C++ declaring double-linked list, traverse list both ways with two for-loops and print

Question

Just beginning with C++ (be gentle, please). I have some code and I have to:

• Add Element* to make doubly-linked list
• Add insert_before() and insert_after() methods to Element
• Traverse list both ways with two separate for-loops
• Print operating number inside each loop

Code

I tried the following:

#include <cstdio>

struct Element {
    Element* next{};
    Element* previous{};

    void insert_after(Element* new_element) {
        new_element -> previous = this;
        new_element -> next = this -> next;
        this -> next = new_element;
    }
    void insert_before(Element* new_element) {
        new_element -> previous = this -> previous;
        new_element -> next = this;
        this -> previous = new_element;
    }
    char prefix[2];
    short operating_number;
};

int main() {
    Element trooper1, trooper2, trooper3, trooper4;
    trooper1.prefix[0] = 'T';
    trooper1.prefix[1] = 'K';
    trooper1.operating_number = 421;
    trooper1.insert_after(&trooper2);
    trooper2.prefix[0] = 'F';
    trooper2.prefix[1] = 'N';
    trooper2.operating_number = 2187;
    trooper2.insert_before(&trooper3);
    trooper3.prefix[0] = 'L';
    trooper3.prefix[1] = 'S';
    trooper3.operating_number = 005;
    trooper3.insert_before(&trooper4);
    trooper4.prefix[0] = 'F';
    trooper4.prefix[1] = 'K';
    trooper4.operating_number = 2602;

    for (Element *cursor = &trooper1; cursor; cursor = cursor -> next) {
        printf("stormtrooper %c%c-%d\n",
                cursor->prefix[0],
                cursor->prefix[1],
                cursor->operating_number);
    }
    for (Element *cursor = &trooper3; cursor; cursor = cursor -> previous) {
        printf("stormtrooper %c%c-%d\n",
               cursor->prefix[0],
               cursor->prefix[1],
               cursor->operating_number);
    }
}

It's very basic, I know. But I'm starting to learn. The code compiles correctly but I get this output:

/home/facundo/Escritorio/C++Projects/cmake-build-debug/C__Projects
stormtrooper TK-421
stormtrooper FN-2187
stormtrooper LS-5
stormtrooper FK-2602
stormtrooper TK-421

Process finished with exit code 0

I really don't understand why it prints only that (I guess there should be 8 lines of output). I was expecting an output that would print the elements like this:

trooper1

trooper4

trooper3

trooper2

trooper2

trooper3

trooper4

trooper1

Some help would be really appreciated. Thanks for taking your time.

Answer 1

In your insert functions, you are not linking up all the pointers correctly:

void insert_after(Element* new_element) {
    new_element -> previous = this;
    new_element -> next = this -> next;
    this -> next = new_element;
    if (new_element -> next)  // this check needed
        // to correctly link the next -> previous
        new_element -> next -> previous = new_element; 
}

and

void insert_before(Element* new_element) {
    new_element -> previous = this -> previous;
    new_element -> next = this;
    this -> previous = new_element;
    if (new_element -> previous) // this check needed
        // to correctly link the previous -> next
        new_element -> previous -> next = new_element;
}

Also, in the second for loop, note that trooper2 is the last Element in the list, not trooper3, so you need to start from there to see all the Elements in reverse.

Here's a demo.