Learn C#: Logic and Conditionals

Boolean Expressions

A boolean expression is any expression that evaluates to, or returns, a boolean value.

// These expressions all evaluate to a boolean value.
// Therefore their values can be stored in boolean variables.
bool a = (2 > 1);
bool b = a && true;
bool c = !false || (7 < 8);

Boolean Type

The bool data type can be either true or false and is based on the concept that the validity of all logical statements must be either true or false.

Booleans encode the science of logic into computers, allowing for logical reasoning in programs. In a broad sense, the computer can encode the truthfulness or falseness of certain statements, and based on that information, completely alter the behavior of the program.

bool skyIsBlue = true;
bool penguinsCanFly = false;
Console.WriteLine($"True or false, is the sky blue? {skyIsBlue}.");
// This simple program illustrates how booleans are declared. However, the real power of booleans requires additional programming constructs such as conditionals.

Logical Operators

Logical operators receive boolean expressions as input and return a boolean value.

The && operator takes two boolean expressions and returns true only if they both evaluate to true.

The || operator takes two boolean expressions and returns true if either one evaluates to true.

The ! operator takes one boolean expression and returns the opposite value.

// These variables equal true.
bool a = true && true;
bool b = false || true;
bool c = !false;

// These variables equal false.
bool d = true && false;
bool e = false || false;
bool f = !true;

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".

Truth Tables

A truth table is a way to visualize boolean logic. Since booleans only have two possible values, that means that we can compactly list out in a table all the possible input and output pairs for unary and binary boolean operators.

The image below gives the truth tables for the AND, OR, and NOT operators. For each row, the last column represents the output given that the other columns were fed as input to the corresponding operator.

Truth tables for AND, OR, and NOT. Each table cell contains either True or False.

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!

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");

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);

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");
}

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.");
}