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SGELS Example Program in Fortran
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* =============================================================================
*
* SGELS Example.
* ==============
*
* Program computes the least squares solution to the overdetermined linear
* system A*X = B with full rank matrix A using QR factorization,
* where A is the coefficient matrix:
*
* 1.44 -7.84 -4.39 4.53
* -9.96 -0.28 -3.24 3.83
* -7.55 3.24 6.27 -6.64
* 8.34 8.09 5.28 2.06
* 7.08 2.52 0.74 -2.47
* -5.45 -5.70 -1.19 4.70
*
* and B is the right-hand side matrix:
*
* 8.58 9.35
* 8.26 -4.43
* 8.48 -0.70
* -5.28 -0.26
* 5.72 -7.36
* 8.93 -2.52
*
* Description.
* ============
*
* The routine solves overdetermined or underdetermined real linear systems
* involving an m-by-n matrix A, or its transpose, using a QR or LQ
* factorization of A. It is assumed that A has full rank.
*
* Several right hand side vectors b and solution vectors x can be handled
* in a single call; they are stored as the columns of the m-by-nrhs right
* hand side matrix B and the n-by-nrhs solution matrix X.
*
* Example Program Results.
* ========================
*
* SGELS Example Program Results
*
* Solution
* -0.45 0.25
* -0.85 -0.90
* 0.71 0.63
* 0.13 0.14
*
* Residual sum of squares for the solution
* 195.36 107.06
*
* Details of QR factorization
* -17.54 -4.76 -1.96 0.42
* -0.52 12.40 7.88 -5.84
* -0.40 -0.14 -5.75 4.11
* 0.44 -0.66 -0.20 -7.78
* 0.37 -0.26 -0.17 -0.15
* -0.29 0.46 0.41 0.24
* =============================================================================
*
* .. Parameters ..
INTEGER M, N, NRHS
PARAMETER ( M = 6, N = 4, NRHS = 2 )
INTEGER LDA, LDB
PARAMETER ( LDA = M, LDB = M )
INTEGER LWMAX
PARAMETER ( LWMAX = 100 )
*
* .. Local Scalars ..
INTEGER INFO, LWORK
*
* .. Local Arrays ..
REAL A( LDA, N ), B( LDB, NRHS ), WORK( LWMAX )
DATA A/
$ 1.44,-9.96,-7.55, 8.34, 7.08,-5.45,
$ -7.84,-0.28, 3.24, 8.09, 2.52,-5.70,
$ -4.39,-3.24, 6.27, 5.28, 0.74,-1.19,
$ 4.53, 3.83,-6.64, 2.06,-2.47, 4.70
$ /
DATA B/
$ 8.58, 8.26, 8.48,-5.28, 5.72, 8.93,
$ 9.35,-4.43,-0.70,-0.26,-7.36,-2.52
$ /
*
* .. External Subroutines ..
EXTERNAL SGELS
EXTERNAL PRINT_MATRIX, PRINT_VECTOR_NORM
*
* .. Intrinsic Functions ..
INTRINSIC INT, MIN
*
* .. Executable Statements ..
WRITE(*,*)'SGELS Example Program Results'
*
* Query the optimal workspace.
*
LWORK = -1
CALL SGELS( 'No transpose', M, N, NRHS, A, LDA, B, LDB, WORK,
$ LWORK, INFO )
LWORK = MIN( LWMAX, INT( WORK( 1 ) ) )
*
* Solve the equations A*X = B.
*
CALL SGELS( 'No transpose', M, N, NRHS, A, LDA, B, LDB, WORK,
$ LWORK, INFO )
*
* Check for the full rank.
*
IF( INFO.GT.0 ) THEN
WRITE(*,*)'The diagonal element ',INFO,' of the triangular '
WRITE(*,*)'factor of A is zero, so that A does not have full '
WRITE(*,*)'rank; the least squares solution could not be '
WRITE(*,*)'computed.'
STOP
END IF
*
* Print least squares solution.
*
CALL PRINT_MATRIX( 'Least squares solution', N, NRHS, B, LDB )
*
* Print residual sum of squares for the solution
*
CALL PRINT_VECTOR_NORM(
$ 'Residual sum of squares for the solution', M-N, NRHS,
$ B( N+1, 1 ), LDB )
*
* Print details of QR factorization.
*
CALL PRINT_MATRIX( 'Details of QR factorization', M, N, A, LDA )
STOP
END
*
* End of SGELS 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 norms of matrix columns.
*
SUBROUTINE PRINT_VECTOR_NORM( DESC, M, N, A, LDA )
CHARACTER*(*) DESC
INTEGER M, N, LDA
REAL A( LDA, * )
*
REAL TEMP
INTEGER I, J
*
WRITE(*,*)
WRITE(*,*) DESC
DO J = 1, N
TEMP = 0.0
DO I = 1, M
TEMP = TEMP + A( I, J )*A( I, J )
END DO
WRITE(*,9998,ADVANCE='NO') TEMP
END DO
WRITE(*,*)
*
9998 FORMAT( 11(:,1X,F6.2) )
RETURN
END
Parent topic: SGELS Example