Try out NAG Library functions

Explore NAG maths and stats routines with interactive demos
Function ID
F07AAF
Name
nagf_lapack_dgesv
Description
Computes the solution to a real system of linear equations
Keywords
DGESV | real, nonsymmetric matrix | LU decomposition
This example solves the equations
 $Ax=b,$
where $A$ is the general matrix
 $A=1.802.882.05-0.895.25-2.95-0.95-3.801.58-2.69-2.90-1.04-1.11-0.66-0.590.80 and b=9.5224.350.77-6.22.$
    Program f07aafe

!     F07AAF Example Program Text

!     Mark 26.1 Release. NAG Copyright 2016.

!     .. Use Statements ..
Use nag_library, Only: dgesv, nag_wp, x04caf
!     .. Implicit None Statement ..
Implicit None
!     .. Parameters ..
Integer, Parameter               :: nin = 5, nout = 6
!     .. Local Scalars ..
Integer                          :: i, ifail, info, lda, ldb, n
!     .. Local Arrays ..
Real (Kind=nag_wp), Allocatable  :: a(:,:), b(:)
Integer, Allocatable             :: ipiv(:)
!     .. Executable Statements ..
Write (nout,*) 'F07AAF Example Program Results'
Write (nout,*)
!     Skip heading in data file
lda = n
ldb = n
Allocate (a(lda,n),b(ldb),ipiv(n))

!     Read A and B from data file

!     Solve the equations Ax = b for x

!     The NAG name equivalent of dgesv is f07aaf
Call dgesv(n,1,a,lda,ipiv,b,ldb,info)

If (info==0) Then

!       Print solution

Write (nout,*) 'Solution'
Write (nout,99999) b(1:n)

!       Print details of factorization

Write (nout,*)
Flush (nout)

!       ifail: behaviour on error exit
!              =0 for hard exit, =1 for quiet-soft, =-1 for noisy-soft
ifail = 0
Call x04caf('General',' ',n,n,a,lda,'Details of factorization',ifail)

!       Print pivot indices

Write (nout,*)
Write (nout,*) 'Pivot indices'
Write (nout,99998) ipiv(1:n)

Else
Write (nout,99997) 'The (', info, ',', info, ')',                      &
' element of the factor U is zero'
End If

99999 Format ((3X,7F11.4))
99998 Format ((3X,7I11))
99997 Format (1X,A,I3,A,I3,A,A)
End Program f07aafe

The NAG Library
The world’s largest collection of robust, documented, tested and maintained numerical algorithms.