Computer Programming in C

Basics and Introduction to C

1.1 History of C

The C programming language was developed by Dennis Ritchie at Bell Laboratories (AT&T) in 1972. It evolved from two earlier languages — BCPL (Basic Combined Programming Language, by Martin Richards, 1967) and B (by Ken Thompson, 1970). C was originally designed to re-implement the UNIX operating system, which had previously been written in assembly language.

1.2 Features of C

• Middle-level language — combines features of high-level and low-level languages
• Structured programming — supports functions and blocks
• Portability — C programs can be compiled on different platforms with minimal changes
• Rich operator set — arithmetic, bitwise, logical, relational, and more
• Pointers — direct memory access and manipulation
• Fast execution — compiled to machine code, close to hardware
• Extensible — new functions can be added to the library
• Recursion — functions can call themselves

1.3 Applications of C

C is used to build: Operating systems (Linux, Windows kernel), Embedded systems (microcontrollers, IoT), Compilers (GCC), Database engines (MySQL, PostgreSQL), Game engines, Device drivers, and Networking tools.

1.4 Structure of a C Program

C
/* Documentation Section (optional) */
/* Preprocessor Directives */
#include <stdio.h>

/* Global Declarations (optional) */
int globalVar = 10;

/* Main Function */
int main() {
    /* Local Declarations */
    int a = 5;

    /* Executable Statements */
    printf("Hello, World!\n");

    return 0;
}

/* User-defined Functions (optional) */

1.5 The C Character Set

1.6 Identifiers and Naming Rules

An identifier is a name given to variables, functions, arrays, or any user-defined item. Rules:

• Must begin with a letter (A-Z, a-z) or underscore (_)
• Can contain letters, digits (0-9), and underscores
• Cannot start with a digit
• Cannot use C keywords as identifiers
• C is case-sensitive: age and Age are different
• No limit on length (but first 31 characters are significant in C89)

1.7 Keywords in C

C has 32 reserved keywords (C89/C90). These cannot be used as identifiers.

1.8 Data Types

C provides several fundamental data types. The size may vary by platform; the table below shows typical sizes on a 32/64-bit system.

C
#include <stdio.h>

int main() {
    printf("Size of char:        %zu byte\n", sizeof(char));
    printf("Size of int:         %zu bytes\n", sizeof(int));
    printf("Size of float:       %zu bytes\n", sizeof(float));
    printf("Size of double:      %zu bytes\n", sizeof(double));
    printf("Size of long long:   %zu bytes\n", sizeof(long long));
    return 0;
}

1.9 Constants

Integer Constants

Decimal: 42, Octal (prefix 0): 052, Hexadecimal (prefix 0x): 0x2A

Suffixes: 10L (long), 10U (unsigned), 10UL (unsigned long), 10LL (long long)

Floating-Point Constants

3.14, 2.0e5 (= 200000.0), 1.5E-3 (= 0.0015). Default type is double; suffix f for float: 3.14f

Character Constants

A single character in single quotes: 'A', '9', '$'. Stored as ASCII integer value.

String Constants

A sequence of characters in double quotes: "Hello". Automatically terminated with \0.

1.10 Variables

A variable is a named location in memory that holds a value which can change during program execution.

C
int age = 21;              // Declaration + Initialization
float gpa;                   // Declaration only (contains garbage)
gpa = 8.75;                 // Assignment
char grade = 'A';          // Character variable
const int MAX = 100;       // Constant — cannot be changed

1.11 Expressions and Statements

An expression is a combination of variables, constants, and operators that evaluates to a value: a + b * c. A statement is a complete instruction ending with a semicolon: x = a + b;

1.12 Arithmetic Operators

C
#include <stdio.h>

int main() {
    int a = 17, b = 5;
    printf("a + b = %d\n", a + b);
    printf("a - b = %d\n", a - b);
    printf("a * b = %d\n", a * b);
    printf("a / b = %d\n", a / b);       // Integer division
    printf("a %% b = %d\n", a % b);
    printf("a / b = %.2f\n", (float)a / b);  // Float division
    return 0;
}

1.13 Unary Operators

C
#include <stdio.h>

int main() {
    int a = 5, b, c;

    b = ++a;   // Pre: a becomes 6, then b = 6
    printf("After ++a: a=%d, b=%d\n", a, b);

    a = 5;
    c = a++;   // Post: c = 5, then a becomes 6
    printf("After a++: a=%d, c=%d\n", a, c);

    printf("!0 = %d, !5 = %d\n", !0, !5);
    printf("~0 = %d\n", ~0);
    return 0;
}

1.14 Relational Operators

Relational operators compare two values and return 1 (true) or 0 (false).

