C Programming Fundamentals: Structure, Variables, and I/O

Posted on Apr 5, 2026 in Computers

1. C Program Structure and Components

Input Section

  • Input Devices: Used to enter data into the computer system.
  • Converts human-understandable input into computer-controllable data.
  • The CPU accepts information from the user through these devices.
  • Examples: Mouse, Keyboard, Touch screen, Joystick.

Output Section

  • Output Devices: Used to send information from the computer to the outside world.
  • Converts data stored as 1s and 0s into human-understandable information.
  • Examples: Monitor, Printer, Plotter, Speakers.

Central Processing Unit (CPU)

  • Arithmetic Logic Unit (ALU): Performs arithmetic calculations (add, subtract, multiply, compare) and logical decisions.
  • Control Unit (CU): Manages all operations, including reading instructions and data from memory.
  • Primary Memory: Also known as main memory; the CPU directly stores and retrieves information from it.

Secondary Memory

Also called auxiliary or external memory, it is used for permanent data storage.

  • Can be modified easily and stores large amounts of data (Terabytes).
  • Examples: Hard disk, CD, DVD, Pen drive.

C Programming Terminology

  • Token: Individual words and punctuation marks in a passage of text (e.g., keywords, identifiers, constants, strings, operators).
  • Keyword: Reserved words with predefined meanings (e.g., int, float, if, else, while, return).
  • Identifier: Names given to program elements like variables, functions, or arrays (e.g., sum, total_marks).
  • Variable: A named memory location for storing data that can change during execution (e.g., int a = 10;).
  • Constant: Fixed values that do not change during execution (e.g., 10, 3.14, 'A').
  • Compiler: System software that translates high-level source code into machine code at once and reports errors.

2. Rules for Defining Variables

  • Identifiers consist of 1 to 31 characters (alphabets, digits, or underscores) without spaces.
  • C is case-sensitive; NUM and num are treated as different variables.
  • Variable names cannot be reserved keywords (e.g., int, float).

3. Type Conversion in C

Type conversion (or Type Casting) is the process of converting one data type to another.

Implicit Type Conversion

Performed automatically by the compiler when necessary.

int a = 20;
double b = 20.5;
printf("%lf", a + b); // Output: 40.500000

Explicit Type Conversion

■ These conversions are done explicitly by users using the predefined functions.

double a = 4.5, b = 4.6, c = 4.9; int result = (int)da + (int)db + (int)dc; printf(“result = %d”, result);

Output:12


Scanf(); scanf() is a function defined in stdio.h file.

Scanf() is used to take input from user. Syntax:

Scanf(“format specified”,&variable);

Example: int a;

                Scanf(“%d”,&a);

Printf(); printf() is a function defined in stdio.h file.

It displays output on standard output mostly monitor.

Message and value of variable can be printed.

gets(); is used to read a string(including space)from the keyword. syntax: gets(string_name);  example:

Char name[20];

gets(name);

puts(); is used to display a string on the screen.

It automatically Move the cursor to the next line.

syntax:puts(string_name);  example:

Char name[ ]

Puts(name);

getch(); is used to read a single character from the keyword without displaying it on the screen. It is commonly used to pause the program. syntax:getch();

Example:#include

                  getch();

putch(); prints one character at a time to the console.

syntax: putch(character); example:#include