c language

This guide covers all the core topics you’ll find on W3Schools’ C Tutorial but in a much shorter, “cheat‑sheet” style. Use it as a quick reference or a refresher before diving into projects.

1. Introduction

• What is C? A general‑purpose, procedural language created by Dennis Ritchie (1972). It’s the foundation for many modern languages and operating systems.
• Why learn C?
  • Low‑level memory control → performance & embedded systems.
  • Portable: compiles to almost every platform.
  • Huge ecosystem of libraries.

2. Program Structure

#include <stdio.h>   // Preprocessor directive – include header files

int main(void) {      // Entry point
    /* code */
    return 0;         // Exit status
}

• #include pulls in declarations from headers (<stdio.h>, <stdlib.h>, etc.).
• main() is mandatory.
• Return 0 signals success.

3. Syntax Basics

Element	Example	Notes
Comments	/* block */ or // line	Used for documentation.
Identifiers	myVar, calculate_sum	Must start with letter/underscore, no spaces.
Semicolon	int x = 5;	Terminates statements.

4. Variables & Data Types

Type	Size (typical)	Range / Example
char	1 byte	'A', '\n'
int	2–4 bytes	-2,147,483,648 … 2,147,483,647 (4-byte)
short	2 bytes	-32768 … 32767
long	4–8 bytes	-9,223,372,036,854,775,808 … 9,223,372,036,854,775,807 (8-byte)
float	4 bytes	3.14f
double	8 bytes	3.1415926535
_Bool	1 byte	true / false (C99)

Tip: Use sizeof(type) to discover exact size on your platform.

5. Operators

Category	Symbols	Example
Arithmetic	+ - * / %	a + b, c / d
Assignment	= += -= *= /= %=	x = y; x += 2;
Comparison	== != < > <= >=	if (a == b)
Logical	&& || !	if (!flag && count)
Bitwise	& | ^ ~ << >>	mask & 0xFF
Increment/Decrement	++ --	i++; --j;

Operator precedence follows the standard C rules (see reference table if needed).

6. Booleans

• Use _Bool or int (0 = false, non‑zero = true).
• Standard header <stdbool.h> defines bool, true, false.

#include <stdbool.h>
bool ok = true;

7. Conditional Statements

if / else if / else

if (score >= 90)      printf("A");
else if (score >= 80) printf("B");
else                  printf("C");

• Ternary operator: char grade = (score >= 60) ? 'P' : 'F';

switch

switch(day) {
    case MONDAY:   /* code */ break;
    case TUESDAY:  /* code */ break;
    default:       /* code */
}

• Use break to exit the switch; omit for fall‑through.

8. Loops

Loop	Syntax	When to use
while	while (cond) { … }	Condition checked before each iteration.
do‑while	do { … } while(cond);	At least one execution; condition after loop.
for	for(init; cond; incr) { … }	Classic counter loops.

// Print 1–10
for (int i = 1; i <= 10; ++i)
    printf("%d ", i);

• break – exit loop immediately.
• continue – skip to next iteration.

9. Arrays

int nums[5] = {1, 2, 3, 4, 5};
printf("%d", nums[0]);   // Access by index (0‑based)

• Multidimensional: int matrix[3][4];
• Size can be inferred: int a[] = {1,2,3}; → size 3.

10. Strings

• In C, strings are arrays of char terminated by '\0'.
• Use <string.h> functions:

#include <string.h>
char s[20];
strcpy(s, "Hello");
printf("%s", s);          // prints Hello

Common functions: strlen, strcmp, strcat (from <string.h>); sprintf (from <stdio.h>).

11. User Input

int age;
scanf("%d", &age);   // Note the address-of operator (&)

• Always check return value of scanf for errors.

12. Pointers

int x = 10;
int *p = &x;          // p holds address of x
printf("%d", *p);     // dereference → 10

• Pointer arithmetic: p++ moves to next element in array type.
• Pointers to arrays: int arr[5]; int *q = arr; (same as &arr[0]).
• Null pointer: NULL (macro from <stddef.h>).

13. Functions

Declaration / Prototype

int add(int a, int b);   // prototype

Definition

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

• Parameters are passed by value (copy).
• Return type can be void if nothing is returned.

Scope & Lifetime

Scope	Example
Local	Inside function – destroyed on exit.
Global	Outside all functions – visible everywhere.

14. Recursion

int fact(int n) {
    return (n <= 1) ? 1 : n * fact(n-1);
}

• Base case prevents infinite recursion.

15. Function Pointers

int (*func_ptr)(int, int) = add;   // points to add()
int result = func_ptr(3, 4);        // calls add(3,4)

Useful for callbacks and generic algorithms.

16. File I/O

Mode	Symbol
"r"	read (text)
"w"	write (truncate)
"a"	append
"rb/wb/ab"	binary

FILE *fp = fopen("data.txt", "r");
if (!fp) { perror("fopen"); exit(1); }

char line[256];
while (fgets(line, sizeof line, fp))
    printf("%s", line);

fclose(fp);

• Use <stdio.h> functions: fprintf, fscanf, fwrite, fread.

17. Structures & Unions

struct Person {
    char name[50];
    int age;
};

struct Person p = {"Alice", 30};

• Unions share memory among members; only one active at a time.

union Data { int i; float f; };

18. Enumerations

enum Color { RED, GREEN, BLUE };
enum Color c = GREEN;

• Underlying type is int by default.

19. Memory Management

Function	Purpose
malloc(size)	Allocate raw memory
calloc(n, size)	Allocate & zero-initialize
realloc(ptr, newsize)	Resize block
free(ptr)	Release memory

int *arr = malloc(10 * sizeof(int));
if (!arr) { /* handle error */ }
...
free(arr);

• Memory leaks: forgetting to free.
• Dangling pointers: using after free.

20. Errors & Debugging

Technique	Description
perror()	Print last system error
assert(expr)	Abort if expression false (debug mode)
#define DEBUG + conditional prints
Valgrind / AddressSanitizer – detect leaks, invalid accesses

21. Miscellaneous

• Macros (#define) for constants or inline code.
• Storage classes: auto, register, static, extern.
• Bitwise operations: useful for flags and low‑level data packing.
• Fixed‑width integers (<stdint.h>): int32_t, uint64_t.

22. Quick Reference Cheat Sheet

/* Header includes */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>

/* Main skeleton */
int main(void) {
    /* code */
    return 0;
}

/* Example: print Hello World */
printf("Hello, World!\n");

/* Example: read integer */
int n; scanf("%d", &n);

/* Example: array loop */
for (int i = 0; i < 5; ++i)
    printf("%d ", arr[i]);

/* Example: function prototype */
int add(int a, int b);

Final Note

This guide is intentionally terse. For deeper explanations, examples, and exercises, refer to the full W3Schools C tutorial or any comprehensive textbook (e.g., The C Programming Language by Kernighan & Ritchie). Happy coding!