Calling function

I don't know is this right room to ask this question of not. If not I am sorry for that. I am new user for the fortran and spending a lot of time for the following stuff.

I have constructed a function called "loglike" which returns a real number depending on two parameters. I want to use this function to construct a mcmc algorithm

which goes like this.

psi = min(0, loglike(propalpha,propbeta) - loglike(currentalpha,currentbeta))

where propalpha = currentalpha + noise, and propbeta = currentbeta + noise, noises are random samples from some distribution.

Now I want to use this algorithm by calling previously constructed function "loglike".

1) how can I call the function 'loglike' for new program called main program
2) how can I use this for the subroutine?

Any help is very great for me. Thanks in advance

EDIT:

module mcmc

implicit none

contains

    subroutine subru(A,B, alphaprop, betaprop)
        real, intent(in)::A,B
        real, intent(out)::alphaprop, betaprop
    end subroutine subru

    real function aloglike(A,B)
        real:: A,B,U, aloglike
        aloglike = U
    end function aloglike

end module mcmc


program  likelihood
    use mcmc

    implicit none

    real :: alpha,beta,dist1,dist2,prob1,prob2
    real:: psi,a(1000),b(1000), u1, u2,u, alphaprop, betaprop
    real, dimension(1:1):: loglike
    integer :: t,i,j,k,l
    real, dimension(1:625):: x
    real, dimension(1:625):: y
    integer, dimension(1:625):: inftime

    alpha = 0.5
    beta = 2.0

    open(10, file = 'epidemic.txt', form = 'formatted')
    do l = 1,625
       read(10,*,end = 200) x(l), y(l), inftime(l)
    enddo
200 continue

    loglike = 0.0
    do t=1,10
        do i=1,625
            if(inftime(i)==0 .or. t < (inftime(i)-1)) then
                dist1 = 0.0
                do  j = 1, 625
                    if(t >= inftime(j) .and. inftime(j)/=0)then
                        dist1 = dist1 + sqrt((x(i) - x(j))**2 + (y(i) - y(j))**2)**(-beta)
                    endif
                enddo
                prob1 = 1 - exp(-alpha * dist1)
                loglike = loglike + log(1 - prob1)
            endif

            if(inftime(i) .eq. (t+1)) then
                dist2 = 0.0
                    do k=1, 625
                    if(t >= inftime(k) .and. inftime(k)/=0) then
                        dist2 = dist2 + sqrt((x(i) - x(k))**2 + (y(i) - y(k))**2)**(-beta)
                    endif
                enddo
                prob2 = 1 - exp(-alpha * dist2)
                loglike = loglike + log(prob2)
            endif
        enddo
    enddo

    do i = 2, 1000
        a(1)= 0.0
        b(1) = 0.0
        call subru(a(i),b(i), alphaprop, betaprop)
        call random_number(u1)
        call random_number(u2)

        alphaprop = a(i-1) + (u1*0.4)-0.2
        betaprop= b(i-1) + (u2*0.4)-0.2

        if(alphaprop> 0 .and. alphaprop < 0.2 .and. betaprop > 0 .and. betaprop < 0.2)then
            psi = min(0.0,aloglike(alphaprop,betaprop)- aloglike(a(i-1),b(i-1)))
            call random_number(u)

            if(u < psi)then
                a(i)= alphaprop
                b(i) = betaprop
            else
                a(i) = a(i-1)
                b(i) = b(i-1)
            endif
        endif
    enddo

    do j = 1, 1000
        print *, A(j), A(j), LOGLIKE
    enddo

end program

Solution

This is the general way it would look. Note that functions return a value by assigning the result to its own name. A return statement is not necessary.

C "loglike" is renamed "aloglike" so implicit typing uses REAL
C A type declaration could be used instead

    function aloglike (alpha, beta)
        aloglike = ... (expression which computes value)
    end

    program whateveryouwanttocallit
       ...
       propalpha = ...
       propbeta = ...
       currentalpha = ...
       currentbeta = ...
       ...
       psi = min(0, aloglike(propalpha,propbeta) - aloglike(currentalpha,currentbeta))
       print *, 'psi = ', psi

    end