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# Count comparisons in sort algorithms??

Posted on 2003-10-27
Medium Priority
711 Views
I'm interested in tracking the number of comparisons in some sort algorithms to compare them.
Im not exactly sure where this counter need to be incremented.  If anyone has any help, thanks.
Some of the sorts I am using include the following.

INSERTION
-------------------------------------------------------------------------------------
template <class T>
void InsertionSort(T A[], int size)
{
for(int j=1; j<size; j++)
{
T key = A[j];
int i = j - 1;
while ((i >= 0) && (A[i] > key))
{
A[i+1] = A[i];
i -= 1;
}
A[i+1] = key;
}
}

HEAPSORT
------------------------------------------------------------------------------
template <class T>
void Heapsort(T A[], int size)
{
BuildHeap(A, size);
for(i=size; i>1; i--)
{
int tmp = A[0];
A[0] = A[i-1];
A[i-1] = tmp;
size--;
Heapify(A, 1, size);
}
}

template <class T>
void BuildHeap(T A[], int size)
{
for(i=size/2; i>0; i--)
Heapify(A, i, size);
}

template <class T>
void Heapify(T A[], int i, int size)
{
int  l = 2 * i;
int  r = 2 * i + 1;
int largest = 0;

if(l <= heapsize && A[l-1] > A[i-1])
largest = l;
else
largest = i;

if(r <= heapsize && A[r-1] > A[largest-1])
largest = r;
if(largest != i)
{
int tmp = A[i-1];
A[i-1] = A[largest-1];
A[largest-1] = tmp;
Heapify(A, largest, heapsize);
}
}

0
Question by:killer455
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LVL 7

Expert Comment

ID: 9631526
Um, you would need to increment a counter everytime you compare elements in the array.

INSERTION - Inside the while loop
HEAPSORT - Twice for each call of Heapify()

I dont think you really want comparisons of non-array elements.
0

Author Comment

ID: 9631890
So if I were to place a counter in the heapify function, is there a way I can use that in the original heapsort function in order to produce output after heapsort is done running?
Does this run into making the variable global? If so how is this done?
0

LVL 7

Expert Comment

ID: 9631900
There are many ways you could do it:
1. Use a global variable to count the number of comparisons
2. Create a property for the class and use that to store the count
3. Return the count from the Heapify function

Decide what you're after and have a go. Im not inclined to do too much until I can see you're trying to figure it out.
0

Author Comment

ID: 9635830
I guess im confused, sorry :(  I modofied it to return the compares??

HEAPSORT
------------------------------------------------------------------------------
template <class T>
void Heapsort(T A[], int size)
{
BuildHeap(A, size);
for(i=size; i>1; i--)
{
int tmp = A[0];
A[0] = A[i-1];
A[i-1] = tmp;
size--;
Heapify(A, 1, size);
}
}

template <class T>
void BuildHeap(T A[], int size)
{
for(i=size/2; i>0; i--)
Heapify(A, i, size);
}

template <class T>
void Heapify(T A[], int i, int size)
{
int compares;
comapres +=2;
int  l = 2 * i;
int  r = 2 * i + 1;
int largest = 0;

if(l <= heapsize && A[l-1] > A[i-1])
largest = l;
else
largest = i;

if(r <= heapsize && A[r-1] > A[largest-1])
largest = r;
if(largest != i)
{
int tmp = A[i-1];
A[i-1] = A[largest-1];
A[largest-1] = tmp;
Heapify(A, largest, heapsize);
}
return compares;
}
0

LVL 7

Accepted Solution

MrNed earned 180 total points
ID: 9637411
You declared the int compares locally inside the Heapify function. This means that when the function exits, that variable is wiped out. To return it successfully, you need to modify the function declaration to return an int rather than a void.

Since you call the Heapify function recursively, you should capture the return value of the Heapify (A, largest, heapsize) call and add that to you compares counter before returning it.
0

Author Comment

ID: 9637633
Well I cant declare it in the heapsort() function because heapify is called both from buildheap and heapsort right?

If I capture it only from Heapify(A, largest, heapsize) won't that make the count on compares inaccurate?

Well sorry for the additional questions, i'm just very confused on how this is done.  If I didnt mention im a beginning programmer so some things that are basically obvious to advanced programmers, are kinda new to me.

Ben

0

LVL 7

Expert Comment

ID: 9647698
This is what im thinking, although there are other ways to do it.

template <class T>
void Heapsort(T A[], int size)
{
int count;
count = BuildHeap(A, size);
for(i=size; i>1; i--)
{
int tmp = A[0];
A[0] = A[i-1];
A[i-1] = tmp;
size--;
count += Heapify(A, 1, size);
}
printf("count = %d\n",count);
}

template <class T>
int BuildHeap(T A[], int size)
{
int count=0;
for(i=size/2; i>0; i--)
count += Heapify(A, i, size);
return count;
}

template <class T>
int Heapify(T A[], int i, int size)
{
int count;
count +=2;
int  l = 2 * i;
int  r = 2 * i + 1;
int largest = 0;

if(l <= heapsize && A[l-1] > A[i-1])
largest = l;
else
largest = i;

if(r <= heapsize && A[r-1] > A[largest-1])
largest = r;
if(largest != i)
{
int tmp = A[i-1];
A[i-1] = A[largest-1];
A[largest-1] = tmp;
count += Heapify(A, largest, heapsize);
}
return count;
}
0

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