Contents

# BLAS Code Examples

Example.
Using BLAS Level 1 Function
The following example illustrates a call to the BLAS Level 1 function
sdot
. This function performs a vector-vector operation of computing a scalar product of two single-precision real vectors
x
and
y
.
Parameters

n
Specifies the number of elements in vectors
x
and
y
.
incx
Specifies the increment for the elements of
x
.
incy
Specifies the increment for the elements of
y
.
#include <stdio.h> #include <stdlib.h> #include "mkl_example.h" int main() { MKL_INT n, incx, incy, i; float *x, *y; float res; MKL_INT len_x, len_y; n = 5; incx = 2; incy = 1; len_x = 1+(n-1)*abs(incx); len_y = 1+(n-1)*abs(incy); x = (float *)calloc( len_x, sizeof( float ) ); y = (float *)calloc( len_y, sizeof( float ) ); if( x == NULL || y == NULL ) { printf( "\n Can't allocate memory for arrays\n"); return 1; } for (i = 0; i < n; i++) { x[i*abs(incx)] = 2.0; y[i*abs(incy)] = 1.0; } res = cblas_sdot(n, x, incx, y, incy); printf("\n SDOT = %7.3f", res); free(x); free(y); return 0; }
As a result of this program execution, the following line is printed:
SDOT = 10.000
Example.
Using BLAS Level 1 Routine
The following example illustrates a call to the BLAS Level 1 routine
scopy
. This routine performs a vector-vector operation of copying a single-precision real vector
x
to a vector
y
.
Parameters

n
Specifies the number of elements in vectors
x
and
y
.
incx
Specifies the increment for the elements of
x
.
incy
Specifies the increment for the elements of
y
.
#include <stdio.h> #include <stdlib.h> #include "mkl_example.h" int main() { MKL_INT n, incx, incy, i; float *x, *y; MKL_INT len_x, len_y; n = 3; incx = 3; incy = 1; len_x = 10; len_y = 10; x = (float *)calloc( len_x, sizeof( float ) ); y = (float *)calloc( len_y, sizeof( float ) ); if( x == NULL || y == NULL ) { printf( "\n Can't allocate memory for arrays\n"); return 1; } for (i = 0; i < 10; i++) { x[i] = i + 1; } cblas_scopy(n, x, incx, y, incy); /* Print output data */ printf("\n\n OUTPUT DATA"); PrintVectorS(FULLPRINT, n, y, incy, "Y"); free(x); free(y); return 0; }
As a result of this program execution, the following line is printed:
Y = 1.00000 4.00000 7.00000
Example.
Using BLAS Level 2 Routine
The following example illustrates a call to the BLAS Level 2 routine
sger
. This routine performs a matrix-vector operation
a := alpha*x*y' + a.
Parameters

alpha
Specifies a scalar
alpha
.
x
m
-element vector.
y
n
-element vector.
a
m
-by-
n
matrix.
#include <stdio.h> #include <stdlib.h> #include "mkl_example.h" int main() { MKL_INT m, n, lda, incx, incy, i, j; MKL_INT rmaxa, cmaxa; float alpha; float *a, *x, *y; CBLAS_LAYOUT layout; MKL_INT len_x, len_y; m = 2; n = 3; lda = 5; incx = 2; incy = 1; alpha = 0.5; layout = CblasRowMajor; len_x = 10; len_y = 10; rmaxa = m + 1; cmaxa = n; a = (float *)calloc( rmaxa*cmaxa, sizeof(float) ); x = (float *)calloc( len_x, sizeof(float) ); y = (float *)calloc( len_y, sizeof(float) ); if( a == NULL || x == NULL || y == NULL ) { printf( "\n Can't allocate memory for arrays\n"); return 1; } if( layout == CblasRowMajor ) lda=cmaxa; else lda=rmaxa; for (i = 0; i < 10; i++) { x[i] = 1.0; y[i] = 1.0; } for (i = 0; i < m; i++) { for (j = 0; j < n; j++) { a[i + j*lda] = j + 1; } } cblas_sger(layout, m, n, alpha, x, incx, y, incy, a, lda); PrintArrayS(&layout, FULLPRINT, GENERAL_MATRIX, &m, &n, a, &lda, "A"); free(a); free(x); free(y); return 0; }
As a result of this program execution, matrix
a
is printed as follows:
Matrix A:
1.50000 2.50000 3.50000
1.50000 2.50000 3.50000
Example.
Using BLAS Level 3 Routine
The following example illustrates a call to the BLAS Level 3 routine
ssymm
. This routine performs a matrix-matrix operation
c := alpha*a*b' + beta*c.
Parameters

