Memory and Storage

A variable is a named storage location in your computer's RAM. When you declare a variable, the computer reserves a specific block of memory for it based on the "Data Type" you choose.

1. Integer Types (int)

Used for whole numbers without any decimal points.

• Standard int: Usually 4 bytes. Range: approximately -2.1 Billion to +2.1 Billion.
• short int: Usually 2 bytes. For when you want to save memory and know the number is small (-32,768 to 32,767).
• long int: 4 or 8 bytes. For larger whole numbers.
• unsigned int: Only positive numbers. It effectively doubles the positive range (0 to 4.2 Billion) by using the bit usually reserved for the negative sign.

2. Floating-Point Types

Used for numbers with decimal points.

• float: 4 bytes. Precision: about 6-7 decimal places. Always use an f suffix like 3.14f to tell C it's a float.
• double: 8 bytes. Precision: about 15-16 decimal places. This is the default type for decimal numbers in C. Use this for most scientific or financial calculations.
• long double: 10, 12, or 16 bytes depending on the hardware. For extreme precision.

3. Character Type (char)

• char: 1 byte. Stores a single character.
• ASCII Encoding: Crucially, C stores characters as numbers. For example, the character 'A' is actually stored as the number 65 in memory. This means you can do math with characters! 'A' + 1 is equal to 'B'.

Variable Declaration & Initialization

In C, you must declare a variable's type before you use it. This allows the compiler to know how much memory to reserve.

int age;           // Declaration (reserves 4 bytes)
age = 25;          // Initialization (stores 25 in those bytes)
float salary = 4500.50f; // Declaration + Initialization
char grade = 'A';  // Single characters always use single quotes

Rules for Identifiers (Names)

• Must start with a letter (a-z, A-Z) or an underscore (_).
• Cannot start with a digit.
• Can contain letters, digits, and underscores.
• Case-sensitive: myVar, MyVar, and myvar are three different variables!
• Cannot be a reserved keyword (like int or while).

Constants

Constants are values that are "read-only" and cannot be changed after they are defined.

1. const keyword: const float PI = 3.14159;
2. #define directive: #define MAX_SPEED 120 (Note: no semicolon or equals sign here).

Format Specifiers Summary

When you want to print or read these types, you use these special codes:

Garbage Values: C does not automatically initialize local variables to zero. A new variable contains "Garbage Value"—whatever random bits were left in that memory spot by a previous program. Always initialize your variables before reading from them!