topic Hej, in Intel® oneAPI Math Kernel Library

cblas_zsyrk returns wrong results

Collins__Andrew — Tue, 10 Apr 2018 08:50:52 GMT

I have the following function and I'm trying to compute C = A*A' using zsyrk from blas, but when I print the C matrix the results are wrong. What am I missing?

double* zsyrk(int N, double *A) {

	int i,j;

	int K;
	K = N;

	const int * alpha;
	int a = 1;
	alpha = &a;

	int lda = N;
	int ldc = N;

	const int * beta;
	int b = 0;
	beta = &b;

	double *C;
	C = (double *)calloc( N*N, sizeof(double));

	for (i = 0; i < N; i++){
		for(j = 0; j < N; j++){
			printf("%lf ", A[i*N + j]);
		}
		printf("\n");
	}

	cblas_zsyrk(CblasRowMajor, CblasUpper, CblasTrans, N, K, alpha, A, lda, beta, C, ldc);

	for (i = 0; i < N; i++){
		for(j = 0; j < N; j++){
			printf("%lf ", C[i*N + j]);
		}
		printf("\n");
	}
	
	return C;
}

The restults are:

Matrix A:

-662.059957 -942.824833
545.621059 -270.699577

Matrix C:
-0.000000 0.000000
0.000000 -0.000000

Hi Andrews,

Ying_H_Intel — Wed, 11 Apr 2018 02:58:21 GMT

Hi Andrews,

are the array A complex number or double array?
How about try cblas_dsyrk to replace the cblas_zsyrk

Best regards,
Ying

BLAS routine names have the following structure:
<character> <name> <mod> ( )
The <character> field indicates the data type:

s	real, single precision
c	complex, single precision

d	real, double precision
z	complex, double precision

Hej,

G__Petru — Wed, 11 Apr 2018 22:51:38 GMT

Hej,

First of all please read the documentation more careful next time.

Instead of int declare both constants, alpha and beta, as double

e.g.

	
				const double * alpha;

	
				09
				    double a = 1;

	
				10
				    alpha = &a;

Also, in order to calculate C as C = A*A' (+yada yada) , you have to use CblasNoTrans instead of CblasTrans (check the documentation).

Now everything will work just as intended.

Regards,

Petru

Hej,

G__Petru — Wed, 11 Apr 2018 23:06:00 GMT

Hej,

First of all, please read the documentation[0] more carefully next time.

In order to calculate correctly you have to:

1. change the alpha and beta constants from int to double.

e.g.

		
					1
					    const double * alpha;
				
					2
					    double a = 1;
				
					3
					    alpha = &a;
				
					4
					 
					5
					    const double * beta;
				
					6
					    double b = 0;
				
					7
					    beta = &b;

 
2. change CblasTrans to CblasNoTrans (check the documentation[0])
[0]:https://software.intel.com/en-us/mkl-developer-reference-c-cblas-syrk
Regards,
Petru