Lesson 2: Introduction to C

C is a general-purpose, structured programming language known for efficiency and close-to-hardware control. It uses economy of expression, modern control structures, and a rich set of operations. Often called a "systems programming language" because it is well-suited for writing operating systems and compilers.

Brief History

• BCPL — developed by Martin Richards in 1967; influenced B.
• B — created by Ken Thompson in 1970 for early UNIX on the DEC PDP-7; typeless (no data types).
• C — designed by Dennis Ritchie in 1972 at AT&T Bell Laboratories; expanded B by adding data types.
• Turbo C — developed by Borland International in 1987 for MS-DOS.

Key Definitions

• Interpreter — reads and executes source code one line at a time.
• Compiler — reads the entire program and converts it to object code.
• Compile time — when compilation happens; syntax errors are caught here.
• Object code — machine-language translation of source code; also called binary or machine code.
• Source code — the human-readable text of a program.
• Run time — when the compiled program executes; run-time (semantic) errors appear here.
• Library — a collection of pre-written functions available for use in programs.

When you write a C program and want to run it, it goes through these stages:

Source code (.c file)
      ↓
  Preprocessor   ← handles #include, #define directives
      ↓
  Compiler       ← converts to object code; catches syntax errors
      ↓
  Object code
      ↓
  Linker         ← combines object code with library code
      ↓
  Executable code

An identifier is a name used to reference a variable, function, or other user-defined element.

Rules for naming identifiers in C:

1. The first character must be a letter or underscore (_).
2. Subsequent characters can be letters, digits, or underscores.
3. Spaces and hyphens are not allowed.
4. The first 63 characters are significant.
5. C is case-sensitive: Sname, SNAME, and sname are three different identifiers.
6. An identifier cannot be the same as a C keyword (e.g., int, float, if, while).

C provides several fundamental (scalar) data types:

Variables

A variable is an identifier that holds a value which can change during program execution.

Declaration syntax:

data_type variable1, variable2;

Examples:

int x, y, z;
char a, b;
double s;
float ave;
char Sname[30];

Initializing Variables

Variables contain garbage values until explicitly assigned. Three ways:

1. Assignment statement:

x = -1;
ch1 = 'A';

2. Using scanf:

scanf("%d", &x);

3. At declaration:

int x = 3;
char y = 'x';
double a, b = 100.00;

Global vs. Local Variables

• Global variables — declared outside all functions; accessible throughout the program; initialized to zero if unspecified.
• Local variables — declared inside a function; accessible only within that function; must be explicitly initialized.

Constants

Arithmetic Operators

Important: Integer division truncates: 11/2 = 5. Modulus gives remainder: 11 % 2 = 1.

Prefix vs. postfix increment:

x = 10;
y = ++x;   // x becomes 11 first, then y = 11
x = 10;
y = x++;   // y = 10 first, then x becomes 11

Shorthand operators:

x += y;   // x = x + y
x -= y;   // x = x - y
x *= y;   // x = x * y
x /= y;   // x = x / y
x %= y;   // x = x % y

Relational Operators

Logical Operators

In C, true = any non-zero value, false = 0.

Ternary Operator

expression1 ? expression2 : expression3;

If expression1 is true, result is expression2; otherwise expression3.

Operator Precedence (highest to lowest)

! ++ --           (unary)
* / %
+ -
< <= > >=
== !=
&&
||
? :
= += -= *= /=

Run the code below. Try changing the values and adding more variables.