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.

program dot_main
real x(10), y(10), sdot, res
integer n, incx, incy, i
external sdot
n = 5
incx = 2
incy = 1
do i = 1, 10
  x(i) = 2.0e0
  y(i) = 1.0e0
end do
res = sdot (n, x, incx, y, incy)
print*, `SDOT = `, res
end

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.

 
program copy_main
real x(10), y(10)
integer n, incx, incy, i
n = 3
incx = 3
incy = 1
do i = 1, 10
  x(i) = i
end do
call scopy (n, x, incx, y, incy)
print*, `Y = `, (y(i), i = 1, n)
end

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.

program ger_main
real a(5,3), x(10), y(10), alpha
integer m, n, incx, incy, i, j, lda
m = 2
 n = 3
lda = 5
incx = 2
incy = 1
alpha = 0.5
do i = 1, 10
  x(i) = 1.0
  y(i) = 1.0
end do
do i = 1, m
  do j = 1, n
    a(i,j) = j
  end do
end do
call sger (m, n, alpha, x, incx, y, incy, a, lda)
print*, `Matrix A: `
do i = 1, m
  print*, (a(i,j), j = 1, n)
end do
end

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.

program symm_main
real a(3,3), b(3,2), c(3,3), alpha, beta
integer m, n, lda, ldb, ldc, i, j
character uplo, side
uplo = 'u'
side = 'l'
m = 3
n = 2
lda = 3
ldb = 3
ldc = 3
alpha = 0.5
beta = 2.0
do i = 1, m
  do j = 1, m
    a(i,j) = 1.0
  end do
end do
do i = 1, m
  do j = 1, n
    c(i,j) = 1.0
    b(i,j) = 2.0
  end do
end do
call ssymm (side, uplo, m, n, alpha,
a, lda, b, ldb, beta, c, ldc)
print*, `Matrix C: `
do i = 1, m
  print*, (c(i,j), j = 1, n)
end do
end

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

Example. Calling a Complex BLAS Level 1 Function from C

In this example, the complex dot product is returned in the structure c.

#define N 5
void main()
{
 
  int n, inca = 1, incb = 1, i;
  typedef struct{ double re; double im; } complex16;
  complex16 a[N], b[N], c;
  void zdotc();
  n = N;
  for( i = 0; i < n; i++ ){
    a[i].re = (double)i; a[i].im = (double)i * 2.0;
    b[i].re = (double)(n - i); b[i].im = (double)i * 2.0;
  }
  zdotc( &c, &n, a, &inca, b, &incb );
  printf( "The complex dot product is: ( %6.2f, %6.2f )\n", c.re, c.im );
}

Note

Instead of calling BLAS directly from C programs, you might wish to use the CBLAS interface; this is the supported way of calling BLAS from C. For more information about CBLAS, see Appendix “CBLAS Interface to the BLAS”, which presents CBLAS, the C interface to the Basic Linear Algebra Subprograms (BLAS) implemented in Intel® MKL.