24. Selection statements#

In programming, it’s common to encounter situations where different actions need to be performed based on different conditions. This is where the concept of ‘selection’ comes into play. Selection allows us to choose different paths of execution based on certain conditions. In C#, this is achieved using selection statements - namely if statements or switch statements.

The `if` statement#

The if statement is the simplest form of selection. It allows the program to choose whether to execute a certain block of code based on whether a condition is true or false.

Here’s an example:

int numberOfMarbles = 10;

if (numberOfMarbles > 0)
{
    Console.WriteLine("There are marbles in the jar.");
}

There are marbles in the jar.

In this example, the message "There are some marbles in the jar." will only be printed if the condition expression numberOfMarbles > 0 evaluates to true.

You can add an else part to an if statement, which provides an alternative block of code that will be executed if the condition is false.

Here’s an example:

int numberOfMarbles = 0;

if (numberOfMarbles > 0)
{
    Console.WriteLine("There are some marbles in the jar.");
}
else
{
    Console.WriteLine("The jar is empty.");
}

The jar is empty.

In this example, if the expression numberOfMarbles > 0 evaluates to true, the message "There are some marbles in the jar." will be printed. If it evaluates to false, the message "The jar is empty." will be printed instead.

If there are several conditions that need to be checked, you can nest if statements by using the else if syntax. This allows you to check multiple conditions and execute different code blocks depending on which condition is true.

Here’s an example:

int numberOfMarbles = 10;

if (numberOfMarbles > 50)
    Console.WriteLine("Wow, that's a lot of marbles!");
else if (numberOfMarbles > 20)
    Console.WriteLine("That's a fair number of marbles.");
else if (numberOfMarbles > 0)
    Console.WriteLine("Not many marbles, but it's a start!");
else
    Console.WriteLine("No marbles in the jar.");

Not many marbles, but it's a start!

In this example, the program first checks if numberOfMarbles > 50. If this condition is not met, it checks the next condition (numberOfMarbles > 20), and so on. Once a condition is met, the corresponding code block is executed, and the rest of the conditions are ignored. If none of the conditions are met, the code block in the else is executed. This allows for a finer control flow, depending on the value of the variable.

Note that in the example above, we omitted the curly braces surrounding each associated block. This is possible when a block consists of a single statement.

The `switch` statement#

The switch statement provides a way to choose between several alternatives. It’s especially useful when you have a variable that can have one of several definite values and you want to perform different actions for each of these values.

Here’s an example using our TrafficLightColor from the chapter on enums.

TrafficLightColor currentColor = TrafficLightColor.Red;

switch (currentColor)
{
    case TrafficLightColor.Red:
        Console.WriteLine("Stop!");
        break;

    case TrafficLightColor.Yellow:
        Console.WriteLine("Get ready...");
        break;

    case TrafficLightColor.Green:
        Console.WriteLine("Go!");
        break;
}

enum TrafficLightColor { Red, Yellow, Green };

Stop!

In this example, the switch statement checks the value of currentColor. If it’s Red, it prints "Stop!". If it’s Yellow, it prints "Get ready...". If it’s Green, it prints "Go!".

The break keyword is used after each case to indicate that the program should exit the switch statement once it has found a matching case and executed the associated block of code. You must use either break, throw, or return in each associated block. We’ll talk about the latter two keywords in later chapters.

The term “Control flow” refers to the order in which the statements, instructions, or function calls of an imperative or a declarative program are executed or evaluated. The control flow of a program is determined by its control structures (like loops, conditional statements, and function calls), hence the name. It’s about deciding what code to execute next. When we use structures like if, else if, and else, we are controlling the flow of the program, deciding which blocks of code to run based on specific conditions.

These selection statements form the basis for making decisions in your code. In the next chapter, we will explore a different aspect of selection in C#: selection expressions. Stay tuned!

24. Selection statements#

The if statement#

The switch statement#

The `if` statement#

The `switch` statement#