Conditionals allow programs to ask questions and act or not act based of the results.
if
An if conditional evaluates a given statement that evaluates to a boolean value and executes code in a block scope defined underneath if it evaluates to true.
var boolValue = true;
if(boolValue)
{
Console.WriteLine("boolValue is true");
}
boolValue = false;
if(boolValue)
{
Console.WriteLine("boolValue is still true");
}
Console.WriteLine("End of the example");
boolValue is true
End of the example
boolValue, which initially stores true, is checked in the first if block. Since if is given the value true, the following block scope is executed and the Console.WriteLine function is called. It is good practice to have a program log it's activity for debugging certain values. The 'Console window' snippet above shows that "boolValue is true" and "End of the example" are printed. Notice that the line Console.WriteLine("boolValue is still true") exists as well, but "boolValue is still true" is not printed. This is because the second if is given the value false, instructing the program to not execute any code inside that conditional's block scope.
Unnecessary Conditionals
Because if conditionals take boolean values, a literal value can also be used:
if(true)
{
Console.WriteLine("This is a tautology");
}
if(false)
{
Console.WriteLine("This is a contradiction");
}
This is a tautology
The first if conditional is known as a tautology, while the second is a contradiction. A tautology is always true, and the code block will always execute. A condtradiction is always false, and its code block will never be executed. While these patterns have no practical purpose in a program, the ability to 'disable' a conditional can be a useful debugging tool, making minimal changes to the code:
if(true /*myVar*/)
{
Console.WriteLine("Is this line run?");
}
myVar is a variable defined somewhere, but the check to see if it has the value true has been commented out and replaced with the literal true in order to explicitlly test the code within the scope of the if block.
Else
An if conditional on its own has the power to decide between action and inaction, but often a decision is made between two different actions. The else conditional is used to execute code when the if condition fails.
if(false)
{
Console.WriteLine("The if ran");
}
else
{
Console.WriteLine("The else ran");
}
The else ran
Under the code block of the first if, there is a code block paired with the Keyword else. The value false if given to the if conditional, and so the else block is executed. In any if-else pair, exactly one of the two code blocks will run.
Else If
While often for any given conditional there are only two outcomes, sometimes there are more. The else if keyword combinations allows the chaining of conditional statements.
var valone = false;
var valtwo = false;
if(valone)
{
Console.WriteLine("valone is true");
}
else if(valtwo)
{
Console.WriteLine("valtwo is true");
}
else
{
Console.WriteLine("neither valone or valtwo are true");
}
neither valone or valtwo are true
In the example above valone is evaluated as false in the first if conditional. Since the first if fails the script continues to the else if and evaluates valtwo as false causing the script to continue to the else condition.
Condensed Conditionals
Conditionals don't always require the use of scope operators. If the code block underneath is only one line long, the { and } become unnecessary.
var valone = false;
var valtwo = false;
if(valone)
Console.WriteLine("valone is true");
else if(valtwo)
Console.WriteLine("valtwo is true");
else
Console.WriteLine("neither valone or valtwo are true");
neither valone or valtwo are true
The above rewrite of the last code block illustrates how scoping brackets can be removed to make code more compact.
WARNING: Removing scope operators isn't always good practice, because they need to be added when a code block is more than one statement. Omitting scope operators can unintentionally change the scope of some code, and yet the script can still sucessfully compile.
Uncondensing else if
The else if conditional is actually an else conditional with an if-else pair afterwards. The fact that scope operators can be removed is the reason that else if is valid. Using this logic we can rewrite the last example with explcit scopes to make this clearer.
var valone = false;
var valtwo = false;
if(valone)
{
Console.WriteLine("valone is true");
}
else
{
if(valtwo)
{
Console.WriteLine("valtwo is true");
}
else
{
Console.WriteLine("neither valone or valtwo are true");
}
}
neither valone or valtwo are true
else if may seem like a seperate keyword than if and else but it is actually just syntactical suger to make code easier to follow.
Boolean Operations
While conditionals a useful their real power comes when used in combination with boolean_operators. It is highly suggested you review this page.