4.1. Intro/Boolean

4.1.1. Selection Statements

Selection statements, sometimes referred to as conditional statements, are one of the three basic constructs (sequence, selection, iteration) in programming. Selection statements provide the logic to perform decision-making: taking different actions based on different conditions. When selecting from possible paths for execution, the decisions are made based on the value of an expression.

C# has the following conditional statements:

• The if statement specifies a block of code to be executed, only if a provided Boolean expression evaluates to true.

• The if-else statement allows you to choose which of the two code paths to follow based on a Boolean expression.

• The else if statement specifies a new condition to test.

• switch statement selects from a list of code to execute based on a pattern match with an expression.

The basic syntax for an if statement is:

Syntax of an if statement

if (condition)
{
  // block of code to be executed if the condition is True
}

where the parenthesized condition of the if statement is a Boolean expression, named after 19th century mathematician George Boole. The parentheses in an if statement delimit the boolean expression; that is, to differentiate the boolean expression from the rest of the if statement; whereas parentheses in an arithmetic expression are used to group expressions together.

4.1.2. Boolean Data Type

The parenthesized condition of the if statement is a Boolean expression, also referred to as relational expression. Boolean expressions are used for control flow statements. In computing, an expression always produce a value. The key characteristic of Boolean expressions is that they always evaluate to either true or false (or Yes vs. No, On vs. Off, or 0 vs. 1). This binary feature is represented as the Boolean binary data type (type bool in C# with possible values of true or false).

In csharprepl, we can perform some quick tests on Boolean data:

> bool isRollaQuiet = true;

> bool isChicagoClose = false;

> isRollaQuiet
true
> isChicagoClose
false
>

You should run a more formal test in VS Code to practice coding. For example, you may create a method for this test like:

Note

In this example, we are calling the method Rolla() in class BooleanExpression by issuing BooleanExpression.Rolla() because it’s a static method. Also note that Rolla() is placed in another class. This is a demonstration to show you that we can call methods in another class as long as they are static.

namespace Program
{

  class Program
  {
    static void Main(string[] args)
    {
      BooleanExpression.Rolla();            // calling static method
    }
  }

  internal class BooleanExpression
  {
    internal static void Rolla()
    {
      bool isRollaQuiet = true;
      bool isChicagoClose = false;
      Console.WriteLine(isRollaQuiet);      // Outputs True
      Console.WriteLine(isChicagoClose);    // Outputs False
    }
  }
}

And the output of this code:

tychen@mac:~/workspaces/workspace/introcscs/Chapter04$ dotnet run
True
False
tychen@mac:~/workspaces/workspace/introcscs/Chapter04$

4.1.3. Boolean Expressions

Since a boolean expression would return a boolean value of either true or false, we can use boolean expressions to return boolean values in conditional testing to build logic in selection/conditional statements. For that we use comparison operators (==, !=, >, <, >=, <=). Note that:

• The < , > , <= , and >= comparison operators are also known as relational operators and are supported by all integral and floating-point numeric types.

• The == (equality) and != (inequality) operators are equality operators and they check if their operands are equal or not. Value types are equal when their value contents (e.g., numerical values or string values) are equal. Reference types are equal when the two variables refer to the same storage.

• The char type also supports comparison operators by comparing the corresponding character codes.

Consider the following sample boolean expressions in the print statements:

Console.WriteLine(7.0 < 5.1);   // output: False
Console.WriteLine(7.0 > 5.1);   // output: True
Console.WriteLine(7.0 <= 5.1);   // output: False
Console.WriteLine(7.0 >= 5.1);   // output: True

int a = 1 + 2 + 3;
int b = 6;
Console.WriteLine(a == b);  // output: True

char c1 = 'a';
char c2 = 'A';
Console.WriteLine(c1 == c2);  // output: False

Reference types will be discussed in subsequent chapters and the following example is provided here for your reference:

public class ReferenceTypesEquality
{
    public class MyClass
    {
        private int id;

        public MyClass(int id) => this.id = id;
    }

    public static void Main()
    {
        var a = new MyClass(1);
        var b = new MyClass(1);
        var c = a;
        Console.WriteLine(a == b);  // output: False
        Console.WriteLine(a == c);  // output: True
    }
}

Understanding boolean expressions above should give you a good sense of how conditional/select statement syntax works:

Syntax of an if statement

if (condition)
{
  // block of code to be executed if the condition is True
}

You should know that the condition is a Boolean expression, which will evaluate to either true or false. The parentheses is a testing construct. If the testing construct results to true, then the code block follows will be executed. Otherwise, the code block follows will not be executed.