Summary: in this tutorial, you will learn about C linked list data structure and how to implement the most commonly used linked list operations.
Table of Contents
- 1 Introduction to linked list data structure
- 2 C Linked List implementation
- 2.1 Add a node at the beginning of the linked list
- 2.2 Traverse the linked list
- 2.3 Count the elements of the linked list
- 2.4 Add a new node at the end of the linked list
- 2.5 Insert a new node after a particular node
- 2.6 Insert a new node before a particular node
- 2.7 Search for a node
- 2.8 Sort a linked list using insertion sort
- 2.9 Reverse linked list
- 2.10 Delete a node from the front of the linked list
- 2.11 Delete a node from the back of the linked list
- 2.12 Delete a node in the middle of the linked list
- 2.13 Delete the whole linked list
- 3 C Linked List Program
Introduction to linked list data structure
A linked list is a data structure that consists of sequence of nodes. Each node is composed of two fields: data field and reference field which is a pointer that points to the next node in the sequence.
Linked List Node
Each node in the list is also called an element. The reference field that contains a pointer which points to the next node is called next pointer or next link.
A head pointer is used to track the first element in the linked list, therefore, it always points to the first element.
The following picture illustrates a linked list
The linked list data structure is designed to be efficient for insertion or removal of elements from any position in the list. However other operations such as getting the last element or finding an element that stores specific data requires scanning most or all the elements in the list.
A linked list is also used to implement other data structure such as stack and queue.
C Linked List implementation
We can model a node of the linked list using a structure as follows:
1 2 3 4 | typedef struct node{ int data; struct node* next; } |
The node structure has two members:
datastores the informationnextpointer holds the address of the next node.
Add a node at the beginning of the linked list
First, we declare a head pointer that always points to the first node of the list.
1 | node* head; |
To add a node at the beginning of the list:
First, we need to create a new node. We will need to create a new node each time we want to insert a new node into the list so we can develop a function that creates a new node and return it.
1 2 3 4 5 6 7 8 9 10 11 12 13 | node* create(int data,node* next) { node* new_node = (node*)malloc(sizeof(node)); if(new_node == NULL) { printf("Error creating a new node.\n"); exit(0); } new_node->data = data; new_node->next = next; return new_node; } |
Second, we need to point the next pointer of the new node to the head pointer and point the head pointer to the new node. It works for both empty and non-empty linked list.
1 2 3 4 5 6 | node* prepend(node* head,int data) { node* new_node = create(data,head); head = new_node; return head; } |
The following picture illustrates how to insert a node when the list is empty:
Add a node into an empty linked list
The following picture illustrates how to insert a node at the beginning of a non-empty linked list:
Add a new node at the beginning of a non-empty linked list
Traverse the linked list
Sometimes we may want to traverse the linked list to get the data stored in each node for further manipulation e.g., display the node information. To traverse the linked list, we start from the first node, and move to the next node until we reach a NULL pointer.
To make the traverse() function more general, we can add a function pointer that points to a function for linked list node manipulation as a parameter of the traverse() function.
We define a call back function for manipulating a node of the linked list:
1 | typedef void (*callback)(node* data); |
The following is the traverse() function:
1 2 3 4 5 6 7 8 9 | void traverse(node* head,callback f) { node* cursor = head; while(cursor != NULL) { f(cursor); cursor = cursor->next; } } |
Count the elements of the linked list
We can use the same traversing technique to count the number of elements in a linked list. See the following count() function:
1 2 3 4 5 6 7 8 9 10 11 | int count(node *head) { node *cursor = head; int c = 0; while(cursor != NULL) { c++; cursor = cursor->next; } return c; } |
Add a new node at the end of the linked list
To add a new node at the end of the linked list:
First, we need to find the last node of the list. Starting the first node which is indicated by the head pointer, we traverse the list until the next pointer reaches NULL.