alpha
Specifies a scalar
alpha
.
beta
Specifies a scalar
beta
.
a
Symmetric matrix
b
m
-by-
n
matrix
c
m
-by-
n
matrix
#include <stdio.h> #include <stdlib.h> #include "mkl_example.h" int main(int argc, char *argv[]) { MKL_INT m, n, i, j; MKL_INT lda, ldb, ldc; MKL_INT rmaxa, cmaxa, rmaxb, cmaxb, rmaxc, cmaxc; float alpha, beta; float *a, *b, *c; CBLAS_LAYOUT layout; CBLAS_SIDE side; CBLAS_UPLO uplo; MKL_INT ma, na, typeA; uplo = 'u'; side = 'l'; layout = CblasRowMajor; m = 3; n = 2; lda = 3; ldb = 3; ldc = 3; alpha = 0.5; beta = 2.0; if( side == CblasLeft ) { rmaxa = m + 1; cmaxa = m; ma = m; na = m; } else { rmaxa = n + 1; cmaxa = n; ma = n; na = n; } rmaxb = m + 1; cmaxb = n; rmaxc = m + 1; cmaxc = n; a = (float *)calloc( rmaxa*cmaxa, sizeof(float) ); b = (float *)calloc( rmaxb*cmaxb, sizeof(float) ); c = (float *)calloc( rmaxc*cmaxc, sizeof(float) ); if ( a == NULL || b == NULL || c == NULL ) { printf("\n Can't allocate memory arrays"); return 1; } if( layout == CblasRowMajor ) { lda=cmaxa; ldb=cmaxb; ldc=cmaxc; } else { lda=rmaxa; ldb=rmaxb; ldc=rmaxc; } if (uplo == CblasUpper) typeA = UPPER_MATRIX; else typeA = LOWER_MATRIX; for (i = 0; i < m; i++) { for (j = 0; j < m; j++) { a[i + j*lda] = 1.0; } } for (i = 0; i < m; i++) { for (j = 0; j < n; j++) { c[i + j*ldc] = 1.0; b[i + j*ldb] = 2.0; } } cblas_ssymm(layout, side, uplo, m, n, alpha, a, lda, b, ldb, beta, c, ldc); printf("\n\n OUTPUT DATA"); PrintArrayS(&layout, FULLPRINT, GENERAL_MATRIX, &m, &n, c, &ldc, "C"); free(a); free(b); free(c); return 0; }
As a result of this program execution, matrix
c
is printed as follows:
Matrix C:
5.00000 5.00000
5.00000 5.00000
5.00000 5.00000
The following example illustrates a call from a C program to the Fortran version of the complex BLAS Level 1 function
zdotc()
. This function computes the dot product of two double-precision complex vectors.
Example.
Calling a Complex BLAS Level 1 Function from C
In this example, the complex dot product is returned in the structure
c
.
#include <cstdio> #include "mkl_blas.h" #define N 5 void main() { int n, inca = 1, incb = 1, i; MKL_Complex16 a[N], b[N], c; void zdotc(); n = N; for( i = 0; i < n; i++ ){ a[i].real = (double)i; a[i].imag = (double)i * 2.0; b[i].real = (double)(n - i); b[i].imag = (double)i * 2.0; } zdotc( &c, &n, a, &inca, b, &incb ); printf( "The complex dot product is: ( %6.2f, %6.2f )\n", c.real, c.imag ); }
Instead of calling BLAS directly from C programs, you might wish to use the C interface to the Basic Linear Algebra Subprograms (CBLAS) implemented in
Intel® oneAPI Math Kernel Library