Unit 2: Constant and Variable

1. Programming Language Levels

Programming languages are categorized based on their proximity to computer hardware.

• Machine Language: The lowest level, consisting of 0s and 1s that the CPU understands directly.
• Assembly Language: Uses mnemonics (symbolic names) instead of binary code; requires an Assembler to translate.
• High-Level Languages: Uses English-like commands (e.g., C, Java); easy for humans to read but requires a Compiler or Interpreter.

2. C Program Structure

Every C program follows a standard template to ensure the compiler can process the code correctly.

/* Documentation Section: Description of program */
#include <stdio.h>  // Link Section: Preprocessor directives

// Global Declaration Section (Optional)

int main()          // Main Function Section
{
    // Declaration part
    // Execution part
    
    return 0;
}
    

3. Character Set & Tokens

The Character Set includes all characters that the C compiler recognizes.

• Letters: A-Z and a-z.
• Digits: 0-9.
• Special Characters: Symbols like +, -, *, /, %, &, etc.
• White Spaces: Blank spaces, tabs, and newlines.

4. Identifiers and Keywords

Keywords

Keywords are reserved words that have a predefined meaning to the C compiler. They cannot be used as names for variables.

Rule: There are 32 standard keywords in C (e.g., int, float, if, else, while, return).

Identifiers

Identifiers are user-defined names given to various program elements like variables, functions, and arrays.

• Rules for Identifiers:
  • Must start with a letter or underscore (_).
  • Can contain letters, digits, and underscores.
  • No special characters (except underscore) or spaces allowed.
  • Cannot be a keyword.

5. Constants and Variables

Variables

A variable is a data name used to store a data value that can change during program execution.

Example: int age = 20; (Here, 'age' is a variable).

Constants

A constant is a value that does not change during the program execution.

6. Exam Focus Enhancements