1 2 3 | node *cursor = head; while(cursor->next != NULL) cursor = cursor->next; |
Second, we create a new node with the next pointer points to NULL , and the next pointer of the cursor points to the new node:
1 2 | node* new_node = create(data,NULL); cursor->next = new_node; |
1 |
Add a new node at the end of the linked list
The following is the append() function that adds a new node at the end of the linked list.
1 2 3 4 5 6 7 8 9 10 11 12 13 | node* append(node* head, int data) { /* go to the last node */ node *cursor = head; while(cursor->next != NULL) cursor = cursor->next; /* create a new node */ node* new_node = create(data,NULL); cursor->next = new_node; return head; } |
Insert a new node after a particular node
To insert a new node after a particular node, we need to:
First, verify if the node exists in the list, we call this node is prev node
1 2 3 | node *cursor = head; while(cursor != prev) cursor = cursor->next; |
If it exists, we point the next pointer of the new node to the next node that the next pointer of the prev node points to, and point the next pointer of the prev node to the new node.
1 2 | node* new_node = create(data,cursor->next); cursor->next = new_node; |
The following picture illustrates how to insert a new node after a particular node in the linked list:
Insert a new after a particular node in the linked list
The insert_after() function is as follows:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 | /* insert a new node after the prev node */ node* insert_after(node *head, int data, node* prev) { /* find the prev node, starting from the first node*/ node *cursor = head; while(cursor != prev) cursor = cursor->next; if(cursor != NULL) { node* new_node = create(data,cursor->next); cursor->next = new_node; return head; } else { return NULL; } } |
Insert a new node before a particular node
To insert a new node before a particular node named nxt we need to:
- First, if the
nxtnode is the first node, we can call theprepend()function to add a new node at the beginning of the list, otherwise, we find the previous node of thenxtnode, suppose it is thecursornode. - Second, point the
nextpointer of the new node to the node that thenextpointer of thecursorpoints to and point thenextpointer of thecursorto the new node.
The following picture illustrates how to insert a node before a particular node:
Insert a node before a particular node
The following is the insert_before() function that inserts a new node before a particular node in the linked list:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 | node* insert_before(node *head, int data, node* nxt) { if(nxt == NULL || head == NULL) return NULL; if(head == nxt) { head = prepend(head,data); return head; } /* find the prev node, starting from the first node*/ node *cursor = head; while(cursor != NULL) { if(cursor->next == nxt) break; cursor = cursor->next; } if(cursor != NULL) { node* new_node = create(data,cursor->next); cursor->next = new_node; return head; } else { return NULL; } } |
Search for a node
To search for a node that stores a given data, we scan the whole list and return the first node that stores the searched data. The following illustrates the search function:
1 2 3 4 5 6 7 8 9 10 11 12 | node* search(node* head,int data) { node *cursor = head; while(cursor!=NULL) { if(cursor->data == data) return cursor; cursor = cursor->next; } return NULL; } |
The search() function returns NULL if no node stores the input data.
Sort a linked list using insertion sort
We can sort a linked list using the insertion sort algorithm. The following is the insertion_sort() function that sorts a linked list:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 | node* insertion_sort(node* head) { node *x, *y, *e; x = head; head = NULL; while(x != NULL) { e = x; x = x->next; if (head != NULL) { if(e->data > head->data) { y = head; while ((y->next != NULL) && (e->data> y->next->data)) { y = y->next; } e->next = y->next; y->next = e; } else { e->next = head; head = e ; } } else { e->next = NULL; head = e ; } } return head; } |
Reverse linked list
To reverse a linked list, you change the next pointer of each node from the next node to the previous node. The following reverse() function reverses a linked list.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 | node* reverse(node* head) { node* prev = NULL; node* current = head; node* next; while (current != NULL) { next = current->next; current->next = prev; prev = current; current = next; } head = prev; return head; } |
Delete a node from the front of the linked list
To delete a node from the front of the linked list is relatively simple. We point the head to the next node and remove the node that the head pointed to.
Notice that if the list has only one node, we should set the head pointer to NULL.
