Flow control in CSharp

Flow control in C#

In this part of the C# tutorial, we will talk about the flow control. We will define several keywords that enable us to control the flow of a C# program.
In C# language there are several keywords that are used to alter the flow of the program. When the program is run, the statements are executed from the top of the source file to the bottom. One by one. This flow can be altered by specific keywords. Statements can be executed multiple times. Some statements are called conditional statements. They are executed only if a specific condition is met.

The if statement

The if statement has the following general form:

if (expression) 
{
    statement;
}

The if keyword is used to check if an expression is true. If it is true, a statement is then executed. The statement can be a single statement or a compound statement. A compound statement consists of multiple statements enclosed by the block. A block is code enclosed by curly brackets.

using System;

public class CSharpApp
{
    static void Main()
    {
        int num = 31;

        if (num > 0)
        {
            Console.WriteLine("num variable is positive");
        }
    }
}

We have a num variable. It is assigned 31. The if keyword checks for a boolean expression. The expression is put between square brackets. 31 > 0 is true, so the statement inside the block is executed.

$ ./ifstatement.exe 
num variable is positive

The condition is met and the message is written to the console.

using System;

public class CSharpApp
{
    static void Main()
    {
        int num = 31;

        if (num > 0)
        {
            Console.WriteLine("num variable is positive");
            Console.WriteLine("num variable equals {0}", num);
        }
    }
}

More statements can be executed inside the block, enclosed by curly brackets.

---

We can use the else keyword to create a simple branch. If the expression inside the square brackets following the if keyword evaluates to false, the statement following the else keyword is automatically executed.

using System;

public class CSharpApp
{
    static void Main()
    {
        string sex = "female";

        if (sex == "male") {
          Console.WriteLine("It is a boy");
        } else {
          Console.WriteLine("It is a girl");
        }
    }
}

We have a sex variable. It has "female" string. The boolean expression evaluates to false and we get "It is a girl" in the console.

$ ./branch.exe 
It is a girl

We can create multiple branches using the else if keyword. The else if keyword tests for another condition, if and only if the previous condition was not met. Note, that we can use multiple else if keywords in our tests.

using System;

public class CSharpApp
{
    static void Main()
    {
        int a = 0;

        if (a < 0) {
          Console.WriteLine("a is negative");
        } else if (a == 0) { 
          Console.WriteLine("a equals to zero");
        } else {
          Console.WriteLine("a is a positive number");
        }
    }
}

We have a numerical variable and we test it, if it is a negative number or positive or if it equals to zero. The first expression evaluates to false. The second condition is met. The program prints 'a equals to zero' to the console. The rest of the branch is skipped. If the previous conditions were not met, than the statement following the else keyword would be executed.

The switch statement

The switch statement is a selection control flow statement. It allows the value of a variable or expression to control the flow of program execution via a multi way branch. It creates multiple branches in a simpler way than using the combination of if, else if statements.
We have a variable or an expression. The switch keyword is used to test a value from the variable or the expression against a list of values. The list of values is presented with the case keyword. If the values match, the statement following the case is executed. There is an optional default statement. It is executed, if no other match is found.

using System;

public class CSharpApp
{
    static void Main()
    {
        string domain = Console.ReadLine();
    
        switch (domain) {
            case "us":
                Console.WriteLine("United States");
            break;
            case "de":
                Console.WriteLine("Germany");
            break;
            case "sk":
                Console.WriteLine("Slovakia");
            break;
            case "hu":
                Console.WriteLine("Hungary");
            break;
            default:
                Console.WriteLine("Unknown");
            break;
        }
    }
}

In our program, we have a domain variable. We read a value for the variable from the command line. We use the case statement to test for the value of the variable. There are several options. If the value equals for example to "us" the "United States" string is printed to the console.

$ ./selectcase.exe 
hu
Hungary

We have entered "hu" string to the console and the program responded with "Hungary".

