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🍚 C — Learn From Zero

Playground: Replit C | JDoodle — click, type, run.

What is C?

C is a low-level, fast systems language. It powers operating systems, embedded devices, and is the foundation of many modern languages.

  • Operating systems (Linux, Windows kernels)
  • Embedded & IoT
  • Game engines & drivers
  • Performance-critical code

Run Online — No Install

Use the playground links above.

🟢 Beginner — Core Basics

Your First Program

c
#include 
int main() {
  printf("Hello, world!\n");
  return 0;
}

Programs start in main().

Variables & Types

c
int age = 36;
float height = 1.78f;
double big = 3.14;
char c = 'A';

Static types: int, float, char.

If / Else

c
int age = 18;
if (age >= 18) printf("Vote\n");
else printf("Too young\n");

No bool keyword in C89; use 0/1.

Loops

c
for (int i = 0; i < 5; i++) printf("%d\n", i);
int n = 3;
while (n-- > 0) printf("%d\n", n);

Classic C-style loops.

Functions

c
int add(int a, int b) {
  return a + b;
}

Declare before use or provide a prototype.

Pointers Intro

c
int x = 10;
int *p = &x;
printf("%d\n", *p);

& gets address, * dereferences.

🟡 Intermediate — Building Real Programs

Arrays

c
int nums[5] = {1, 2, 3, 4, 5};
for (int i = 0; i < 5; i++) printf("%d\n", nums[i]);

Arrays decay to pointers when passed.

Strings

c
char name[] = "Ada";
printf("%s\n", name);

Strings are char arrays ending in \0.

Structs

c
struct Person { char name[20]; int age; };
struct Person p = {"Ada", 36};

Group related data in structs.

Dynamic Memory

c
int *arr = malloc(5 * sizeof(int));
arr[0] = 1;
free(arr);

Always free() what you malloc().

File I/O

c
FILE *f = fopen("a.txt", "w");
fprintf(f, "Hi");
fclose(f);

Use fopen/fclose.

🔴 Advanced — Pro Concepts

Function Pointers

c
int (*op)(int,int) = add;
printf("%d\n", op(2,3));

Pointers to functions enable callbacks.

Preprocessor

c
#define MAX 100
#ifdef DEBUG
  printf("debug\n");
#endif

Macros and conditional compilation.

Bitwise Ops

c
int flags = 1 | 4;
if (flags & 1) printf("bit0\n");

Manipulate individual bits.

Unions

c
union Data { int i; float f; };
union Data d; d.i = 10;

Share memory between members.

What's Next?

  • Build a tiny project: Calculator, To-Do App, Weather Widget
  • Read the official docs and a good book
  • Solve Exercism exercises
  • Join a community & share your code

Practice daily for 20 minutes. Consistency > intensity.