Remove from the front of the list
The following is the remove_front() function:
1 2 3 4 5 6 7 8 9 10 11 12 13 | node* remove_front(node* head) { if(head == NULL) return NULL; node *front = head; head = head->next; front->next = NULL; /* is this the last node in the list */ if(front == head) head = NULL; free(front); return head; } |
Delete a node from the back of the linked list
To remove a node from the back of the linked list, we need to:
- Use two pointers:
cursorandbackto track the node. - Start from the first node until the
cursorpointer reaches the last node and thebackpointer reaches the node before the last node. - Set the
nextpointer of thebacktoNULLand delete the node that thecursorpoints to. - If the node has only 1 element, set the
headpointer toNULLbefore removing the node.
See the following picture:
Delete a node from the back of the linked list
The following is the function to remove a node from the back of the linked list:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 | node* remove_back(node* head) { if(head == NULL) return NULL; node *cursor = head; node *back = NULL; while(cursor->next != NULL) { back = cursor; cursor = cursor->next; } if(back != NULL) back->next = NULL; /* if this is the last node in the list*/ if(cursor == head) head = NULL; free(cursor); return head; } |
Delete a node in the middle of the linked list
To delete a node in the middle of the linked list:
If the node is the first node, call remove_front() function to remove it.
If the node is the last node, call remove_back() function to remove it.
If the node is in the middle of the list:
- Traverse from the first node, use the
cursorpointer to point to the node before the node that needs to be removed e.g., in the picture, the node that needs to be removed is2and thecursorpointer points to node5. - Use a
tmppointer to point to the node that needs to be removed. - Set the
nextpointer of thecursorpoint to the node that thenextpointer of thetmppoints to. - Remove the node that the
tmppointer points to.
The following picture illustrates how to delete a node in the middle of the linked list:
Remove the middle node of a linked list
The following is the function that can be used to remove any node in the linked list:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 | node* remove_any(node* head,node* nd) { /* if the node is the first node */ if(nd == head) { head = remove_front(head); return head; } /* if the node is the last node */ if(nd->next == NULL) { head = remove_back(head); return head; } /* if the node is in the middle */ node* cursor = head; while(cursor != NULL) { if(cursor->next = nd) break; cursor = cursor->next; } if(cursor != NULL) { node* tmp = cursor->next; cursor->next = tmp->next; tmp->next = NULL; free(tmp); } return head; } |
Delete the whole linked list
It is important to remove all nodes of the linked list when you no longer use it. The following dispose() function releases memory allocated for all the nodes in a linked list:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 | void dispose(node *head) { node *cursor, *tmp; if(head != NULL) { cursor = head->next; head->next = NULL; while(cursor != NULL) { tmp = cursor->next; free(cursor); cursor = tmp; } } } |
C Linked List Program
We can write a program to test the linked list functions that we’ve developed in the above section.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 | /* & File : main.c * Author : zentut.com * Purpose : C Linked List Data Structure * Copyright: @ zentut.com */ #include <stdio.h> #include <stdlib.h> typedef struct node { int data; struct node* next; } node; typedef void (*callback)(node* data); /* create a new node initialize the data and next field return the newly created node */ node* create(int data,node* next) { node* new_node = (node*)malloc(sizeof(node)); if(new_node == NULL) { printf("Error creating a new node.