Lesson 4: Control Flow

Logic is at the core of almost all programming. Control flow refers to how we cause the program to behave in different ways depending on some truth value.

In TypeScript, and most other programming languages, the type used to express a truth value is called a boolean. A boolean can either have a value of true or false:

let my_bool_1: boolean = true;
let my_bool_2: boolean = false;

If Statements

The most fundamental control flow statement is the if statement, that tells our program to do one thing only if a specific boolean value evaluates to true:

Example #4.0: If Statement

let my_bool: boolean = true;
if (my_bool)
{
    console.log('My bool was true!')
}

This statement is a little more complicated than the console.log statement we learnt about earlier, and comes with both a set of parantheses () and curly brackets {}.

Inside the parantheses we insert a boolean value, and inside the curly brackets we can insert as much code as we want that should execute if the boolean is true.

Else statements

When we create an if statement, we can also follow it up with an else statement, that the program should take if the first path was not taken:

Example #4.1: Else Statement

let my_bool: boolean = true;

if (my_bool)
{
  console.log("My bool was true!");
}
else
{
  console.log("My bool was false!");
}

Else if statements

If statements can also be followed by else if statements, that adds an additional boolean check if the previous path was not chosen:

Example #4.2: Else-if Statement

let my_bool: boolean = false;
let my_other_bool: boolean = true;

if (my_bool)
{
  console.log("my_bool was true!");
}
else if(my_other_bool)
{
  console.log("my_bool was false, but my_other_bool was true!");
}
else
{
  console.log("Both my_bool and my_other_bool were false!");
}

We can have as many else if statements as we want, but we can only have one else statement (try to think about why that is).

Example 4.3: Multiple Else-if Statements

let my_bool: boolean = false;
let my_other_bool: boolean = false;
let my_third_bool: boolean = true;

if (my_bool)
{
  console.log("my_bool was true!");
}
else if(my_other_bool)
{
  console.log("my_bool was false, but my_other_bool was true!");
}
else if(my_third_bool)
{
  console.log("Both my_bool and my_other_bool were false, but my_third_bool was true!");
}
else
{
  console.log("all my bools were false!");
}

Boolean Operators

Just like with arithmetic, there are boolean operators that can transform expressions into boolean values. This is much of what makes programming so powerful.

The == operator

== is a powerful operator that checks if two different expressions have the same value, and resolves into a boolean true/false value that can be used in if statements.

== is especially powerful, as it doesn’t require that its operands are themselves booleans, but can be both strings, numbers or almost any other type. Even when its operands are not booleans, the expression resolves into a boolean value.

Example 4.4: The == Operator

let my_first_string = "a";
let my_second_string = "b";

if (my_first_string == my_second_string)
{
  console.log("The two strings are the same");
}

The == operator has a few siblings that behave similarly:

• < works on numbers and checks if one number is smaller than the other.
• > works on numbers and checks if one number is larger than the other
• != works on any type and checks if one value is not equal to another value

Assignment #4.0: Change the above code to print out a message when the two strings are not equal (assignment) (solution)

The && and || operators

The && operator is used to combine two boolean values into a single value that is only true if both the operands are also true. This operator is called the “and” operator.

Example #4.5: The && Operator

let my_first_bool = true;
let my_second_bool = false;

// This line reads like "my_first_bool AND my_second_bool"
if (my_first_bool && my_second_bool)
{
  console.log("Both my_first_bool and my_second_bool are true!")
}

The && operator has a sibling called ||, which combines into a boolean value that is true if either of its operands are true. This operator is called the “or” operator.

Assignment #4.1: Change the above code to print out a message when either my_first_bool or my_second_bool are equal (assignment) (solution)