C Data Structures: Arrays, Structs, Unions, and More

Arrays

Arrays are a data structure that stores a fixed-size sequential collection of elements of the same type. An array is used to store a collection of data.

Structures

To define a structure, use the struct statement. This statement defines a new data type with multiple members. Structures can contain array members and can also be treated as arrays.

Unions

A union is a special data type that allows storing different data types in the same memory location. A union can have multiple members, but only one member can hold a value at any given time. Unions provide an efficient way to use the same memory location for multiple purposes.

Enumerations

Enumeration (or enum) is a user-defined data type in C. It is mainly used to assign names to integral constants, making a program easier to read and maintain.

Pointers

A pointer is a variable whose value is the address of another variable. Like any variable, you must declare a pointer before using it to store a variable address. Pointers can be declared using *. A null pointer has a special value and does not point to any address.

String Functions

The string.h header defines a variable type, a macro, and various functions for manipulating arrays of characters.

• The function int strcmp(const char *str1, const char *str2) compares the string pointed to by str1 to the string pointed to by str2.
• The function char *strncat(char *dest, const char *src, size_t n) appends the string pointed to by src to the end of the string pointed to by dest, up to n characters long.
• strncpy() prevents buffer overflow, while strcpy() can cause buffer overflow.
• The function char *strcpy(char *dest, const char *src) copies the string pointed to by src to dest.

Pointer Arithmetic

You can perform various arithmetic operations on pointers, such as increment, decrement, and comparison.

Typedef

The C programming language provides the keyword typedef, which you can use to give a type a new name. For example, to define BYTE for one-byte numbers:

typedef unsigned char BYTE;
BYTE b1, b2;

Memory Allocation

The function malloc() reserves a block of memory of a specified size and returns a pointer of type void, which can be cast into a pointer of any form.

ptr = (cast-type*) malloc(byte-size);
ptr = (int*) malloc(100 * sizeof(int));

The only difference between malloc() and calloc() is that malloc() allocates a single block of memory, whereas calloc() allocates multiple blocks of memory, each of the same size, and sets all bytes to zero.

ptr = (float*) calloc(25, sizeof(float));

Dynamically allocated memory created with either calloc() or malloc() does not get freed on its own. You must explicitly use free() to release the space.

free(ptr);

Linked Lists

A linked list is an extensible data structure consisting of nodes. Each node contains application-specific data and a pointer to another node. The program only needs to remember the pointer to the first node.

#include<stdio.h>
#include<stdlib.h>
struct node {
  int data;
  struct node *next;
};
struct node *head = NULL;
struct node *current = NULL;
void printList(){
  struct node *ptr = head;
  printf("\n[head] =>");
  while(ptr != NULL){
    printf(" %d =>",ptr->data);
    ptr = ptr->next;
  }
  printf(" [null]\n");
}

C Functions

A function is a group of statements that together perform a task. Every C program has at least one function, which is main(). Most programs define additional functions.

A function declaration tells the compiler about a function’s name, return type, and parameters. A function definition provides the actual body of the function.

Iterators

An iterator is a mechanism to advance through some data structure without revealing its underlying implementation.