C introduction: why C matters, where it's used, compiling, your first program, program structure, data types and printf

C is one of the most influential programming languages ever created. Developed in the early 1970s by Dennis Ritchie at Bell Labs to build the Unix operating system, it still runs much of the world's technology: operating system kernels (Linux, much of Windows and macOS), embedded devices (microcontrollers in cars, ATMs, POS machines, medical devices, smart meters), routers, databases, and the interpreters of languages like Python. Learning C teaches you how computers really work: memory, addresses, bytes and the cost of every operation. It's also a core course in most Kenyan computer science and engineering programmes.

Why learn C?

Trade-offs: no built-in safety nets (no automatic bounds checking or garbage collection), so mistakes can crash programs or create security holes. That's why understanding C deeply is valuable.

Where C is used

From source code to running program

hello.c → [preprocessor] → [compiler] → hello.o (object code) → [linker] → hello (executable) → run

1. Preprocessor: handles #include and #define (copies in header files, replaces macros).
2. Compiler: translates C into machine code (object file).
3. Linker: combines object files and libraries (like the one containing printf) into an executable.

Setting up

Compile and run in a terminal:

gcc hello.c -o hello -Wall    # -Wall shows helpful warnings
./hello                        # Windows: hello.exe

Your first program

#include <stdio.h>

int main(void) {
    printf("Habari, Kenya!\n");
    printf("Learning C on the Marzley learning hub.\n");
    return 0;
}

Line by line:

• #include <stdio.h>: includes the standard input/output header, which declares printf.
• int main(void): the main function, where execution starts; it returns an int.
• { ... }: the function body.
• printf(...): prints formatted text; \n is a new line.
• return 0;: tells the operating system the program finished successfully (non-zero means an error).
• Every statement ends with ;.

Data types

#include <stdio.h>

int main(void) {
    int students = 45;
    float height = 1.72f;
    double balance = 12500.75;
    char grade = 'A';
    long population = 53000000L;

    printf("Students: %d\n", students);
    printf("Height: %.2f m\n", height);
    printf("Balance: KSh %.2f\n", balance);
    printf("Grade: %c (stored as the number %d)\n", grade, grade);
    printf("Population: %ld\n", population);
    printf("int is %d bytes, double is %d bytes, char is %d byte\n",
           (int)sizeof(int), (int)sizeof(double), (int)sizeof(char));
    return 0;
}

Sizes can vary by platform; sizeof tells you on your machine, and <limits.h> defines limits such as INT_MAX (2,147,483,647 for a 4-byte int). C has no built-in bool or string types in old standards: <stdbool.h> provides bool, and strings are arrays of char (see the arrays and strings lesson).

Variables and constants

#include <stdio.h>
#define VAT_RATE 0.16          /* preprocessor constant: replaced before compiling */

int main(void) {
    int max_students = 50;
    double price = 1000.0;
    double vat = price * VAT_RATE;
    int enrolled = 47;

    printf("VAT on KSh %.0f is KSh %.2f\n", price, vat);
    printf("Seats left: %d of %d\n", max_students - enrolled, max_students);
    return 0;
}

You can also make a typed constant with const, which the compiler protects from changes (try it with GCC on your computer):

const int MAX_STUDENTS = 50;
MAX_STUDENTS = 60;   /* error: assignment of read-only variable */

Rules: names start with a letter or underscore, contain letters/digits/underscores, are case-sensitive, and can't be keywords (int, return, while...). Always initialise variables: an uninitialised local variable holds garbage (whatever was in memory).

printf format specifiers

#include <stdio.h>

int main(void) {
    printf("[%d]\n", 42);
    printf("[%5d]\n", 42);          /* width 5, right-aligned */
    printf("[%-5d]\n", 42);         /* left-aligned */
    printf("[%05d]\n", 42);         /* zero-padded */
    printf("[%.2f]\n", 3.14159);    /* 2 decimal places */
    printf("[%8.2f]\n", 160.5);     /* width 8 */
    printf("[%s]\n", "Kisumu");
    printf("[%-10s|]\n", "Sugar");  /* padded string for tables */
    printf("[%x] [%o]\n", 255, 8);  /* hexadecimal, octal */
    printf("100%% sure\n");         /* %% prints % */
    return 0;
}

Reading input with scanf

Run this on your own computer (the online runner can't type input):

#include <stdio.h>

int main(void) {
    int age;
    double fee;
    char name[50];

    printf("Name: ");
    scanf("%49s", name);          /* reads one word; 49 limits length to avoid overflow */
    printf("Age: ");
    scanf("%d", &age);            /* & gives scanf the address of age */
    printf("Fee: ");
    scanf("%lf", &fee);           /* %lf for double in scanf */

    printf("%s, %d years, fee KSh %.2f\n", name, age, fee);
    return 0;
}

The & (address-of operator) lets scanf store the value in your variable; pointers are explained later. For whole lines with spaces, use fgets.

Operators and integer division

#include <stdio.h>

int main(void) {
    int a = 17, b = 5;
    printf("%d %d %d %d %d\n", a + b, a - b, a * b, a / b, a % b);   /* 22 12 85 3 2 */
    printf("%.2f\n", (double)a / b);                                  /* 3.40 */

    int count = 10;
    count++;
    count += 5;
    printf("count = %d\n", count);                                    /* 16 */

    int total = 245, n = 4;
    printf("Wrong average: %d\n", total / n);                         /* 61 */
    printf("Right average: %.2f\n", (double)total / n);               /* 61.25 */
    return 0;
}

int / int gives an int (decimals dropped). Cast one operand to double for decimal results.

Comments

/* Multi-line comment
   (works in all C versions) */
// Single-line comment (C99 and later)

Common beginner errors

Compile with -Wall -Wextra to catch many of these early.

Summary

• C (1972, Dennis Ritchie) powers operating systems, embedded devices, networking and language runtimes; it teaches how computers work.
• Source code goes through the preprocessor, compiler and linker; use GCC/Clang with -Wall.
• Programs start at int main(void), use #include <stdio.h> for printf, end statements with ; and return 0 on success.
• Types: char, int, long, float, double, unsigned; use sizeof; constants with #define or const; always initialise variables.
• printf formats output (%d, %f, %.2f, %c, %s); scanf reads input with &; watch integer division.