Stack using doubly linked list for the first time in C, with no printing

February 14, 2023

I have implemented my stack using doubly linked list and I don’t know why it’s not printing, I think it has something to do with the right C syntax or something?

#include <stdio.h>
#include <stdlib.h>

typedef struct Node {
    int data;
    struct Node* prev;
    struct Node* next;
} Node;

//---------------------Stack---------------------

typedef struct Stack {
    int size;
    Node* head;
    Node* tail;
    int top;
} Stack;

const Stack stack_init = { .size = 0, .head = NULL, .tail = NULL, .top = -1 };

Node* create_node(int elm) {
    Node* node = malloc(sizeof * node);
    if (!node) return node;
    node->data = elm;
    node->prev = NULL;
    node->next = NULL;
    return node;
}

void push(Stack s, int elm) {
    Node* updated_head = create_node(elm);
    if (!s.head) {
        s.head = updated_head;
        s.tail = s.head;
    }
    else {
        updated_head->next = s.head;
        s.head->prev = updated_head;
        s.head = updated_head;
    }
    s.size++;
    s.top = s.head->data;
    // printf("%d ", s.tail->data);
}

int pop(Stack s) {
    Node* node = s.head;
    int elm = node->data;
    s.head = s.head->next;
    if (s.head) {
        s.head->prev = NULL;
        s.top = s.head->data;
    }
    else {
        s.tail = NULL;
        s.top = -1;
    }
    s.size--;
    free(node);
    return elm;
}

int top(Stack s) {
    return s.top;
}

void clear_s(Stack s) {
    while (s.tail)
        pop(s);
}

Stack reverse_s(Stack s) { // iterative
    Stack s2= stack_init;
    while (s.tail) {
        push(s2, pop(s));
    }
    while (s2.tail) {
        push(s, pop(s2));
    }
    return s;
}

int is_empty_s(Stack s) {
    return s.tail == NULL;
}

void print_s(Stack s) {
    Node* trav = s.head;
    while (trav) {
        printf("%d ", trav->data);
        trav = trav->next;
    }
    printf("\n");
}

int main() {

    Stack s1 = stack_init;
    push(s1, 5);
    push(s1, 4);
    push(s1, 3);
    push(s1, 2);
    push(s1, 1);
    print_s(s1);
    
    return 0;
}

As you can see, the stack is based off my doubly linked list which is functional and working, but the print function when starting from the head node doesn’t output anything.

>Solution :

C uses call by value which means that push(s1, 5) will pass a copy of s1 and 5 are to the function. As you want to mutation you need to pass the address of s1:

#include <stdio.h>
#include <stdlib.h>

typedef struct Node {
    int data;
    struct Node* prev;
    struct Node* next;
} Node;

typedef struct Stack {
    int size;
    Node* head;
    Node* tail;
    int top;
} Stack;

const Stack stack_init = { .size = 0, .head = NULL, .tail = NULL, .top = -1 };

Node* create_node(int elm) {
    Node* node = malloc(sizeof * node);
    if (node) {
        node->data = elm;
        node->prev = NULL;
        node->next = NULL;
    }
    return node;
}

void push(Stack *s, int elm) {
    Node* updated_head = create_node(elm);
    if (!s->head) {
        s->head = updated_head;
        s->tail = s->head;
    } else {
        updated_head->next = s->head;
        s->head->prev = updated_head;
        s->head = updated_head;
    }
    s->size++;
    s->top = s->head->data;
}

void print_s(Stack *s) {
    Node* trav = s->head;
    while (trav) {
        printf("%d ", trav->data);
        trav = trav->next;
    }
    printf("\n");
}

int main() {
    Stack s1 = stack_init;
    push(&s1, 5);
    push(&s1, 4);
    push(&s1, 3);
    push(&s1, 2);
    push(&s1, 1);
    print_s(&s1);
}

and the resulting output is: