---

Deleting at the start of a list

---

• Deleting elements from a linked list: Java vs. C
  • Java's way to delete the first element from a linked list:

    List delete ( List head ) { return( head.next ); // Make the second element the new head }

    ---

  • Why this will not work in C:

    The deleted linked list elements becomes garbage (inaccessible): What will happen sooner or later: Eventually (after many deletions), the computer memory will become exhausted When computer memory is exhausted (no more unused memory cells), the Java run time system will invoke the garbage collector and reclaim the inaccessible memory cells In a C program, when computer memory is exhausted, the C program will crash (because C does not have any garbage collector) You (the C programmer) is responsible for the un-reserving of memory cells used for storing the deleted list elements !!!!

  ---

  ---

  ---

  ---

• Delete the first element in a list in Java
  • In Java, you should have seen some thing like this to delete the first element from a linked list:

    class List { int value; List next; } public class delete { /* ==================================================== delete(h): delete the first elem from the list h return new list ==================================================== */ public static List delete (List h) { if ( h != null ) return (h.next); // Return new list that starts at h.next else return null; } public static void main(String[] args) { List head = makeList( ); // Make an list for demo print(head); /* ------------------------------------------------- Delete the first list element from list at head ------------------------------------------------- */ head = delete(head); // Delete first elem in head print(head); /* ------------------------------------------------------- Delete the first list element from list at head AGAIN ------------------------------------------------------- */ head = delete(head); // Delete first elem in head print(head); } // Make a list static List makeList( ) { List h = null; List p; for ( int i = 1; i <= 5; i++ ) { p = new List( ); p.value = i; p.next = h; h = p; } return h; } // Print the list public static void print(List h) { while (h != null) { System.out.printf("%d ", h.value); h = h.next; } System.out.printf("\n\n"); } }

    ---

  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/C/Linked-list/delete.java

    How to run the program:

    To compile:   javac delete.java          To run: java delete

    I will again show you the same code in C.

    But now notice how we must de-allocate the deleted List object before we return from the function !!!

  ---

  ---

  ---

  ---

• Delete the first element in a list in C
  • This is the same program as above, but in the C language.

    I don't want to point out the syntax differences (it was pointed out in the insert( ) function.

    I highlighted the part where you must de-allocate the deleted List element:

    #include <stdio.h> #include <stdlib.h> struct List { int value; struct List *next; }; struct List *makeList( ); // Declare these help functions to void print(struct List *h); // avoid compile error /* ==================================================== delete(h): delete the first elem from the list h return new list ==================================================== */ struct List *delete (struct List *h) { struct List *newHead; if ( h != NULL) { newHead = h->next; // Save the value h->next // Because free(h) MAY corrupt h->next !!! free( h ); // *** De-allocate the deleted list object at head h !!! // *** free(h) MAY corrupt h->value and h->next !! return (newHead); // Return new list that starts at h.next } else return NULL; } int main(int argc, char *argv[]) { struct List *head = makeList( ); // Make an list for demo print(head); /* ------------------------------------------------- Delete the first list element from list at head ------------------------------------------------- */ head = delete(head); // Delete first elem in head print(head); /* ------------------------------------------------------- Delete the first list element from list at head AGAIN ------------------------------------------------------- */ head = delete(head); // Delete first elem in head print(head); } // Make a list struct List *makeList( ) { struct List *h = NULL; struct List *p; for ( int i = 1; i <= 5; i++ ) { p = malloc( sizeof(struct List) ); p->value = i; p->next = h; h = p; } return h; } // Print the list void print(struct List *h) { while (h != NULL) { printf("%d ", h->value); h = h->next; } printf("\n\n"); }

    ---

  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/C/Linked-list/delete.c

    How to run the program:

    To compile:   gcc delete.c          To run: ./a.out