least squares minimisation fortran 77 - fortran

Question

Trying to get a one-parameter least squares minimisation working in fortran77. Here's the code; it compiles and seems to work except....it gets caught in an infinite loop between values of h1= 1.8E-2 and 3.5E-2.
Having a look now but, odds are, I'm not going to have much luck sussing the issue on my own. All help welcome!
PROGRAM assignment
! A program designed to fit experiemental data, using the method
! of least squares to minimise the associated chi-squared and
! obtain the four control parameters A,B,h1 and h2.
!*****************************************************************
IMPLICIT NONE
INTEGER i
DOUBLE PRECISION t(17),Ct(17),eCt(17)
DOUBLE PRECISION h1loop1,h1loop2,deltah,Cs
DOUBLE PRECISION chisqa,chisqb,dchisq
OPEN(21, FILE='data.txt', FORM='FORMATTED', STATUS='OLD')
DO i=1,17
READ(21,*)t(i),Ct(i),eCt(i)
END DO
CLOSE(21)
!Read in data.txt as three one dimensional arrays.
!*****************************************************************
!OPEN(21, FILE='outtest.txt', FORM='FORMATTED', STATUS='NEW')
!DO i=1,17
! WRITE(21,*)t(i),Ct(i),eCt(i)
!END DO
!CLOSE(21)
!
!Just to check input file is being read correctly.
!*****************************************************************
!**********************Minimising Lamda1 (h1)*********************
deltah= 0.0001
h1loop2= 0.001
h1loop1= 0.0 !Use initial value of 0 to calculate start-point chisq
DO 10
chisqa= 0.0
DO 20 i= 1, 17
Cs= exp(-h1loop1*t(i))
chisqa= chisqa + ((Ct(i) - Cs)/eCt(i))**2
20 END DO
chisqb= 0.0
DO 30 i= 1, 17
h1loop2= h1loop2 + deltah
Cs= exp(-h1loop2*t(i))
chisqb= chisqb + ((Ct(i) - Cs)/eCt(i))**2
30 END DO
!Print the two calculated chisq values to screen.
WRITE(6,*) 'Chi-squared a=',chisqa,'for Lamda1=',h1loop1
WRITE(6,*) 'Chi-squared b=',chisqb,'for Lamda1=',h1loop2
dchisq= chisqa - chisqb
IF (dchisq.GT.0.0) THEN
h1loop1= h1loop2
ELSE
deltah= deltah - ((deltah*2)/100)
END IF
IF (chisqb.LE.6618.681) EXIT
10 END DO
WRITE(6,*) 'Chi-squared is', chisqb,' for Lamda1 = ', h1loop2
END PROGRAM assignment
EDIT: Having looked at it again I've decided I have no clue what's screwing it up. Should be getting a chi-squared of 6618.681 from this, but it's just stuck between 6921.866 and 6920.031. Help!

Answer 1

do i=1
is not starting a loop, for a loop you need to specify an upper bound as well:
do i=1,ub
that's why you get the error message about the doi not having a type, in fixed format spaces are insignificant...
Edit: If you want to have an infinite loop, just skip the "i=" declaration completely. You can use an exit statement to leave the loop, when a certain criterion has been reached:
do
if (min_reached) EXIT
end do
Edit2: I don't know why you stick to F77 fixed format. Here is your program in free format, with some fixes of places, which looked weird, without digging too much into the details:
PROGRAM assignment
! A program designed to fit experiemental data, using the method
! of least squares to minimise the associated chi-squared and
! obtain the four control parameters A,B,h1 and h2.
!*****************************************************************
IMPLICIT NONE
integer, parameter :: rk = selected_real_kind(15)
integer, parameter :: nd = 17
integer :: i,t0
real(kind=rk) :: t(nd),t2(nd),Ct(nd),eCt(nd),Ctdiff(nd),c(nd)
real(kind=rk) :: Aa,Ab,Ba,Bb,h1a,h1b,h2a,h2b,chisqa,chisqb,dchisq
real(kind=rk) :: deltah,Cs(nd)
OPEN(21, FILE='data.txt', FORM='FORMATTED', STATUS='OLD')
DO i=1,nd
READ(21,*) t(i),Ct(i),eCt(i)
END DO
CLOSE(21)
!Read in data.txt as three one dimensional arrays.
!*****************************************************************
!OPEN(21, FILE='outtest.txt', FORM='FORMATTED', STATUS='NEW')
!DO i=1,17
! WRITE(21,*)t(i),Ct(i),eCt(i)
!END DO
!CLOSE(21)
!
!Just to check input file is being read correctly.
!*****************************************************************
!****************************Parameters***************************
Aa= 0
Ba= 0
h1a= 0
h2a= 0
!**********************Minimising Lamda1 (h1)*********************
deltah= 0.001_rk
h1b= deltah
minloop: DO
chisqa= 0
DO i= 1,nd
Cs(i)= exp(-h1a*t(i))!*Aa !+ Ba*exp(-h2a*t(i))
Ctdiff(i)= Ct(i) - Cs(i)
c(i)= Ctdiff(i)**2/eCt(i)**2
chisqa= chisqa + c(i)
h1a= h1a + deltah
END DO
! Use initial h1 value of 0 to calculate start-point chisq.
chisqb= 0
DO i= 1,nd
h1b= h1b + deltah
Cs(i)= exp(-h1b*t(i))!*Ab !+ Bb*exp(-h2b*t(i))
Ctdiff(i)= Ct(i) - Cs(i)
c(i)= Ctdiff(i)**2/eCt(i)**2
chisqb= chisqb + c(i)
END DO
! First-step h1 used to find competing chisq for comparison.
WRITE(6,*) 'Chi-squared a=', chisqa,'for Lamda1=',h1a
WRITE(6,*) 'Chi-squared b=', chisqb,'for Lamda1=',h1b
! Prints the two calculated chisq values to screen.
dchisq= chisqa - chisqb
IF (dchisq.GT.0) THEN
h1a= h1b
ELSE IF (dchisq.LE.0) THEN
deltah= (-deltah*2)/10
END IF
IF (chisqb.LE.6000) EXIT minloop
END DO minloop
WRITE(6,*) 'Chi-squared is', chisqb,'for Lamda1=',h1b
END PROGRAM assignment