I converted a c++ vector data-set into a c array. I pass the same data-set into a function that should give the same output

Question

This code should generate the same output. But the c output seems to be rounded up while c++ is not. I don't understand what is going on here. Please explain.

Upto the point where I show the code, same values are passed to both c and c++ code.

c code:

unsigned int sz1=sz+1; 
double* bMat = (double*)calloc(bMatSize,bMatSize * sizeof(double));

for (unsigned int ii = 0; ii<sz; ii++)
    for (unsigned int jj = 0; jj <= ii; jj++)
    {
       bMat[jj*sz1 + ii] = simgms_math_ddot(numBFunc, queueTest[ii], queueTest[jj] );
       bMat[ii*sz1 + jj] = bMat[jj*sz1 + ii];
    }

c++ code:

std::vector<double> bMat;
unsigned int sz1=sz+1;
bMat.resize(sz1*sz1);
for(unsigned int ii=0; ii<sz; ii++)
    for(unsigned int jj=0; jj<=ii; jj++)
    {
        bMat.at(jj*sz1+ii)=simgms_math_ddot(numBFunc, queueTest.at(ii).data(), queueTest.at(jj).data());
        bMat.at(ii*sz1+jj)=bMat.at(jj*sz1+ii);
    }

for (int i = 0; i < bMat.size(); ++i) {
   std::cout << bMat[i] << ' ';
}
std::cout << std::endl;

common function

double simgms_math_ddot(unsigned int numElements, double aMat[], double bMat[])

{
    const MKL_INT n=numElements;

    double res = cblas_ddot(n, aMat, 1, bMat, 1);

    return res;
}

c coutput (bMat)

0 0 -1 0 0 -1 -1 -1 0

c++ coutput (bMat)

 1.05e-05 0.00134 -1 0.00134 1.11 -1 -1 -1 0

And the error matrix contains values:

1.78e-15 0.387 1.3 -0.48 -17.5 2.28 -0.387 -1.78e-15 1.48 -0.33 -17.3 2.4 -1.3 -1.48 -1.11e-16 1.08 -0.926 2.41 0.48 0.33 -1.08 0 0.551 0.74 17.5 17.3 0.926 -0.551 0 -0.384 -2.28 -2.4 -2.41 -0.74 0.384 0

std::deque< double* > queueTest;

....Its filled with errorMatrix elements, which are 2d arrays,

 e.g: queueTest.push_front(errorMatrix);

how do you print your result on the C side? if your `printf` format is wrong, that would explain a lot. — Jean-François Fabre, Jun 21 '17 at 19:44
@Olaf don't shoot the guy: he's using `vector` in C++ instead of `new`, that's a good start! — Jean-François Fabre, Jun 21 '17 at 19:45
@Jean-FrançoisFabre: How is your comment in any way related to mine? — too honest for this site, Jun 21 '17 at 19:46
@Olaf: Your sentence was in the imperative and included an exclamation mark! Which might be interpreted as shouting! You see?! — einpoklum, Jun 21 '17 at 19:47
@Olaf don't take it the wrong way. I was just pointing out that OP uses correct C++ code. And casting the return of `malloc` in C is not a very serious offence, it's just useless. — Jean-François Fabre, Jun 21 '17 at 19:47
Maybe I'm missing something but I can't see how this is printed in the C version — Support Ukraine, Jun 21 '17 at 19:48
@4386427 exactly, and that's probably the key of the difference. — Jean-François Fabre, Jun 21 '17 at 19:48
As in *there's literally no code that outputs anything*? Well, yeah. Must be missing :) — , Jun 21 '17 at 19:49
I have the code in an 'extern c'. So I still use c++ std::cout to print. So its the same code that I use to print in c++ — MadHatter, Jun 21 '17 at 19:53
@Jean-FrançoisFabre for (unsigned int i = 0; i < bMatSize; ++i) { std::cout << bMat[i] << ' '; } std::cout << std::endl; — MadHatter, Jun 21 '17 at 19:53
@Jean-FrançoisFabre not *just useless*, I think it is *a bit* harmful in that it makes casting of pointer types look like a normal thing to do. But indeed, nothing to "shout" about ;) — , Jun 21 '17 at 19:53
where is `queueTest` defined/allocated in the C version? also does `simgms_math_ddot` has a valid prototype in the C version? — Jean-François Fabre, Jun 21 '17 at 19:55
note that it's not just rounded. Some values don't match their rounded counterparts (ex: 1.11 vs 0) — Jean-François Fabre, Jun 21 '17 at 19:57
@Jean-FrançoisFabre: Such caasts can shadow errors, e.g. typos or missing headers. It is very well considered harmful. It also is a good indicator if the code is **really** compiled with a C compiler or not. There are way too many people thinking compiling C coding style with a C++ compiler is identical to C code. Oh, and shouting is more LIKE THIS! — too honest for this site, Jun 21 '17 at 20:00
@Jean-FrançoisFabre std::deque< double* > queueTest; ....Its filled with errorMatrix elements, which are 2d arrays, e.g: queueTest.push_front(errorMatrix); — MadHatter, Jun 21 '17 at 20:00
@einpoklum: The Exclamation mark just adds some emphasis to the sentence. Shouting would be ALL UPPERCASE. — too honest for this site, Jun 21 '17 at 20:02
And the error matrix contains values, ,1.78e-15 0.387 1.3 -0.48 -17.5 2.28 -0.387 -1.78e-15 1.48 -0.33 -17.3 2.4 -1.3 -1.48 -1.11e-16 1.08 -0.926 2.41 0.48 0.33 -1.08 0 0.551 0.74 17.5 17.3 0.926 -0.551 0 -0.384 -2.28 -2.4 -2.41 -0.74 0.384 0 — MadHatter, Jun 21 '17 at 20:02
@MadHatter - Don't put information in comments - update the question using the `edit` link just below the question — Support Ukraine, Jun 21 '17 at 20:05
@4386427 put "edit" between square brackets to provide the [edit] link. — Jean-François Fabre, Jun 21 '17 at 20:07
@MadHatter correct me if I'm wrong, but a double ended queue isn't contiguous in memory. So in C it will fail. — Jean-François Fabre, Jun 21 '17 at 20:08

Jean-François Fabre · Answer 1 · 2017-06-21T20:17:34.503

The problem is probably because of the way you're storing queueTest in a deque.

queueTest.at(ii).data()

reaches the iith element without problem (but with O(n) complexity, and can make you think that it's indexed access whereas it's not), but if you pass the address of the first element of the std::deque to your C program, adding an offset won't work because, unlike std::vector memory, std::deque memory isn't contiguous (it's a linked list) so you're getting random data for your coefficients.

From http://www.cplusplus.com/reference/deque/deque/:

unlike vectors, deques are not guaranteed to store all its elements in contiguous storage locations: accessing elements in a deque by offsetting a pointer to another element causes undefined behavior.

The quickfix to be able to use it from the C side is to convert your deque to a vector.

I'll try this and get back to you, thanks! – MadHatter Jun 21 '17 at 20:16 — MadHatter, Jun 21 '17 at 20:16

I converted a c++ vector data-set into a c array. I pass the same data-set into a function that should give the same output

1 Answers1