Program f08fqfe

!     F08FQF Example Program Text

!     Mark 30.1 Release. NAG Copyright 2024.

!     .. Use Statements ..
      Use nag_library, Only: nag_wp, x04daf, zheevd, zscal
!     .. Implicit None Statement ..
      Implicit None
!     .. Parameters ..
      Integer, Parameter               :: nin = 5, nout = 6
!     .. Local Scalars ..
      Integer                          :: i, ifail, info, k, lda, liwork,      &
                                          lrwork, lwork, n
      Character (1)                    :: job, uplo
!     .. Local Arrays ..
      Complex (Kind=nag_wp), Allocatable :: a(:,:), work(:)
      Real (Kind=nag_wp), Allocatable  :: rwork(:), w(:)
      Integer, Allocatable             :: iwork(:)
!     .. Intrinsic Procedures ..
      Intrinsic                        :: abs, cmplx, conjg, maxloc
!     .. Executable Statements ..
      Write (nout,*) 'F08FQF Example Program Results'
!     Skip heading in data file
      Read (nin,*)
      Read (nin,*) n
      lda = n
      liwork = 5*n + 3
      lrwork = 2*n*n + 5*n + 1
      lwork = n*(n+2)
      Allocate (a(lda,n),work(lwork),rwork(lrwork),w(n),iwork(liwork))
      Read (nin,*) uplo

!     Read A from data file

      If (uplo=='U') Then
        Read (nin,*)(a(i,i:n),i=1,n)
      Else If (uplo=='L') Then
        Read (nin,*)(a(i,1:i),i=1,n)
      End If

      Read (nin,*) job

!     Calculate all the eigenvalues and eigenvectors of A
!     The NAG name equivalent of zheevd is f08fqf
      Call zheevd(job,uplo,n,a,lda,w,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 so that the element of largest absolute
!       value is real.
        Do i = 1, n
          rwork(1:n) = abs(a(1:n,i))
          k = maxloc(rwork(1:n),1)
          Call zscal(n,conjg(a(k,i))/cmplx(abs(a(k,i)),kind=nag_wp),a(1,i),1)
        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,a,lda,'Eigenvectors',ifail)

      End If

99999 Format (3X,I5,5X,2F8.4)
    End Program f08fqfe