C# Studying#

What is a variable?#

A variable is a named space in memory that stores a value. In C#, every variable must have a type, which determines:

• how much memory it uses
• what kind of values it can store

Value Types#

Common value types include:

Example:

1
int level = 100;
2
float speed = 3.14f;
3
bool isActive = true;
4
char grade = 'A';

Key point: The type decides what operations are allowed. You cannot treat an int like a bool, even if the value “looks similar”.

Variable Initialization#

Variables can be:

• initialized immediately
• declared first, assigned later

Example:

1
int a1, a2, a3;
2
a1 = 10;
3

4
int b = 20;

If you forget to initialize, the compiler will usually stop you. This is intentional: C# avoids “garbage values”.

Arithmetic Operators#

From the operator lecture :

• + addition
• - subtraction
• * multiplication
• / division
• % remainder

Example:

1
int num1 = 5;
2
int num2 = 2;
3

4
int sum = num1 + num2;   // 7
5
int mod = num1 % num2;   // 1

Division and Casting#

If both operands are integers, integer division happens.

1
int a = 5;
2
int b = 2;
3
float result = a / b;   // result = 2 (not 2.5)

Correct way:

1
float result = (float)a / b; // 2.5

if / else#

Conditionals allow the program to choose a path.

1
int num = 11;
2

3
if (num > 10)
4
{
5
    Debug.Log("num is greater than 10");
6
}
7
else
8
{
9
    Debug.Log("num is 10 or less");
10
}

Conditions must evaluate to bool.

Logical Operators#

• && AND
• || OR

Example:

1
if (num > 0 && num < 20)
2
{
3
    Debug.Log("num is between 1 and 19");
4
}

Important detail: C# uses short-circuit evaluation. The second condition may not be evaluated if the first already determines the result.

switch and enum#

Used when states are discrete and named.

1
enum STATE
2
{
3
    NONE = 0,
4
    INIT, //1
5
    PLAY = 100, //100
6
    OVER, //101
7

8
}
9

10
private int num = 0;
11
    private STATE currentState = STATE.INIT;
12

13
switch(num)
14
{
15
    case 100:
16
        {
17
            Debug.Log("switch 100");
18
        }
19
        break;
20
    case 90:
21
        {
22
            Debug.Log("switch 90");
23
        }
24
        break;
25
    default:
26
        Debug.Log("switch default");
27
        break;
28
}

This appears in the later control-flow section.

for loop#

Used when the number of iterations is known.

1
for (int i = 0; i < 5; i++)
2
{
3
    Debug.Log(i); // 0 1 2 3 4
4
}

while / do-while#

Used when the stopping condition is more important than the count.

Key difference:

• while checks first
• do-while runs at least once

1
int num1 = 0;
2
while (num1 < 10)
3
{
4
    Console.WriteLine(num1);
5
    num1++;
6
}
7

8
int num2 = 0;
9
do
10
{
11
    Console.WriteLine(num2);
12
    num2++;
13
}
14
while (num2 < 5);

break and continue#

1
for (int i = 0; i < 10; i++)
2
{
3
    if (i % 2 == 0) continue;
4
    if (i > 8) break;
5
    Debug.Log(i);
6
}

Why arrays exist#

Arrays store multiple values of the same type in one structure.

1
int[] scores = { 90, 70, 50 };

Access by index:

1
scores[0]; // 90

Looping is almost always required:

1
for (int i = 0; i < scores.Length; i++)
2
{
3
    Debug.Log(scores[i]);
4
}

2D Arrays#

Used when data has rows and columns.

1
int[,] grid = new int[2, 3];

Access:

1
grid[0, 1];

Why methods matter#

Methods:

• reduce duplication
• make code readable
• isolate logic

Important rule: A method should do one thing. If it does more, it probably should be split.

Example#

1
void Print()
2
{
3
    Debug.Log("Hello World");
4
}
5

6
int MaxInt()
7
{
8
    return int.MaxValue;
9
}
10

11
long Sum(int a, int b)
12
{
13
    return a + b;
14
}