\n"); exit(0); } new_node->data = data; new_node->next = next; return new_node; } /* add a new node at the beginning of the list */ node* prepend(node* head,int data) { node* new_node = create(data,head); head = new_node; return head; } /* add a new node at the end of the list */ node* append(node* head, int data) { if(head == NULL) return NULL; /* go to the last node */ node *cursor = head; while(cursor->next != NULL) cursor = cursor->next; /* create a new node */ node* new_node = create(data,NULL); cursor->next = new_node; return head; } /* insert a new node after the prev node */ node* insert_after(node *head, int data, node* prev) { if(head == NULL || prev == NULL) return NULL; /* find the prev node, starting from the first node*/ node *cursor = head; while(cursor != prev) cursor = cursor->next; if(cursor != NULL) { node* new_node = create(data,cursor->next); cursor->next = new_node; return head; } else { return NULL; } } /* insert a new node before the nxt node */ node* insert_before(node *head, int data, node* nxt) { if(nxt == NULL || head == NULL) return NULL; if(head == nxt) { head = prepend(head,data); return head; } /* find the prev node, starting from the first node*/ node *cursor = head; while(cursor != NULL) { if(cursor->next == nxt) break; cursor = cursor->next; } if(cursor != NULL) { node* new_node = create(data,cursor->next); cursor->next = new_node; return head; } else { return NULL; } } /* traverse the linked list */ void traverse(node* head,callback f) { node* cursor = head; while(cursor != NULL) { f(cursor); cursor = cursor->next; } } /* remove node from the front of list */ node* remove_front(node* head) { if(head == NULL) return NULL; node *front = head; head = head->next; front->next = NULL; /* is this the last node in the list */ if(front == head) head = NULL; free(front); return head; } /* remove node from the back of the list */ node* remove_back(node* head) { if(head == NULL) return NULL; node *cursor = head; node *back = NULL; while(cursor->next != NULL) { back = cursor; cursor = cursor->next; } if(back != NULL) back->next = NULL; /* if this is the last node in the list*/ if(cursor == head) head = NULL; free(cursor); return head; } /* remove a node from the list */ node* remove_any(node* head,node* nd) { if(nd == NULL) return NULL; /* if the node is the first node */ if(nd == head) return remove_front(head); /* if the node is the last node */ if(nd->next == NULL) return remove_back(head); /* if the node is in the middle */ node* cursor = head; while(cursor != NULL) { if(cursor->next == nd) break; cursor = cursor->next; } if(cursor != NULL) { node* tmp = cursor->next; cursor->next = tmp->next; tmp->next = NULL; free(tmp); } return head; } /* display a node */ void display(node* n) { if(n != NULL) printf("%d ", n->data); } /* Search for a specific node with input data return the first matched node that stores the input data, otherwise return NULL */ node* search(node* head,int data) { node *cursor = head; while(cursor!=NULL) { if(cursor->data == data) return cursor; cursor = cursor->next; } return NULL; } /* remove all element of the list */ void dispose(node *head) { node *cursor, *tmp; if(head != NULL) { cursor = head->next; head->next = NULL; while(cursor != NULL) { tmp = cursor->next; free(cursor); cursor = tmp; } } } /* return the number of elements in the list */ int count(node *head) { node *cursor = head; int c = 0; while(cursor != NULL) { c++; cursor = cursor->next; } return c; } /* sort the linked list using insertion sort */ node* insertion_sort(node* head) { node *x, *y, *e; x = head; head = NULL; while(x != NULL) { e = x; x = x->next; if (head != NULL) { if(e->data > head->data) { y = head; while ((y->next != NULL) && (e->data> y->next->data)) { y = y->next; } e->next = y->next; y->next = e; } else { e->next = head; head = e ; } } else { e->next = NULL; head = e ; } } return head; } /* reverse the linked list */ node* reverse(node* head) { node* prev = NULL; node* current = head; node* next; while (current != NULL) { next = current->next; current->next = prev; prev = current; current = next; } head = prev; return head; } /* display the menu */ void menu() { printf("--- C Linked List Demonstration --- \n\n"); printf("0.menu\n"); printf("1.prepend an element\n"); printf("2.append an element\n"); printf("3.search for an element\n"); printf("4.insert after an element\n"); printf("5.insert before an element\n"); printf("6.remove front node\n"); printf("7.remove back node\n"); printf("8.remove any node\n"); printf("9.sort the list\n"); printf("10.Reverse