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SSYSV Example Program in Fortran
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* =============================================================================
*
* SSYSV Example.
* ==============
*
* The program computes the solution to the system of linear equations
* with a real symmetric matrix A and multiple right-hand sides B,
* where A is the coefficient matrix:
*
* -5.86 3.99 -5.93 -2.82 7.69
* 3.99 4.46 2.58 4.42 4.61
* -5.93 2.58 -8.52 8.57 7.69
* -2.82 4.42 8.57 3.72 8.07
* 7.69 4.61 7.69 8.07 9.83
*
* and B is the right-hand side matrix:
*
* 1.32 -6.33 -8.77
* 2.22 1.69 -8.33
* 0.12 -1.56 9.54
* -6.41 -9.49 9.56
* 6.33 -3.67 7.48
*
* Description.
* ============
*
* The routine solves for X the real system of linear equations A*X = B,
* where A is an n-by-n symmetric matrix, the columns of matrix B are
* individual right-hand sides, and the columns of X are the corresponding
* solutions.
*
* The diagonal pivoting method is used to factor A as A = U*D*UT or
* A = L*D*LT , where U (or L) is a product of permutation and unit upper
* (lower) triangular matrices, and D is symmetric and block diagonal with
* 1-by-1 and 2-by-2 diagonal blocks.
*
* The factored form of A is then used to solve the system of equations A*X = B.
*
* Example Program Results.
* ========================
*
* SSYSV Example Program Results
*
* Solution
* 1.17 0.52 -0.86
* -0.71 1.05 -4.90
* -0.63 -0.52 0.99
* -0.33 0.43 1.22
* 0.83 -1.22 1.96
*
* Details of factorization
* -5.86 0.00 0.00 0.00 0.00
* -0.68 7.18 0.00 0.00 0.00
* 1.01 -0.20 -2.82 0.00 0.00
* 0.48 0.35 11.93 4.21 0.00
* -1.31 1.37 0.02 0.16 6.22
*
* Pivot indices
* 1 2 -4 -4 5
* =============================================================================
*
* .. Parameters ..
INTEGER N, NRHS
PARAMETER ( N = 5, NRHS = 3 )
INTEGER LDA, LDB
PARAMETER ( LDA = N, LDB = N )
INTEGER LWMAX
PARAMETER ( LWMAX = 100 )
*
* .. Local Scalars ..
INTEGER INFO, LWORK
*
* .. Local Arrays ..
INTEGER IPIV( N )
REAL A( LDA, N ), B( LDB, NRHS ), WORK( LWMAX )
DATA A/
$ -5.86, 3.99,-5.93,-2.82, 7.69,
$ 0.00, 4.46, 2.58, 4.42, 4.61,
$ 0.00, 0.00,-8.52, 8.57, 7.69,
$ 0.00, 0.00, 0.00, 3.72, 8.07,
$ 0.00, 0.00, 0.00, 0.00, 9.83
$ /
DATA B/
$ 1.32, 2.22, 0.12,-6.41, 6.33,
$ -6.33, 1.69,-1.56,-9.49,-3.67,
$ -8.77,-8.33, 9.54, 9.56, 7.48
$ /
*
* .. External Subroutines ..
EXTERNAL SSYSV
EXTERNAL PRINT_MATRIX, PRINT_INT_VECTOR
*
* .. Intrinsic Functions ..
INTRINSIC INT, MIN
*
* .. Executable Statements ..
WRITE(*,*)'SSYSV Example Program Results'
*
* Query the optimal workspace.
*
LWORK = -1
CALL SSYSV( 'Lower', N, NRHS, A, LDA, IPIV, B, LDB, WORK, LWORK,
$ INFO )
LWORK = MIN( LWMAX, INT( WORK( 1 ) ) )
*
* Solve the equations A*X = B.
*
CALL SSYSV( 'Lower', N, NRHS, A, LDA, IPIV, B, LDB, WORK, LWORK,
$ INFO )
*
* Check for the exact singularity.
*
IF( INFO.GT.0 ) THEN
WRITE(*,*)'The element of the diagonal factor '
WRITE(*,*)'D(',INFO,',',INFO,') is zero, so that'
WRITE(*,*)'D is singular; the solution could not be computed.'
STOP
END IF
*
* Print solution.
*
CALL PRINT_MATRIX( 'Solution', N, NRHS, B, LDB )
*
* Print details of factorization.
*
CALL PRINT_MATRIX( 'Details of factorization', N, N, A, LDA )
*
* Print pivot indices.
*
CALL PRINT_INT_VECTOR( 'Pivot indices', N, IPIV )
STOP
END
*
* End of SSYSV Example.
*
* =============================================================================
*
* Auxiliary routine: printing a matrix.
*
SUBROUTINE PRINT_MATRIX( DESC, M, N, A, LDA )
CHARACTER*(*) DESC
INTEGER M, N, LDA
REAL A( LDA, * )
*
INTEGER I, J
*
WRITE(*,*)
WRITE(*,*) DESC
DO I = 1, M
WRITE(*,9998) ( A( I, J ), J = 1, N )
END DO
*
9998 FORMAT( 11(:,1X,F6.2) )
RETURN
END
*
* Auxiliary routine: printing a vector of integers.
*
SUBROUTINE PRINT_INT_VECTOR( DESC, N, A )
CHARACTER*(*) DESC
INTEGER N
INTEGER A( N )
*
INTEGER I
*
WRITE(*,*)
WRITE(*,*) DESC
WRITE(*,9999) ( A( I ), I = 1, N )
*
9999 FORMAT( 11(:,1X,I6) )
RETURN
END
Parent topic: SSYSV Example