Program f08hqfe

!     F08HQF Example Program Text

!     Mark 30.1 Release. NAG Copyright 2024.

!     .. Use Statements ..
      Use nag_library, Only: dznrm2, nag_wp, x04daf, zhbevd
!     .. Implicit None Statement ..
      Implicit None
!     .. Parameters ..
      Integer, Parameter               :: nin = 5, nout = 6
!     .. Local Scalars ..
      Complex (Kind=nag_wp)            :: scal
      Integer                          :: i, ifail, info, j, k, kd, ldab, ldz, &
                                          liwork, lrwork, lwork, n
      Character (1)                    :: job, uplo
!     .. Local Arrays ..
      Complex (Kind=nag_wp), Allocatable :: ab(:,:), work(:), z(:,:)
      Real (Kind=nag_wp), Allocatable  :: rwork(:), w(:)
      Integer, Allocatable             :: iwork(:)
!     .. Intrinsic Procedures ..
      Intrinsic                        :: abs, conjg, max, maxloc, min
!     .. Executable Statements ..
      Write (nout,*) 'F08HQF Example Program Results'
!     Skip heading in data file
      Read (nin,*)
      Read (nin,*) n, kd
      ldab = n
      ldz = n
      liwork = 5*n + 3
      lrwork = 2*n*n + 5*n + 1
      lwork = 2*n*n
      Allocate (ab(ldab,n),work(lwork),z(ldz,n),rwork(lrwork),w(n),            &
        iwork(liwork))

!     Read A from data file

      Read (nin,*) uplo
      If (uplo=='U') Then
        Do i = 1, n
          Read (nin,*)(ab(kd+1+i-j,j),j=i,min(n,i+kd))
        End Do
      Else If (uplo=='L') Then
        Do i = 1, n
          Read (nin,*)(ab(1+i-j,j),j=max(1,i-kd),i)
        End Do
      End If

      Read (nin,*) job

!     Calculate all the eigenvalues and eigenvectors of A
!     The NAG name equivalent of zhbevd is f08hqf
      Call zhbevd(job,uplo,n,kd,ab,ldab,w,z,ldz,work,lwork,rwork,lrwork,iwork, &
        liwork,info)

      Write (nout,*)
      If (info>0) Then
        Write (nout,*) 'Failure to converge.'
      Else

!       Print eigenvalues and eigenvectors

        Write (nout,*) 'Eigenvalues'
        Do i = 1, n
          Write (nout,99999) i, w(i)
        End Do
        Write (nout,*)
        Flush (nout)

!       Normalize the eigenvectors, largest element real
        Do i = 1, n
          rwork(1:n) = abs(z(1:n,i))
          k = maxloc(rwork(1:n),1)
          scal = conjg(z(k,i))/abs(z(k,i))/dznrm2(n,z(1,i),1)
          z(1:n,i) = z(1:n,i)*scal
        End Do

!       ifail: behaviour on error exit
!              =0 for hard exit, =1 for quiet-soft, =-1 for noisy-soft
        ifail = 0
        Call x04daf('General',' ',n,n,z,ldz,'Eigenvectors',ifail)

      End If

99999 Format (3X,I5,5X,2F8.4)
    End Program f08hqfe