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In my intro CS class we're reviewing data structures. I'm currently working on implementing a queue using a linked list (FIFO) in C. I'd appreciate a review of the implementation as well as of my understanding of how a queue should work

// This program is implementation of queue data structure// via linked list (FIFO)#include <stdio.h>#include <stdlib.h>#include <string.h>typedef struct node{    int number;    struct node *next;} node;node *enqueue(int element, node *temp);node *dequeue(int n, node *temp);void display(node *temp);void destroy_queue(node *temp);int main(){    node *queue = NULL;    // add some elements to queue    for (int i = 0; i < 5; i++)    {        queue = enqueue((i + 1), queue);    }    // display queue    display(queue);    // dequeue 2 elements from queue    printf("Removing two elements from queue...\n");    queue = dequeue(2, queue);    //display queue    display(queue);    // free memory    destroy_queue(queue);}node *enqueue(int element, node *temp){    node *newElement = malloc(sizeof(node));    node *head = temp;    if (newElement == NULL)    {        fprintf(stderr, "No memory for the new queue element");        return temp;    }    newElement->number = element;    newElement->next = NULL;    if (head == NULL)    {        return newElement;    }    else    {        while ((temp->next) != NULL)        {            temp = temp->next;        }        temp->next = newElement;    }    return head;}node *dequeue(int n, node *temp){    node *aux;    for (int i = 0; i < n; i++)    {        if (!temp)        {            fprintf(stderr,"No elements to remove!");            return temp;        }        aux = temp->next;        free(temp);        temp = aux;    }    return temp;}void display(node *temp){    while (temp)    {        printf("Elements in queue are: %i\n", temp->number);        temp = temp->next;    }}void destroy_queue(node *temp){    node *aux = temp;    while (temp)    {        aux = temp->next;        free(temp);        temp = aux;    }}
askedOct 10, 2018 at 19:16
Ivan Stimac's user avatar
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1 Answer1

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  • Having a separatequeue structure

    typedef struct {    node * head;    node * tail;} queue;

    will bring you several benefits:

    • Most important,enqueue would take a constant time (vs linear in queue length you have now).

    • The client would be relieved from responsibility to maintain the head node.

    • enqueue doesn't inform the client that it failed the allocation.fprintf is good for the human, but does nothing for the calling routine. Having thequeue structure enables you to return an error code.

  • The specialized utility functions, likeenqueue anddequeue should not print anything. They must communicate success/error to the caller, and let it act appropriately.

  • else inenqueue is redundant.

  • I don't see why do you#include <string.h>.
answeredOct 10, 2018 at 23:06
vnp's user avatar
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  • \$\begingroup\$Constant timeenqueue() anddequeue() does not require 2 pointers inqueue. One is sufficient. Simple have member.tail and let the tail of the queue point to the head. Adding to either end of queue isO(1) Removing from the head is alsoO(1).\$\endgroup\$CommentedOct 11, 2018 at 5:30
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    \$\begingroup\$@chux Right. Circular queue is better. I was pondering that as well, but decided against for purely didactical reasons.\$\endgroup\$CommentedOct 11, 2018 at 5:36

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