getting mergesort to work on linked-list? - c++

Question

Apologies if this is a silly / simple question.. but I'm very lost. I'm having trouble getting this program to run. I've written this program to read in 2 values, the first being a number of elements in a linked list, and the second to be the maximum random value that can be put into each element.
It should then use the merge sort algorithm included to sort and reprint the sorted list.
Ok, so I'm getting errors like:
base operand of `->' has non-pointer type `LIST'
and
request for member `element' in `conductor', which is of non-aggregate type `LIST *'
...(and a few others).
Yes this is for a class.. I've written the program but I'm not sure what I've done wrong here or why I'm getting errors? Any help is appreciated! Thank you
#include <cstdlib>
#include <iostream>
#include <math.h>
#include <sys/time.h>
using namespace std;
typedef struct LIST {
int element;
LIST *next;
};
LIST split(LIST list)
{
LIST pSecondCell;
if (list == NULL)
return NULL;
else if (list.next == NULL)
return NULL;
else {
pSecondCell = list.next;
list.next = pSecondCell.next;
pSecondCell.next = split(pSecondCell->next);
return pSecondCell;
}
}
LIST merge(LIST list1, LIST list2)
{
if (list1 == NULL)
return list2;
else if (list2 == NULL)
return list1;
else if (list1.element <= list2.element) {
list1.next = merge(list1.next, list2);
return list1;
} else {
list2.next = merge(list1, list2.next);
}
}
LIST MergeSort(LIST list)
{
LIST SecondList;
if (list == NULL)
return NULL;
else if (list.next == NULL)
return list;
else {
SecondList = split(list);
return merge(MergeSort(list), MergeSort(SecondList));
}
}
int main(int argCount, char *argVal[])
{
int i, number, max;
struct timeval time1;
struct timeval time2;
//check for correct number of arguments
if (argCount != 3) {
cout << "Incorrect number of arguments" << endl;
return 0;
}
// initialize read in n and max values
number = atoi(argVal[1]);
max = atoi(argVal[2]);
// create list and fill with random numbers
LIST *conductor;
LIST *root = new LIST;
conductor = root;
for (i = 0; i < number; i++) {
conductor.element = rand() % max;
conductor.next = new LIST;
conductor = conductor.next;
}
// time how long it takes to sort array using mergeSort
gettimeofday(&time1, NULL);
mergeSort(root);
gettimeofday(&time2, NULL);
// print name, sorted array, and running time
cout << "Heather Wilson" << endl;
conductor = root;
for (i = 0; i < number - 2; i++) {
cout << conductor.element << ", ";
conductor = conductor.next;
}
double micro1 = time1.tv_sec * 1000000 + time1.tv_usec;
double micro2 = time2.tv_sec * 1000000 + time2.tv_usec;
cout << conductor.element << endl;
cout << "Running time: " << micro2 - micro1 << " microseconds" << endl;
return 0;
}

Answer 1

For base operand of->' has non-pointer type LIST'
Replace the -> with a .. You want to access a member of a local LIST, not a member of a pointed at object.
request for memberelement' in conductor', which is of non-aggregate type LIST *
This is the opposite. Replace the . with a ->. You want to access a member of the pointed at LIST, not a member of the pointer.
For clarification, I didn't read the code. There's too much of it. But those are the usual ways to address those specific errors. parapura seems to have actually read the code.

Answer 2

First: you should never have let the code grow this big with so many errors. You should start small and simple, then build up, testing at every stage, and never add to code that doesn't work.
Here's a stripped-down beginning of your code, with some bugs fixed:
#include <iostream>
using namespace std;
typedef struct LIST{
int element;
LIST *next;
};
int main(){
int i, number, max;
number = 5;
max = 100;
// create list and fill with random numbers
LIST *conductor;
LIST *root = new LIST;
conductor = root;
for(i=0; i<number; i++){
conductor->element = rand() % max;
cout << "element " << i << " is " << conductor->element << endl;
conductor->next = new LIST;
conductor = conductor->next;
}
conductor = root; // Forgot this, didn't you!
for(i=0; i<number-2;i++){
cout << conductor->element << ", ";
conductor = conductor->next;
}
return 0;
}
Take a look at this, verify that it works, make sure you understand the changes I made, then you can take a crack at implementing your split, merge and MergeSort functions and the I/O (one at a time, and testing at every stage, naturally).

Answer 3

I think all the places you are passing
LIST merge ( LIST list1 , LIST list2 )
it should be
LIST* merge ( LIST* list1 , LIST* list2 )