Question

I’ve tried, but I can’t get it to work. I need to delete an element number from a linked list. Here’s what I’ve done so far:

class MatrixLL
{
private:
    struct MatrixLLElem
    {
        Matrix elem;
        MatrixLLElem* next;
        MatrixLLElem(const Matrix& m1): elem(m1), next(NULL)
        { }
    };
    MatrixLLElem* start;
public:
    MatrixLL();
    Matrix& elem(const int index);
    int getlength();
    void append(const Matrix& m1);
    void deleteelem(const int index);
    ~MatrixLL();
};

My other code is of no concern, as it works perfectly, so here’s the code for the deleteelem(); function:

void MatrixLL::deleteelem(const int index)
{
    if(index < 1)
        throw "Invalid index specified.";

    if(start == NULL)
        throw "No element at specified location.";

    MatrixLLElem* currP = start;
    MatrixLLElem** prevP = NULL;

    for(int i = 1; i < index; i++)
    {
        prevP = &currP;
        if((*currP).next != NULL)
            currP = (*currP).next;
        else
            throw "No element at specified location.";
    }
    if(prevP == NULL)
    {
        start = NULL;
    }
    else
    {
        (*prevP) = (*currP).next;
    }
    delete currP;
}

EDIT: It reduces the length from 2 to 0 if I check… And the length function seems to be working fine if I append and then check etc. The index is supposed to start from 1.

Answer 1

The problem is when you want to delete the first element (index=1).

instead of

start = NULL; // wrong

the correct one is:

start = (*currP).next; // the second element now becomes the first element