valarray.sum() gives unclean results

[Oliver Block]

Hi everybody,

using the following code leads to unclean result of sum() function:

/* va is a nxn array (valarray<int> (0,n*n) ) filled with integers 1 ..
5 */
int sum = 0;

for(int i = 0; i < n; i++) {
std::valarray<int> col = va[std::slice(i*n, n, n)];
cout << "sum: " << col.sum();
}

This leads to results like 14993885 even if the maximum should be 15

I have no clue what the problem might be.

I using a SuSE Linux Box with gcc 3.3 (as far as I remember the
version).

bye,

oliver

 

[John Harrison]

Hi everybody,

using the following code leads to unclean result of sum() function:

/* va is a nxn array (valarray<int> (0,n*n) ) filled with integers 1 ..
5 */
int sum = 0;

for(int i = 0; i < n; i++) {
std::valarray<int> col = va[std::slice(i*n, n, n)];
cout << "sum: " << col.sum();
}

This leads to results like 14993885 even if the maximum should be 15

I have no clue what the problem might be.

I using a SuSE Linux Box with gcc 3.3 (as far as I remember the
version).

bye,

oliver

Because it accesses outside of the bounds of the valarray. Either you
meant this

std::valarray<int> col = va[std::slice(i*n, n, 1)];

or this

std::valarray<int> col = va[std::slice(i, n, 1)];

John

 

[John Harrison]

std::valarray<int> col = va[std::slice(i, n, 1)];

Typo

std::valarray<int> col = va[std::slice(i, n, n)];

john

 

[Oliver Block]

n is the dimension of the nxn array.

i*n is 1*n, 2*n ... which is supposed to be the first element of each
column of my (logical) 2dim array:

for 3x3:

each column has 3 elements and the distance (stride) is also 3.

11 12 13
21 22 23
31 32 33

is saved in va like this

11 21 31 12 22 32 13 23 33

start:

0*3 = 0; 1*3 = 3; 2*3 = 6

You are right: stride should be 1.

Thanks,

Oliver