Learn C#: Conditionals

If Statements

In C#, an if statement executes a block of code based on whether or not the boolean expression provided in the parentheses is true or false.

If the expression is true then the block of code inside the braces, {}, is executed. Otherwise, the block is skipped over.

if (true) {
  // This code is executed.
  Console.WriteLine("Hello User!");
}

if (false) {
  // This code is skipped.
  Console.WriteLine("This won't be seen :(");
}

Break Keyword

One of the uses of the break keyword in C# is to exit out of switch/case blocks and resume program execution after the switch code block. In C#, each case code block inside a switch statement needs to be exited with the break keyword (or some other jump statement), otherwise the program will not compile. It should be called once all of the instructions specific to that particular case have been executed.

string color = "blue";

switch (color) {
  case "red":
    Console.WriteLine("I don't like that color.");
    break;
  case "blue":
    Console.WriteLine("I like that color.");
    break;
  default:
    Console.WriteLine("I feel ambivalent about that color.");
    break;
}
// Regardless of where the break statement is in the above switch statement, 
// breaking will resume the program execution here.
Console.WriteLine("- Steve");

Comparison Operators

A comparison operator, as the name implies, compares two expressions and returns either true or false depending on the result of the comparison. For example, if we compared two int values, we could test to see if one number is greater than the other, or if both numbers are equal. Similarly, we can test one string for equality against another string.

int x = 5;
Console.WriteLine(x < 6); // Prints "True" because 5 is less than 6.
Console.WriteLine(x > 8); // Prints "False" because 5 is not greater than 8.

string foo = "foo";
Console.WriteLine(foo == "bar"); // Prints "False" because "foo" does not equal "bar".

Switch Statements

A switch statement is a control flow structure that evaluates one expression and decides which code block to run by trying to match the result of the expression to each case. In general, a code block is executed when the value given for a case equals the evaluated expression, i.e, when == between the two values returns true. switch statements are often used to replace if else structures when all conditions test for equality on one value.

// The expression to match goes in parentheses.
switch (fruit) {
  case "Banana":
    // If fruit == "Banana", this block will run.
    Console.WriteLine("Peel first.");
    break;
  case "Durian":
    Console.WriteLine("Strong smell.");
    break;
  default:
    // The default block will catch expressions that did not match any above.
    Console.WriteLine("Nothing to say.");
    break;
}

// The switch statement above is equivalent to this:
if (fruit == "Banana") {
  Console.WriteLine("Peel first.");
} else if (fruit == "Durian") {
  Console.WriteLine("Strong smell.");  
} else {
  Console.WriteLine("Nothing to say.");
}

Else Clause

An else followed by braces, {}, containing a code block, is called an else clause. else clauses must always be preceded by an if statement.

The block inside the braces will only run if the expression in the accompanying if condition is false. It is useful for writing code that runs only if the code inside the if statement is not executed.

if (true) {
  // This block will run.
  Console.WriteLine("Seen!");
} else {
  // This will not run.
  Console.WriteLine("Not seen!");
}

if (false) {
  // Conversely, this will not run.
  Console.WriteLine("Not seen!");
} else {
  // Instead, this will run.
  Console.WriteLine("Seen!");
}

If and Else If

A common pattern when writing multiple if and else statements is to have an else block that contains another nested if statement, which can contain another else, etc. A better way to express this pattern in C# is with else if statements. The first condition that evaluates to true will run its associated code block. If none are true, then the optional else block will run if it exists.

int x = 100, y = 80;
 
if (x > y)
{
  Console.WriteLine("x is greater than y");
} 
else if (x < y) 
{
  Console.WriteLine("x is less than y");
} 
else
{
  Console.WriteLine("x is equal to y");
}

Conditional Control

Conditional statements or conditional control structures allow a program to have different behaviors depending on certain conditions being met.

Intuitively, this mimics the way humans make simple decisions and act upon them. For example, reasoning about whether to go outside might look like:

Condition: Is it raining outside?
- If it is raining outside, then bring an umbrella.
- Otherwise, do not bring an umbrella.

We could keep adding clauses to make our reasoning more sophisticated, such as “If it is sunny, then wear sunscreen”.

Control Flow

In programming, control flow is the order in which statements and instructions are executed. Programmers are able to change a program’s control flow using control structures such as conditionals.

Being able to alter a program’s control flow is powerful, as it lets us adapt a running program’s behavior depending on the state of the program. For example, suppose a user is using a banking application and wants to withdraw $500. We certainly want the application to behave differently depending on whether the user has $20 or $1000 in their bank account!

Ternary Operator

In C#, the ternary operator is a special syntax of the form: condition ? expression1 : expression2.

It takes one boolean condition and two expressions as inputs. Unlike an if statement, the ternary operator is an expression itself. It evaluates to either its first input expression or its second input expression depending on whether the condition is true or false, respectively.

bool isRaining = true;
// This sets umbrellaOrNot to "Umbrella" if isRaining is true,
// and "No Umbrella" if isRaining is false.
string umbrellaOrNot = isRaining ? "Umbrella" : "No Umbrella";

// "Umbrella"
Console.WriteLine(umbrellaOrNot);