Conditionals and Comparison

So far our programs have been quite linear going step-by-step in order without any options or alternatives. We do often want options in our software. Meaning that functionality is dependant on the state of variables in the program.

For the program to branch we will need a conditional statement in the program.

Console.WriteLine("Hello world!");
if (true) 
{
    Console.WriteLine("This is always printed!");
}

Hello world!
This is always printed!

On the other hand we could make part of our code unreachable.

Console.WriteLine("Hello world!");
if (false) 
{
    Console.WriteLine("This is never printed!");
}

Hello world!

A conditional statement begins with a keyword if followed by the round brackets (). Inside the round brackets is the expression that is evaluated. The result of the evaluation is a truth value. In the examples above there was no need for evaluation as they already were truth values.

The brackets are followed by a code block enclosed in the curly brackets {}. The code inside the block is run if the expression inside the round brackets is evaluated as true.

Let us examine an example where we compare integers.

int number = 11;
if (number > 10) 
{
    Console.WriteLine("The number was larger than 10");
}

If this conditional statement is evaluated as true: ‘if the value in the variable number is greater than 10’ the code execution moves inside the block defined within the conditional statement.

If the statement would be false the code execution moves to the next line after the closing curly bracket of the code block.

Notice that there is no semicolon after the if clause as the statement does not end after the conditional part.

Reminder of Code Indent

The code inside a block should be indented. For example the code inside an if statement should be indented more than the keyword if in the code. The ending curly braket } should be at the same level as the if.

int number = 11;
if (number > 10) 
{
Console.WriteLine("This indention is wrong");
}

int number = 11;
if (number > 10) 
{
    Console.WriteLine("This indention is right");
}

Relational Operators

Below are the relational operators and an example how they are used.

int number = 55;

if (number != 0) 
{
    Console.WriteLine("The number was not equal to zero");
}

if (number >= 1000) 
{
    Console.WriteLine("The number was at least 1000");
}

The number was not equal to zero

Else Option

If the expression inside the if clause evaluates as false the code execution continues to the next statement. This is not always desired, but we want to have an option for the cases when the if is evaluated as false.

This can be achieved with an else statement that is combined with the if statement.

int number = 4;

if (number > 5) 
{
    Console.WriteLine("Your number is greater than five!");
} 
else 
{
    Console.WriteLine("Your number is 5 or less!");
}

Your number is 5 or less!

If the conditional statement has an else branch the code block defined for the else is run incase the if clause is evaluated as false. Notice the indentation!

More Options

If you want to have more than one option use else if structure. It is similar to else, but has an if conditional. There can be multiple of them and they come after if and before else.

int number = 3;

if (number == 1) 
{
    Console.WriteLine("Number is one!");
} 
else if (number == 2) 
{
    Console.WriteLine("Number is two!");
} 
else if (number == 3) 
{
    Console.WriteLine("Number is three!");
} 
else 
{
    Console.WriteLine("Number is something else!");
}

Number is three!

In the example we first check if the number is equal to 1. As it is not we move to the first else-if and compare the number to value of 2. As this is not the case we move forward and compare our variable value to 3. As this is true we execute the code inside the code block and print the message shown above. We do not go into the else statement because an earlier statement evaluated as true.

Order of Comparison

As any code the comparisons are done in order, from top to bottom, left to right. When we reach a conditional which evaluates to true we execute that block and end comparison.

int number = 42;

if (number == 0) 
{
    Console.WriteLine("The number is 0.");
} 
else if (number > 0) 
{
    Console.WriteLine("The number is greater than 0.");
} 
else if (number > 2) 
{
    Console.WriteLine("The number is greater than 2.");
} 
else 
{
    Console.WriteLine("The number is smaller than 0.");
}

The number is greater than 0.

In the example the condition number > 0 is evaluated as true so we execute the code block related to that and end comparison. Even if the next statement would also evaluate to true we do not reach that part of the code.

Conditional Statement and Boolean Variable

The evaluated value of an expression inside the if () round brackets must be a boolean. Boolean is a representation of a truth value and is either true or false.

bool truthValue = true;
Console.WriteLine("The value of truthValue is " + truthValue);

The value of truthValue is True

With a conditional.

bool truthValue = true;
if (truthValue)
{
    Console.WriteLine("This is awesome!");
}

This is awesome!

Comparison can be used outside a statement. Then the value of a boolean is stored into a boolean variable until further use.

int first = 1;
int second = 3;
bool isFirstLargerThanSecond = first > second;

Now the value of isFirstLargerThanSecond is false. Let’s change the example a bit and continue.

int first = 1;
int second = 3;
bool isFirstSmallerThanSecond = first < second;

if (isFirstSmallerThanSecond)
{
    Console.WriteLine(first + " is less than " + second + "!");
}

1 is less than 3!

Remainder

Remainder is not used as frequently, but is a nice tool, especially if we want to check if something is divisible by some other number.

int remainder = 7 % 2;
Console.WriteLine(remainder); // prints 1
Console.WriteLine(5 % 3); // prints 2
Console.WriteLine(7 % 4); // prints 3
Console.WriteLine(8 % 4); // prints 0
Console.WriteLine(1 % 2); // prints 1

As remainder is an operation similar to other calculations we can use it in an if clause.

string userInput = Console.ReadLine();
int number = Convert.ToInt32(userInput);

if ((number % 400) == 0) 
{
    System.out.println("The number " + number + " is divisible by four hundred");
} 
else 
{
    System.out.println("The number " + number + " is not divisible by four hundred");
}

Conditionals an Equality of Variables

In most programming languages string is of reference type where as integer, boolean and double are value types.

In some of the programming languages this means that string comparison has to be done differently from other variables. C# programming is more forgiving. We can compare two strings with == operator. At least at this point of the course.

For example the following works in C# programming while in most other languages it would not.

string a = "word";
string b = "word";
Console.WriteLine(a == b);

True

Other way to compare strings is with the Equals method. The method compares the value of an object. We will go deeper into that later. For now it is enough for you to know such a method exists.

string a = "word";
string b = "word"; 

Console.WriteLine(a.Equals(b));

True

You can now do the exercises for Conditionals and Comparison.