Basics of C, Variables & Data Types

Basics of C, Variables & Data Types

C is a foundational, middle-level, procedural programming language developed by Dennis Ritchie at Bell Labs in 1972. It was originally created to develop the UNIX operating system. Because of its incredible speed and close-to-hardware capabilities, C is often called the 'mother of all programming languages'.

1. The Structure of a C Program

• #include <stdio.h>: Instructs the preprocessor to include the Standard Input/Output header file, which contains definitions for printf() and scanf().
• int main(): The entry point of every C program. Execution always begins here.
• printf(): A built-in function used to print output to the console.
• return 0;: Indicates that the program terminated successfully. A non-zero return indicates an error.

2. Compilation Process in C

1. Preprocessing: Removes comments, expands macros (#define), and includes header files. Output is an expanded .i file.
2. Compilation: Converts the expanded code into Assembly code (.s file).
3. Assembly: Converts the assembly code into pure machine code/object code (.o or .obj file).
4. Linking: Combines one or more object files and library files to generate the final executable file (.exe).

3. C Tokens

1. Keywords: 32 reserved words with predefined meanings (e.g., int, if, while, return).
2. Identifiers: Names given to variables, functions, and arrays by the programmer. Rules:
   • Must start with a letter or underscore (_).
   • Cannot contain spaces or special symbols (except _).
   • Cannot be a keyword.
3. Constants: Fixed values that do not change during execution (e.g., 10, 'A', 3.14).
4. Strings: A sequence of characters enclosed in double quotes (e.g., "Hello").
5. Operators: Symbols that perform mathematical or logical operations (e.g., +, -, &&).
6. Special Symbols: Characters with special syntactic meaning (e.g., {}, (), ;, ,).

4. Data Types

4.1 Primary (Primitive) Data Types

• int: Used for integers (whole numbers).
  • Size: 2 or 4 bytes (depending on compiler/architecture).
  • Format Specifier: %d or %i
• char: Used for single characters. Internally stored as ASCII integer values.
  • Size: 1 byte.
  • Format Specifier: %c
• float: Used for single-precision decimal numbers.
  • Size: 4 bytes. Precision: 6 decimal places.
  • Format Specifier: %f
• double: Used for double-precision decimal numbers.
  • Size: 8 bytes. Precision: 15 decimal places.
  • Format Specifier: %lf
• void: Means 'no value'. Used to indicate a function returns nothing.

4.2 Derived & User-Defined Types

• Derived: Arrays, Pointers, Functions.
• User-Defined: struct, union, enum, typedef.

5. Variables & Constants

• Variable: A named memory location whose value can change during execution. Declaration: int age = 25;
• Constant: A value that cannot be altered. Created using the const keyword or #define macro.
  • Example: const float PI = 3.14;

6. Operators in C

1. Arithmetic: +, -, *, /, % (Modulus: returns remainder. Cannot be used with floats).
2. Relational: ==, !=, >, <, >=, <= (Returns 1 for true, 0 for false).
3. Logical:
   • && (Logical AND): True if both operands are true.
   • || (Logical OR): True if at least one operand is true.
   • ! (Logical NOT): Reverses the logical state.
4. Assignment: =, +=, -=, *=, /=. (e.g., a += 5 is a = a + 5).
5. Increment/Decrement: ++, --.
   • Pre-increment (++a): Increase first, then use.
   • Post-increment (a++): Use first, then increase.
6. Bitwise: Operates on bits (&, |, ^, ~, <<, >>).
7. Ternary (Conditional): condition ? expression1 : expression2; (Shorthand for if-else).