Deleting an entire node from a singly linked list

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开发者_如何学Python 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.

---

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

---

I got it right. If it helps someone, this is the code:

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)
    {
        (*prevP).next = (*currP).next;
    }
    else
    {
        start = (*start).next;
    }

    delete currP;
}

Just for reference, the start index is 1, not 0.

---

(*prevP) = (*currP).next;

should be

(*prevP).next = (*currP).next;

---

Looks like the condition and the changing pointers part are wrong. Should be like this:

MatrixLLElem* prevP = NULL; // Why ** ?

for(int i = 1; i <= index; ++i) // Here
{
    prevP = currP;
    if((*currP).next != NULL)
        currP = (*currP).next;
    else
        throw "No element at specified location.";
}
if(currP == start) // This would be better
{
    start = NULL;
}
else
{
    (*prevP).next = (*currP).next; // And here
}
delete currP;