The pause and return commands of fortran 90 to matlab

Hi,

I am trying to translate a fortran 90 code into Matlab. I do it solely for my own practice and in order to understand a code provided by academic journal. The particular section, is taken from the well known book of "numerical recipes 77", I slightly modified (simplified the code), to the following form:

PROGRAM PAUSEE 
IMPLICIT NONE 
real*8:: saf1=10, saf2=-15, tol, r

r = zbrent(saf1,saf2, 10.0215, 20.03256, 0.00003)

print*, 'this is what we get'
print*, r 

contains 

FUNCTION zbrent (saf1,saf2,x1,x2,tol)

    real(kind=1), intent(in) :: x1, x2, tol
    integer, parameter:: itmax=300
    real*8, parameter:: epsz=3.e-8
    integer:: iter
    real*8:: a,b,c,d,e,fa,fb,fc,p,q,r,s,tol1,xm, zbrent, saf1,saf2

    a=x1
    b=x2
    fa=saf1
    fb=saf2
    if(fa*fb>0.) pause 'root must be bracketed for zbrent'
     c=b
     fc=fb
    do iter=1,itmax
     if (fb*fc>0.) then
       c=a
      fc=fa
       d=b-a
       e=d
    end if
     if (abs(fc)<abs(fb)) then
       a=b
       b=c
       c=a
      fa=fb
      fb=fc
      fc=fa
    end if
       tol1=2.*epsz*abs(b)+0.5*tol
         xm=.5*(c-b)
         if (abs(xm)<=tol1 .or. fb==0.) then
            zbrent=b
            return
         end if
         if (abs(e)>=tol1 .and. abs(fa)>abs(fb)) then
            s=fb/fa
            if(a==c) then
               p=2.*xm*s
               q=1.-s
            else
               q=fa/fc
               r=fb/fc
               p=s*(2.*xm*q*(q-r)-(b-a)*(r-1.))
               q=(q-1.)*(r-1.)*(s-1.)
            end if
            if(p>0.) q=-q
            p=abs(p)
            if (2.*p<min(3.*xm*q-abs(tol1*q),abs(e*q))) then
               e=d
               d=p/q
            else
               d=xm
               e=d
            end if
         else
            d=xm
            e=d
         end if
         a=b
         fa=fb
         if(abs(d)>tol1) then
            b=b+d
         else
            b=b+sign(tol1,xm)
         end if
         fb=saf2
      end do

      pause 'zbrent exceeding maximum iterations'
      zbrent=b
      return

    END FUNCTION zbrent

END PROGRAM PAUSEE

My basic questions concerns the the meaning of "pause" and "return" commands (I noticed that in the first use of the pause THERE IS NO "END IF" ) and whether those do have any equivalent in Matlab. I checked and find out, that they do exist such commands but I am not sure whether are the equivalent to fortran. Second, I really wonder whether the "pause" and "return" commands in the code, are of any value if someone translates this code into Matlab. In other words, can the algorithm here go through in matlab without the "pause" and "return" commands of the above code ? I tried a bit myself, with and without the "pause" and "return" commands, for some particular inputs, and seems to produce the same results. However, I am still concerned whether more generally those can be omitted or not.

many thanks

Answer 1

[James Tursa]

The PAUSE statement does just what the name implies, pauses execution with a message until user does something (e.g., presses Enter) to resume execution. Looks like Image Analyst has a good solution for that if you want the same behavior. I might suggest just using a warn( ) instead of a pause and simply let the routine continue on its own.

Regarding your question about a missing "endif", in Fortran, the following syntaxes mean the same thing:

if( condition ) then
    statement
endif

if( condition ) statement

That is, you can either bracket the statement with a "then-endif", or you can simply include the statement on the same line without the "then-endif".

The RETURN means exactly the same thing as in MATLAB, simply returning control to the calling routine. In your Fortran above, the RETURN near the end of the zbrent function is simply returning control back to the calling routine, which in this case is PROGRAM PAUSEE.

[msh]

Thank you for your answer. I believe i got the main point. Just a short clarification, in the code above, if I make the translation WITHOUT the "pause" or "return" commands I do not lose something in particular, am i correct? The "pause" in the above code seems to me as a warning over something. In particular, this is a code for root-finding algorithm based on Brent method, so the warning is what the message in the strings message says. Given your experience, what would you suggest ? to include those equivalents (i.e pause and return) or not in the intended translation of mine ?

[dpb]

...would you suggest ? to include those equivalents (i.e pause and return) or not...

Well, clearly the warnings are there for a reason; it would be unwise to not make the user aware of an issue. HOW you choose to do that is a design decision. The above ensures there is a response from the user; whether your code is that onerous or not is your choice.

On the IF structure, NB: that the one-line IF in Fortran includes only the one statement on the same line in that form be careful to not accidentally either leave something out or include something that shouldn't be.

[James Tursa]

@Safis: Since you asked, I would replace the pause with either a warn( ) or an error( ) depending on how severe you think the problem is. I would also keep the return statement just because I always explicitly include them in my MATLAB functions, but it is really optional whether you include it or not.

Answer 2

[Image Analyst]

return is probably the same meaning. It's been about 30 years since I've used Fortran but from the looks of your code:

pause 'root must be bracketed for zbrent'

I'd guess the equivalent in MATLAB would be

uiwait(helpdlg('root must be bracketed for zbrent'));

[Image Analyst]

Regarding your latest comment. I would leave the returns in there because that seems to be the normal (no error) mode of exiting this code. For the pause, I'd use my code since it will popup the information in a dialog box like the original FORTRAN code. If you think it should be more of a warning than an information, you can use helpdlg() or msgbox() instead of helpdlg(). Using the uiwait will wait for the user to click on it, otherwise it just throws up the message and continues on with the code.