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Inserting and deleting at the start of a list

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• Function to insert a list element at the start of a list
  • We will write the following function:

    ListElem* insertHead( ListElem* head, ListElem* elem): Inserts the list element elem into the list head at the start of the list The function returns the address (reference) of the head element of the new list

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  • Previously, we have seen how to insert a new list element at the start of a list:

    /* ======================================================== Insert at head technique: 1. create new list element p put values in the payload area... 2. make p point to first element in list 3. make head point to new list element ======================================================== */ p = malloc( sizeof(struct ListElem) ); // (1) p->value = 123; // Payload... p->next = head; // (2) head = p; // (3)

    We just need to put these statements into a function

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  • Function that insert a new list element at start of a list:

    struct ListElem* insertHead(struct ListElem* head, struct ListElem* elem) { elem->next = head; // make "elem" point to "head" return(elem); // return address of new head }

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link(s) and save in a scratch directory To compile:   gcc insert-list2.c To run:          ./a.out

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• 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 }

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  • 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 !!!!

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• Review: free(): un-reserve memory cells
  • Syntax to un-reserve memory cells:

    free( pointerVariable ) ;

    Effect:

    The memory cells pointed to by the pointerVariable are un-reserved.

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  • Example:

    int main(int argc, char *argv[]) { struct BankAccount* p; p = malloc( sizeof(struct BankAccount) ); // (1) (*p).accNum = 123; // Long form to access a member variable p->balance= 400; // Short form to access a member variable printf("p->accNum = %d p->balance = %f\n", p->accNum, p->balance); /* ================================================== Unreserve memory that was allocated at step (1) ================================================== */ free(p); }

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• Deleting elements from a linked list in C
  • How to delete an element from a linked list in C:

    First, do what you have learned (in CS170) to delete the element from the linked list Before you delete the element from the linked list (i.e., before the element become inaccessible) record the address of the deleted list element !!! Then, use the address that you have saved in the previous step to un-reserve the memory cells for the deleted list element

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  • Example: delete the first element from a linked list (in C)

    struct ListElem* deleteHead ( struct ListElem* h ) { struct ListElem* deleted; struct ListElem* newHead; if ( h == NULL ) return(NULL); // Nothing to delete.... deleted = h; // Save the location of the deleted element newHead = h->next; // We will return this value free(deleted); // Un-reserve memory cells return( newHead ); // Make the second element the new head }

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link(s) and save in a scratch directory To compile:   gcc delete-list1.c To run:          ./a.out

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• Note
  • Deleting from tail or from any position in the linked list will not be discussed in this course.

    I don't want to waste time teaching you deleting from linked list again....

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    You just apply what you have learned in CS170 to delete the list element; but make sure you save the address (reference) of the deleted element

    Then use the free( deleted_element ); statement to un-reserve the memory cells used before the function return.