The while statement

The while statement is a control flow statement that allows code to be executed repeatedly based on a given boolean condition.
This is the general form of the while loop:

while (expression)
{
    statement;
}

The while keyword executes the statements inside the block enclosed by the curly brackets. The statements are executed each time the expression is evaluated to true.

using System;

public class CSharpApp
{
    static void Main()
    {
        int i = 0;
        int sum = 0;

        while (i < 10)
        {
            i++;
            sum += i;   
        }
        Console.WriteLine(sum);        
    }
}

In the code example, we calculate the sum of values from a range of numbers.
The while loop has three parts. Initialization, testing and updating. Each execution of the statement is called a cycle.

int i = 0;

We initiate the i variable. It is used as a counter.

while (i < 10)
{
   ...
}

The expression inside the square brackets following the whilekeyword is the second phase, the testing. The statements in the body are executed, until the expression is evaluated to false.

i++;

The last, third phase of the while loop. The updating. We increment the counter. Note that improper handling of the while loops may lead to endless cycles.

---

It is possible to run the statement at least once. Even if the condition is not met. For this, we can use the do while keywords.

using System;

public class CSharpApp
{
    static void Main()
    {
        int count = 0;
        
        do { 
            Console.WriteLine(count);
        } while (count != 0);      
    }
}

First the iteration is executed and then the truth expression is evaluated.

The for statement

When the number of cycles is know before the loop is initiated, we can use the for statement. In this construct we declare a counter variable, which is automatically increased or decreased in value during each repetition of the loop.

using System;

public class CSharpApp
{
    static void Main()
    {
        for (int i=0; i<9; i++)
        {
            Console.WriteLine(i);
        }
    }
}

In this example, we print numbers 0..9 to the console.

for (int i=0; i<9; i++)
{
    Console.WriteLine(i);
}

There are three phases. First, we initiate the counter i to zero. This phase is done only once. Next comes the condition. If the condition is met, the statement inside the for block is executed. Then comes the third phase; the couter is increased. Now we repeat the 2, 3 phases until the condition is not met and the for loop is left. In our case, when the counter i is equal to 9, the for loop stops executing.

The foreach statement

The foreach construct simplifies traversing over collections of data. It has no explicit counter. The foreach statement goes through the array or collection one by one and the current value is copied to a variable defined in the construct.

using System;

public class CSharpApp
{
    static void Main()
    {
        string[] planets = { "Mercury", "Venus", 
            "Earth", "Mars", "Jupiter", "Saturn", 
            "Uranus", "Neptune" };

        foreach (string planet in planets)
        {
            Console.WriteLine(planet);
        }
    }
}

In this example, we use the foreach statement to go through an array of planets.

foreach (string planet in planets)
{
    Console.WriteLine(planet);
}

The usage of the foreach statement is straightforward. The planets is the array, that we iterate through. The planet is the temporary variable, that has the current value from the array. The foreach statement goes through all the planets and prints them to the console.

$ ./planets.exe 
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune

Running the above C# program gives this output.

The break, continue statements

The break statement can be used to terminate a block defined by while, foror switch statements.

using System;

public class CSharpApp
{
    static void Main()
    {
        Random random = new Random();

        while (true)
        {            
            int num = random.Next(1, 30);
            Console.Write(num + " ");
            
            if (num == 22)
                break;                 
        }

        Console.Write('\n');
    }
}

We define an endless while loop. We use the break statement to get out of this loop. We choose a random value from 1 to 30. We print the value. If the value equals to 22, we finish the endless while loop.

$ ./break.exe 
29 21 26 6 29 3 10 3 18 6 3 22 

We might get something like this.

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The continue statement is used to skip a part of the loop and continue with the next iteration of the loop. It can be used in combination with for and while statements.
In the following example, we will print a list of numbers, that cannot be divided by 2 without a remainder.

using System;

public class CSharpApp
{
    static void Main()
    {
        int num = 0;
         
        while (num < 1000)
        {
            num++;

            if ((num % 2) == 0)
                continue;
            
            Console.Write(num + " "); 
        }

        Console.Write('\n');
    }
}

We iterate through numbers 1..999 with the while loop.

if ((num % 2) == 0)
    continue;

If the expression num % 2 returns 0, the number in question can be divided by 2. The continue statement is executed and the rest of the cycle is skipped. In our case, the last statement of the loop is skipped and the number is not printed to the console. The next iteration is started.
In this part of the C# tutorial, we were talking about control flow structures.