Re: Promoting unsigned long int to long int

pereges <Broli00@xxxxxxxxx> wrote:
Hello, I'm trying to sort an array of pointers based on the values
they point to. I'm using the quick sort method. The array of pointers
is parent_vpa and left, right represent the indices. The axis can take
value 0,1,2 and hence i decided to use unsigned char in order to save
some space. I noticed that the program fails and terminates abnormally
when I use left and right as unsigned long but works well when its
data type is declared as long. I would have preferred unsigned long
because the array indices should not be negative and in the calling
function unsigned integers (left = 0, right = n - 1) are passed. I
think the error occurs because of when there is exactly one element in
the array , the j-- can cause j to assume garbage values. My question
is am I breaking any rule or commiting some illegal action by passing
an unsigned long int as a parameter in calling function but using a
long int to receive the variable ? My compiler doesn't complain.

Using unsigned long should be completely ok.

#define ulong unsigned long int
#define uchar unsigned char

void quicksort(vector **parent_vpa, ulong left, ulong right, uchar
axis)
{
ulong i, j;
vector *pivotpoint;
vector *tempstore;

i = left;
j = right;

Lets look at the case you say seems to be the one where things
fail, left = right = 0. In that case i = j = 0

pivotpoint = parent_vpa[(left+right)/2];

Since i and j are equal you get into this loop.

while (i <= j)
{

The next two inner loops shouldn't do anything.

while (parent_vpa[i]->coord[axis] < pivotpoint->coord[axis])
{
i++;
}
while (parent_vpa[j]->coord[axis] > pivotpoint->coord[axis])
{
j--;
}

if (i <= j)
{
tempstore = parent_vpa[i];
parent_vpa[i] = parent_vpa[j];
parent_vpa[j] = tempstore;

This is useless if i == j.

i++;

Now i gets set to 1.

j--;

And j gets set to ULONG_MAX (unsigned integers "wrap around").
That will keep you in this loop for a loooong time and you will
try to access elements of your array that don't exist, rather
likely resulting in a crash.

}
}

if (left < j)
{
quicksort(parent_vpa, left, j, axis);
}

if (i < right)
{
quicksort(parent_vpa, i, right, axis);
}

return;
}

The simplest solution is to just bail out of the function if
left is equal to right - there's nothing to sort in that case.

Regards, Jens
--
\ Jens Thoms Toerring ___ jt@xxxxxxxxxxx
\__________________________ http://toerring.de
.

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