1.15 Logical Operators

1.16 Assignment Operators

1.17 Conditional (Ternary) Operator

C
int a = 10, b = 20;
int max = (a > b) ? a : b;   // max = 20
printf("Max = %d\n", max);

// Equivalent to:
if (a > b) max = a;
else max = b;

1.18 Bitwise Operators

Bitwise operators work on individual bits of integer values.

C
#include <stdio.h>

int main() {
    int a = 5, b = 3;  // a = 0101, b = 0011
    printf("a & b  = %d\n", a & b);   // 0001 = 1
    printf("a | b  = %d\n", a | b);   // 0111 = 7
    printf("a ^ b  = %d\n", a ^ b);   // 0110 = 6
    printf("~a     = %d\n", ~a);      // -6
    printf("a << 1 = %d\n", a << 1);  // 1010 = 10
    printf("a >> 1 = %d\n", a >> 1);  // 0010 = 2
    return 0;
}

1.19 Operator Precedence and Associativity

1.20 Type Casting

Implicit Casting (Widening / Type Promotion)

C automatically converts a smaller type to a larger type: char → int → long → float → double

C
int x = 10;
float y = 3.5;
float result = x + y;  // x is promoted to float: 10.0 + 3.5 = 13.5

Explicit Casting (Narrowing)

C
float pi = 3.14159;
int truncated = (int)pi;    // truncated = 3 (decimal lost!)

int a = 7, b = 2;
float div = (float)a / b;  // 7.0 / 2 = 3.5 (not 3!)

C
#include <stdio.h>

int main() {
    float celsius, fahrenheit;
    int choice;

    printf("=== Temperature Converter ===\n");
    printf("1. Celsius to Fahrenheit\n");
    printf("2. Fahrenheit to Celsius\n");
    printf("Enter choice: ");
    scanf("%d", &choice);

    if (choice == 1) {
        printf("Enter Celsius: ");
        scanf("%f", &celsius);
        fahrenheit = (celsius * 9.0 / 5.0) + 32;
        printf("%.2f°C = %.2f°F\n", celsius, fahrenheit);
    } else if (choice == 2) {
        printf("Enter Fahrenheit: ");
        scanf("%f", &fahrenheit);
        celsius = (fahrenheit - 32) * 5.0 / 9.0;
        printf("%.2f°F = %.2f°C\n", fahrenheit, celsius);
    }
    return 0;
}

C
#include <stdio.h>

int main() {
    double a, b, result;
    char op;

    printf("Enter expression (e.g. 5 + 3): ");
    scanf("%lf %c %lf", &a, &op, &b);

    switch (op) {
        case '+': result = a + b; break;
        case '-': result = a - b; break;
        case '*': result = a * b; break;
        case '/':
            if (b != 0) result = a / b;
            else { printf("Error: Division by zero!\n"); return 1; }
            break;
        case '%': result = (int)a % (int)b; break;
        default: printf("Invalid operator!\n"); return 1;
    }
    printf("%.2lf %c %.2lf = %.2lf\n", a, op, b, result);
    return 0;
}

C
#include <stdio.h>

// Device status flags (each bit = one flag)
#define FLAG_POWER    (1 << 0)  // Bit 0: 0x01
#define FLAG_WIFI     (1 << 1)  // Bit 1: 0x02
#define FLAG_BLUETOOTH (1 << 2) // Bit 2: 0x04
#define FLAG_GPS      (1 << 3)  // Bit 3: 0x08
#define FLAG_ERROR    (1 << 7)  // Bit 7: 0x80

void printStatus(unsigned char reg) {
    printf("Status: Power=%d WiFi=%d BT=%d GPS=%d Error=%d\n",
        (reg & FLAG_POWER) ? 1 : 0,
        (reg & FLAG_WIFI) ? 1 : 0,
        (reg & FLAG_BLUETOOTH) ? 1 : 0,
        (reg & FLAG_GPS) ? 1 : 0,
        (reg & FLAG_ERROR) ? 1 : 0);
}

int main() {
    unsigned char deviceReg = 0x00;  // All off

    // SET flags (turn ON): use OR
    deviceReg |= FLAG_POWER;
    deviceReg |= FLAG_WIFI;
    printf("After power + wifi ON:\n");
    printStatus(deviceReg);

    // CLEAR a flag (turn OFF): use AND NOT
    deviceReg &= ~FLAG_WIFI;
    printf("After wifi OFF:\n");
    printStatus(deviceReg);

    // TOGGLE a flag: use XOR
    deviceReg ^= FLAG_GPS;
    printf("After GPS toggle:\n");
    printStatus(deviceReg);

    // CHECK a flag
    if (deviceReg & FLAG_POWER)
        printf("Device is powered ON\n");
    return 0;
}