Week 2 - Structured Programming

Table of contents

Control Flow
Loops

In this topic, we will learn about structured programming in C#. Structured programming is a programming paradigm that uses a sequence of statements to perform a task.

Structure programming involves the use of a code block, which is a sequence of statements enclosed in curly braces { }.

// This is a code block
{
    // This is a statement in the code block
    Console.WriteLine("Hello World!");
}

Control Flow

Boolean Expressions and Boolean Operators

A boolean expression is an expression that evaluates to a boolean value and can be chained together using boolean operators.

int min = 0;
int max = 10;
int x = 5;

// Check if x is greater than min and less than max
bool result = x > min && x < max;

// Check if x is equal to min or max
result = x == min || x == max;

// Check if min and max are different
result = min != max;

Some common boolean operators are:

Operator	Description
`&&`	Logical AND
`\|\|`	Logical OR
`!`	Logical NOT

`if` Statements

An if statement is a control flow statement that allows the program to execute a block of code if a condition is true.

// Usually, the condition involves a variable
int x = 5;
bool isPositive = x > 0;

// Check if x is greater than 0
if (isPositive)
{
    // This code block will be executed if x is greater than 0
    Console.WriteLine("x is greater than 0");
}

`else` Statements and nested `if` Statements

An else statement is a control flow statement that allows the program to execute a block of code if a condition is false.

int x = 5;

// Check if x is greater than 0. Notice that the condition is not stored in a variable this time.
if (x > 0)
{
    // This code block will be executed if x is greater than 0
    Console.WriteLine("x is greater than 0");
}
else
{
    // This code block will be executed if x is less than or equal to 0
    Console.WriteLine("x is less than or equal to 0");
}

An else statement can be used in conjunction with an if statement to create a nested if statement.

int x = 5;

// Check if x is greater than 0
if (x > 0)
{
    // This code block will be executed if x is greater than 0
    Console.WriteLine("x is greater than 0");
}
else
{
    // Check if x is less than 0
    if (x < 0)
    {
        // This code block will be executed if x is less than 0
        Console.WriteLine("x is less than 0");
    }
    else
    {
        // This code block will be executed if x is equal to 0
        Console.WriteLine("x is equal to 0");
    }
}

Cascading with `else if`

An else if statement is a control flow statement that allows the program to execute a block of code if a condition is true, and the previous conditions are false.

int year = 2021;

// Check if year is a leap year. 
// A leap year is a year that is divisible by 4, but not by 100, unless it is also divisible by 400.
if (year % 4 == 0)
{
    Console.WriteLine("{0} is a leap year", year);
}
else if (year % 100 == 0)
{
    Console.WriteLine("{0} is not a leap year", year);
}
else if (year % 400 == 0)
{
    Console.WriteLine("{0} is a leap year", year);
}
else
{
    Console.WriteLine("{0} is not a leap year", year);
}

`switch` Statements

A switch statement is a control flow statement that allows the program to execute a block of code based on the value of a variable.

int gpa = 5;

// Returns a description of the GPA:
switch (gpa)
{
    case 7:
        Console.WriteLine("High Distinction");
        break;
    case 6:
        Console.WriteLine("Distinction");
        break;
    case 5:
        Console.WriteLine("Credit");
        break;
    case 4:
        Console.WriteLine("Pass");
        break;
    case 3:
    case 2:
    case 1:
        Console.WriteLine("Fail");
        break;
    default:
        Console.WriteLine("Invalid");
        break;
}

Use the or operator to combine multiple cases together instead of cascading case statements.

int gpa = 5;

// Returns a description of the GPA:
switch (gpa)
{
    case 7:
        Console.WriteLine("High Distinction");
        break;
    case 6:
        Console.WriteLine("Distinction");
        break;
    case 5:
        Console.WriteLine("Credit");
        break;
    case 4:
        Console.WriteLine("Pass");
        break;
    case 3 or 2 or 1:
        Console.WriteLine("Fail");
        break;
    default:
        Console.WriteLine("Invalid");
        break;
}

Loops

A loop is a control flow statement that allows the program to execute a block of code repeatedly.

`while` Loops

A while loop is a control flow statement that allows the program to execute a block of code repeatedly while a condition is true.

// Ask the user to enter a whole number until they enter a parsable number
Console.Write("Enter a whole number: ");
string input = Console.ReadLine();

while(!int.TryParse(input, out int number))
{
    Console.Write("Invalid input. Enter a whole number: ");
    input = Console.ReadLine();
}

`do while` Loops

A do while loop is a control flow statement that allows the program to execute a block of code repeatedly while a condition is true. The difference between a while loop and a do while loop is that the do while loop will always execute the block of code at least once.

// Ask the user to enter a whole number until they enter a parsable number
string input;
do
{
    Console.Write("Enter a whole number: ");
    input = Console.ReadLine();
} while (!int.TryParse(input, out int number));

`for` Loops

A for loop is a control flow statement that allows the program to execute a block of code repeatedly for a specified number of times. There are three parts to a for loop, separated by semicolons:

The initialization part, which is executed once before the loop starts.
The condition part, which is executed repeatedly while the condition is true.
The update part, which is executed repeatedly after each iteration.

// Print the numbers 1 to 10

// Initialization (int i = 1): set i to 1
// Condition (i <= 10): check if i is less than or equal to 10
// Update (i++): increment i by 1
for (int i = 1; i <= 10; i++)
{
    // The second part of the for loop is executed repeatedly while the condition is true
    Console.WriteLine(i);
}

Note that this is equivalent to the following while loop:

int i = 1;
while (i <= 10)
{
    Console.WriteLine(i);
    i++;
}

`foreach` Loops

A foreach loop is a control flow statement that allows the program to execute a block of code repeatedly for each element in a collection.

string[] names = { "Jane", "Mary", "Dan" };

// Print each name in the array
foreach (string name in names)
{
    Console.WriteLine(name);
}

`break` and `continue`

The break statement is a control flow statement that allows the program to exit any loop prematurely.

// Print the numbers 1 to 10, but exit prematurely if i is equal to 5
for (int i = 1; i <= 10; i++)
{
    // Exit the loop if i is equal to 5
    if (i == 5)
    {
        break;
    }

    Console.WriteLine(i);
}

The continue statement is a control flow statement that allows the program to skip the rest of the current iteration of a loop.

// Print the numbers 1 to 10, skipping 5
for (int i = 1; i <= 10; i++)
{
    // Skip the rest of the current iteration if i is equal to 5
    if (i == 5)
    {
        continue;
    }

    Console.WriteLine(i);
}

Week 2 - Structured Programming

Control Flow

Boolean Expressions and Boolean Operators

if Statements

else Statements and nested if Statements

Cascading with else if

switch Statements

Loops

while Loops

do while Loops

for Loops

foreach Loops

break and continue