Placing the contents of a linked list in an array reference - c++

Question

My instructions are...
// Description: Places the payload contents of the list in the
// array referenced by 'populateMeWithElements'.
// Returns the number of elements that were placed
// in the provided memory location.
// Precondition: Enough memory has been allocated to the provided
// memory location to hold the full contents
// of the list.
// Postcondition: The memory allocated for 'populateMeWithElements'
// has been deallocated after the completion of
// this method call.
int getListElements(int* populateMeWithElements);
I have this written...
int OOLList::getListElements(int* populateMeWithElements) {
int count = 0;
OOLNode* iterator = this->start;
int* populateMeWithElements = new int[getListSize()];
for (int i = 0; iterator->next != NULL; i++) {
populateMeWithElements[i] = iterator->payload;
iterator = iterator->next;
count++;
}
return count;
}
but I am not sure if it is correct and if it is...
How do I display the contents in my driver
Where I deallocate the memory (do I do this in the driver or in the function that I wrote?)
Thanks in advance for any suggestions or help.

Answer 1

Your function is declaring a local populateMeWithElements variable that shadows the input populateMeWithElements parameter. You are filling the array that you allocate locally, you are not populating the caller's array at all.
For that matter, your function should not be using new[] at all, since the instructions clearly state that the precondition of the function is that the caller has already allocated an array of sufficient size beforehand, and will deallocate that array after the function exits. So, your function's job is merely to fill the caller's array, nothing more.
And on that task, your loop is wrong. iterator->next != NULL needs to be iterator != NULL, otherwise if the list is empty then the 1st access of iterator->next will fail, and if the list is not empty then the payload of the tail node will be skipped.
Try this instead:
int OOLList::getListElements(int* populateMeWithElements) {
int count = 0;
OOLNode* iterator = this->start;
for (int i = 0; iterator != NULL; i++) {
populateMeWithElements[i] = iterator->payload;
iterator = iterator->next;
++count;
}
return count;
}
OOLList list;
// populate list as needed...
int* elements = new int[list.getListSize()];
int count = list.getListElements(elements);
for(int i = 0; i < count; ++i) {
// use elements[i] as needed...
}
delete[] elements;
The function can be simplified a bit further:
int OOLList::getListElements(int* populateMeWithElements) {
int count = 0;
for (OOLNode* iterator = this->start; iterator != NULL; iterator = iterator->next) {
populateMeWithElements[count++] = iterator->payload;
}
return count;
}
Or even:
int OOLList::getListElements(int* populateMeWithElements) {
int* ptr = populateMeWithElements;
for (OOLNode* iterator = this->start; iterator != NULL; iterator = iterator->next) {
*ptr++ = iterator->payload;
}
return ptr - populateMeWithElements;
}