Program f08gbfe
! F08GBF Example Program Text
! Mark 29.0 Release. NAG Copyright 2023.
! .. Use Statements ..
Use nag_library, Only: blas_damax_val, dspevx, nag_wp, x04caf
! .. Implicit None Statement ..
Implicit None
! .. Parameters ..
Real (Kind=nag_wp), Parameter :: zero = 0.0E+0_nag_wp
Integer, Parameter :: nin = 5, nout = 6
Character (1), Parameter :: uplo = 'U'
! .. Local Scalars ..
Real (Kind=nag_wp) :: abstol, r, vl, vu
Integer :: i, ifail, il, info, iu, j, k, ldz, &
m, n
! .. Local Arrays ..
Real (Kind=nag_wp), Allocatable :: ap(:), w(:), work(:), z(:,:)
Integer, Allocatable :: iwork(:), jfail(:)
! .. Executable Statements ..
Write (nout,*) 'F08GBF Example Program Results'
Write (nout,*)
! Skip heading in data file
Read (nin,*)
Read (nin,*) n
ldz = n
m = n
Allocate (ap((n*(n+1))/2),w(n),work(8*n),z(ldz,m),iwork(5*n),jfail(n))
! Read the lower and upper bounds of the interval to be searched,
! and read the upper or lower triangular part of the matrix A
! from data file
Read (nin,*) vl, vu
If (uplo=='U') Then
Read (nin,*)((ap(i+(j*(j-1))/2),j=i,n),i=1,n)
Else If (uplo=='L') Then
Read (nin,*)((ap(i+((2*n-j)*(j-1))/2),j=1,i),i=1,n)
End If
! Set the absolute error tolerance for eigenvalues. With ABSTOL
! set to zero, the default value is used instead
abstol = zero
! Solve the symmetric eigenvalue problem
! The NAG name equivalent of dspevx is f08gbf
Call dspevx('Vectors','Values in range',uplo,n,ap,vl,vu,il,iu,abstol,m, &
w,z,ldz,work,iwork,jfail,info)
If (info>=0) Then
! Print solution
Write (nout,99999) 'Number of eigenvalues found =', m
Write (nout,*)
Write (nout,*) 'Eigenvalues'
Write (nout,99998) w(1:m)
Flush (nout)
! Normalize the eigenvectors: largest element positive
Do i = 1, m
Call blas_damax_val(n,z(1,i),1,k,r)
If (z(k,i)<zero) Then
z(1:n,i) = -z(1:n,i)
End If
End Do
! ifail: behaviour on error exit
! =0 for hard exit, =1 for quiet-soft, =-1 for noisy-soft
ifail = 0
Call x04caf('General',' ',n,m,z,ldz,'Selected eigenvectors',ifail)
If (info>0) Then
Write (nout,99999) 'INFO eigenvectors failed to converge, INFO =', &
info
Write (nout,*) 'Indices of eigenvectors that did not converge'
Write (nout,99997) jfail(1:m)
End If
Else
Write (nout,99999) 'Failure in DSPEVX. INFO =', info
End If
99999 Format (1X,A,I5)
99998 Format (3X,(8F8.4))
99997 Format (3X,(8I8))
End Program f08gbfe