the linked list\n"); printf("-1.quit\n"); } int main() { int command = 0; int data; node* head = NULL; node* tmp = NULL; callback disp = display; menu(); while(1) { printf("\nEnter a command(0-10,-1 to quit):"); scanf("%d",&command); if(command == -1) break; switch(command) { case 0: menu(); break; case 1: printf("Please enter a number to prepend:"); scanf("%d",&data); head = prepend(head,data); traverse(head,disp); break; case 2: printf("Please enter a number to append:"); scanf("%d",&data); head = append(head,data); traverse(head,disp); break; case 3: printf("Please enter a number to search:"); scanf("%d",&data); tmp = search(head,data); if(tmp != NULL) { printf("Element with value %d found.",data); } else { printf("Element with value %d not found.",data); } break; case 4: printf("Enter the element value where you want to insert after:"); scanf("%d",&data); tmp = search(head,data); if(tmp != NULL) { printf("Enter the element value to insert after:"); scanf("%d",&data); head = insert_after(head,data,tmp); if(head != NULL) traverse(head,disp); } else { printf("Element with value %d not found.",data); } break; case 5: printf("Enter the element value where you want to insert before:"); scanf("%d",&data); tmp = search(head,data); if(tmp != NULL) { printf("Enter the element value to insert before:"); scanf("%d",&data); head = insert_before(head,data,tmp); if(head != NULL) traverse(head,disp); } else { printf("Element with value %d not found.",data); } break; case 6: head = remove_front(head); if(head != NULL) traverse(head,disp); break; case 7: head = remove_back(head); if(head != NULL) traverse(head,disp); break; case 8: printf("Enter the element value to remove:"); scanf("%d",&data); tmp = search(head,data); if(tmp != NULL) { remove_any(head,tmp); if(head != NULL) traverse(head,disp); } else { printf("Element with value %d not found.",data); } break; case 9: head = insertion_sort(head); if(head != NULL) traverse(head,disp); break; case 10: head = reverse(head); if(head != NULL) traverse(head,disp); break; } } dispose(head); return 0; } |
The following is the output when we ran the C linked list program above:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 | --- C Linked List Demonstration --- 0.menu 1.prepend an element 2.append an element 3.search for an element 4.insert after an element 5.insert before an element 6.remove front node 7.remove back node 8.remove any node 9.sort the list 10.Reverse the linked list -1.quit Enter a command(0-10,-1 to quit):1 Please enter a number to prepend:1 1 Enter a command(0-10,-1 to quit):1 Please enter a number to prepend:3 3 1 Enter a command(0-10,-1 to quit):1 Please enter a number to prepend:2 2 3 1 Enter a command(0-10,-1 to quit):1 Please enter a number to prepend:5 5 2 3 1 Enter a command(0-10,-1 to quit):2 Please enter a number to append:6 5 2 3 1 6 Enter a command(0-10,-1 to quit):2 Please enter a number to append:9 5 2 3 1 6 9 Enter a command(0-10,-1 to quit):2 Please enter a number to append:8 5 2 3 1 6 9 8 Enter a command(0-10,-1 to quit):2 Please enter a number to append:0 5 2 3 1 6 9 8 0 Enter a command(0-10,-1 to quit):3 Please enter a number to search:2 Element with value 2 found. Enter a command(0-10,-1 to quit):3 Please enter a number to search:4 Element with value 4 not found. Enter a command(0-10,-1 to quit):4 Enter the element value where you want to insert after:3 Enter the element value to insert after:4 5 2 3 4 1 6 9 8 0 Enter a command(0-10,-1 to quit):5 Enter the element value where you want to insert before:1 Enter the element value to insert before:-1 5 2 3 4 -1 1 6 9 8 0 Enter a command(0-10,-1 to quit):6 2 3 4 -1 1 6 9 8 0 Enter a command(0-10,-1 to quit):7 2 3 4 -1 1 6 9 8 Enter a command(0-10,-1 to quit):8 Enter the element value to remove:1 2 3 4 -1 6 9 8 Enter a command(0-10,-1 to quit):9 -1 2 3 4 6 8 9 Enter a command(0-10,-1 to quit):10 9 8 6 4 3 2 -1 Enter a command(0-10,-1 to quit):-1 |
You can download the C linked list program via the following link:
C Linked List (4038 downloads) In this tutorial, we have introduced you to C linked list and how to implement commonly used linked list operations such as prepend, append, traverse, count, insert, remove, sort, and dispose.