!   E04YAF Example Program Text
!   Mark 23 Release. NAG Copyright 2011.

    MODULE e04yafe_mod

!      E04YAF Example Program Module:
!             Parameters and User-defined Routines

!      .. Use Statements ..
       USE nag_library, ONLY : nag_wp
!      .. Implicit None Statement ..
       IMPLICIT NONE
!      .. Parameters ..
       INTEGER, PARAMETER                  :: liw = 1, mdec = 15, ndec = 3,    &
                                              nin = 5, nout = 6
       INTEGER, PARAMETER                  :: ldfjac = mdec
       INTEGER, PARAMETER                  :: lw = 3*ndec + mdec + mdec*ndec
!      .. Local Arrays ..
       REAL (KIND=nag_wp)                  :: t(mdec,ndec), y(mdec)
    CONTAINS
       SUBROUTINE lsqfun(iflag,m,n,xc,fvec,fjac,ldfjac,iw,liw,w,lw)

!         Routine to evaluate the residuals and their 1st derivatives

!         .. Implicit None Statement ..
          IMPLICIT NONE
!         .. Scalar Arguments ..
          INTEGER, INTENT (INOUT)             :: iflag
          INTEGER, INTENT (IN)                :: ldfjac, liw, lw, m, n
!         .. Array Arguments ..
          REAL (KIND=nag_wp), INTENT (INOUT)  :: fjac(ldfjac,n), w(lw)
          REAL (KIND=nag_wp), INTENT (OUT)    :: fvec(m)
          REAL (KIND=nag_wp), INTENT (IN)     :: xc(n)
          INTEGER, INTENT (INOUT)             :: iw(liw)
!         .. Local Scalars ..
          REAL (KIND=nag_wp)                  :: denom, dummy
          INTEGER                             :: i
!         .. Executable Statements ..
          DO i = 1, m
             denom = xc(2)*t(i,2) + xc(3)*t(i,3)

             IF (iflag/=1) THEN
                fvec(i) = xc(1) + t(i,1)/denom - y(i)
             END IF

             IF (iflag/=0) THEN
                fjac(i,1) = 1.0E0_nag_wp
                dummy = -1.0E0_nag_wp/(denom*denom)
                fjac(i,2) = t(i,1)*t(i,2)*dummy
                fjac(i,3) = t(i,1)*t(i,3)*dummy
             END IF

          END DO

          RETURN

       END SUBROUTINE lsqfun
    END MODULE e04yafe_mod
    PROGRAM e04yafe

!      E04YAF Example Main Program

!      .. Use Statements ..
       USE nag_library, ONLY : e04yaf, nag_wp
       USE e04yafe_mod, ONLY : ldfjac, liw, lsqfun, lw, mdec, ndec, nin, nout, &
                               t, y
!      .. Implicit None Statement ..
       IMPLICIT NONE
!      .. Local Scalars ..
       INTEGER                             :: i, ifail, m, n
!      .. Local Arrays ..
       REAL (KIND=nag_wp)                  :: fjac(ldfjac,ndec), fvec(mdec),   &
                                              w(lw), x(ndec)
       INTEGER                             :: iw(liw)
!      .. Executable Statements ..
       WRITE (nout,*) 'E04YAF Example Program Results'

!      Skip heading in data file
       READ (nin,*)

       n = ndec
       m = mdec

!      Observations of TJ (J = 1, 2, ..., n) are held in T(I, J)
!      (I = 1, 2, ..., m)

       DO i = 1, m
          READ (nin,*) y(i), t(i,1:n)
       END DO

!      Set up an arbitrary point at which to check the 1st
!      derivatives

       x(1:n) = (/ 0.19E0_nag_wp, -1.34E0_nag_wp, 0.88E0_nag_wp/)

       WRITE (nout,*)
       WRITE (nout,*) 'The test point is'
       WRITE (nout,99999) x(1:n)

       ifail = -1
       CALL e04yaf(m,n,lsqfun,x,fvec,fjac,ldfjac,iw,liw,w,lw,ifail)

       IF (ifail>=0 .AND. ifail/=1) THEN

          SELECT CASE (ifail)
          CASE (0)
             WRITE (nout,*)
             WRITE (nout,*) &
                '1st derivatives are consistent with residual values'
          CASE (2)
             WRITE (nout,*)
             WRITE (nout,*) 'Probable error in calculation of 1st derivatives'
          END SELECT

          WRITE (nout,*)
          WRITE (nout,*) 'At the test point, LSQFUN gives'
          WRITE (nout,*)
          WRITE (nout,*) '      Residuals                   1st derivatives'
          WRITE (nout,99998) (fvec(i),fjac(i,1:n),i=1,m)
       END IF

99999  FORMAT (1X,4F10.5)
99998  FORMAT (1X,1P,4E15.3)
    END PROGRAM e04yafe