Unit 3: Constants, Variables, Data Types, and Operators

1. C Fundamentals

Character Set

The C Character Set is the set of all valid characters you can use in a C program. This includes:

• Alphabets: A-Z, a-z
• Digits: 0-9
• Special Characters: + - * / % & # ! ? , ; : . ( ) { } [ ] _ etc.
• Whitespace: Space, Newline (\n), Tab (\t)

C Tokens

A token is the smallest individual unit in a C program. A line of code is made up of multiple tokens. The main tokens are:

1. Keywords
2. Identifiers
3. Constants
4. Strings
5. Operators
6. Special Symbols (e.g., ;, ,)

Keywords and Identifiers

• Keywords: These are reserved words that have a special meaning to the compiler (e.g., int, if, else, while, return). You cannot use them as variable names.
• Identifiers: These are the names you give to variables, functions, and arrays (e.g., sum, myVariable, main).
  • Rules: Must start with a letter (a-z, A-Z) or an underscore (_). Can be followed by letters, underscores, or digits. Case-sensitive (age and Age are different).

2. Constants

Constants are values that do not change during program execution.

• Integer Constants: 10, -5, 0
• Floating-point Constants: 3.14, -0.005
• Character Constants: 'a', '5', '' (single quotes)
• String Constants: "Hello" (double quotes)

Symbolic Constants

You can give a name to a constant using the #define preprocessor directive. This makes code more readable and easier to maintain.

#include  // This is a symbolic constant
#define PI 3.14159 int main() { float area = PI * 5 * 5; // Compiler replaces PI with 3.14159 return 0;
}

3. Variables

A variable is a named location in memory used to store data. Its value can be changed during program execution.

Declaration of Variables

You must declare a variable before using it. This tells the compiler the variable's type and name.

int age; // Declares a variable 'age' of type int
float marks; // Declares a variable 'marks' of type float
char grade; // Declares a variable 'grade' of type char

Assigning Values to Variables

You assign a value using the assignment operator (=). This can be done at declaration (initialization) or later.

// Initialization
int age = 20; // Assignment
float marks;
marks = 95.5; 

4. Data Types

Data types define the type and size of data a variable can store.

5. Operators and Expressions

An expression is a combination of variables, constants, and operators that evaluates to a single value (e.g., a + 5 * b).

Arithmetic Operators

Used for mathematical calculations.

• + (Addition), - (Subtraction), * (Multiplication), / (Division)
• % (Modulus): Gives the remainder of a division. (e.g., 10 % 3 is 1).

Relational Operators

Used to compare two values. They return 1 (true) or 0 (false).

• == (Equal to)
• != (Not equal to)
• > (Greater than), < (Less than)
• >= (Greater than or equal to), <= (Less than or equal to)

Logical Operators

Used to combine two or more conditions.

• && (Logical AND): True only if both conditions are true.
• || (Logical OR): True if at least one condition is true.
• ! (Logical NOT): Reverses the truth value (true becomes false, false becomes true).

Assignment Operators

Used to assign values to variables.

• = (Simple assignment): a = 10;
• Shorthand operators: a += 5 (same as a = a + 5). Also -=, *=, /=.

Increment and Decrement Operators

• ++ (Increment): Increases value by 1.
• -- (Decrement): Decreases value by 1.
• Pre-increment (++x): Increments first, then uses the value.
• Post-increment (x++): Uses the value first, then increments.

Conditional Operator (Ternary Operator)

A shorthand for an if-else statement.

(condition) ? value_if_true : value_if_false;

int a = 10, b = 20;
int max = (a > b) ? a : b; // 'max' will be 20

Arithmetic Expressions

These are combinations of variables, constants, and arithmetic operators. C follows operator precedence (like BODMAS) to evaluate them. (e.g., * and / have higher precedence than + and -).