C++ :: Linked List - Implementation At Nth Node

I have to write a program which has the user be able to enter a specific value at a specific position of the linkedlist, replacing that node with the user defined value

EX: Enter the value 5 at 2nd node, which will override the old value at the 2nd node with the new one

I am getting a compiler error which terminates my program right after the user presses the return key after he/she has given a position to change the value

Error: Unhandled exception at 0x013C50C1 in Linked(1).exe: 0xC0000005: Access violation reading location 0x0000812B.

I am not going to show the whole code as the problem resides solely on the insert function:

void TheNode::insert(double num, int choice) {
int post = 0;
MyNode *ptr = new MyNode;
(*ptr).value = num;
MyNode *previous = head;
MyNode *current = head->next;

[Code] .....