten similar lines, differing only in the printed line number (see Figure 20-5).
../images/458464_2_En_20_Chapter/458464_2_En_20_Fig5_HTML.jpg
Figure 20-5

Outputting something similar ten times

Solution Without a Loop

Again, you can start with a solution without a loop to appreciate the importance of loops.

Here’s the code:
static void Main(string[] args)
{
    // Output
    Console.WriteLine("My main to-do list:");
    Console.WriteLine("1. To learn");
    Console.WriteLine("2. To learn");
    Console.WriteLine("3. To learn");
    Console.WriteLine("4. To learn");
    Console.WriteLine("5. To learn");
    Console.WriteLine("6. To learn");
    Console.WriteLine("7. To learn");
    Console.WriteLine("8. To learn");
    Console.WriteLine("9. To learn");
    Console.WriteLine("10. To learn");
    // Waiting for Enter
    Console.ReadLine();
}

Solution Using a Loop

Loops can efficiently solve this type of problem. Actually, you will find yourself incorporating loops to repeat similar activities more often than to repeat precisely the same ones.
static void Main(string[] args)
{
    // Output
    Console.WriteLine("My main to-do list:");
    for (int taskNumber = 1; taskNumber <= 10; taskNumber++)
    {
        Console.WriteLine(taskNumber.ToString() + ". To learn");
    }
    // Waiting for Enter
    Console.ReadLine();
}

Discussion

The following sections discuss the program.

Control Variable

The core of the solution is to use the value of the loop’s control variable inside its body. In this program, you name the variable taskNumber, and you use its value for output.

This is how you achieve displaying one in the first passage of the loop, two in the second passage, and so on.

Check the situation yourself using your debugging tools.

The Loop Starts at 1

The previous exercise (repeatedly throwing a die) used the loop with its control variable running from 0 to 19. Contrary to that, this time it was more convenient to start at 1 rather than at 0. This change also caused the loop condition to change. You used a “less than or equal” test rather than a “less than” one.

Summary

The chapter introduced you to the topic of loops, which are a mighty programming tool allowing you to specify repetitions of the same or, more often, a similar activity.

For loops, C# offers several programming constructs; you learned about the most fundamental for loop in this chapter. In the code, the for loop consists of a header controlling the loop and a body consisting of the statements to be repeated surrounded with braces. The header itself consists of three parts separated by semicolons:

  • The initializer is the statement to be executed once before the loop starts “revolving.”

  • The loop condition is the condition evaluated before each turn of the loop. If it is fulfilled (i.e., evaluated to true), another round of statements of the loop’s body is executed. If it is not fulfilled (i.e., evaluated to false), the loop is terminated, and the program’s execution continues to the statements after the loop.

  • The iterator is the statement to be executed after each turn of the loop.

To get a deeper understanding of how for loops work, definitely use debugging tools like stepping and memory inspection.

The for loop is most often controlled by a variable working more or less like a counter of loop turns. This variable is called the control variable. In the last task, you learned how to use the value of the control variable also inside the loop’s body.

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