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.

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

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.

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.

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!

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”.

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.

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.

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.

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.

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.

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.