difference between dowhile and do while in fortran90

Question

In a f90 code there is the following section of a code

dowhile: do while <conditions>


 ....

end do dowhile 

I never came across with such a loop, and I cannot understand what would be the difference if i omit the "dowhile ... end dowhile" and simply use a normal "do while" loop.

Can anybody here help me to clarify this point ?

If i have to convert this loop into a matlab, does the simple matlab while loop be sufficient ?

Many thanks

Answer 1

Okay, now I think I got your question:

the dowhile: statement gives a label or name to the loop, and consequently the loop called dowhile is terminated using the enddo statement. You could have used a different name for the loop as well:

mainloop: do while <conditions>
  ! ...
enddo mainloop

This labeling is optional, you can leave it away.

The comments put up a good point: The use of exit or cycle with a label. You could prematurely exit or cycle the loop if it has a label. This is especially useful for nested loops:

outer: do while <condition>
  do i=1,10
    if <some condition> then
      cycle ! cycles the inner loop
    elseif <some other condition> then
      cycle outer ! cycles the outer loop
    endif
  enddo ! i
enddo outer

If such constructs are used, you cannot easily remove the loop names.

I don't think Matlab allows labeling loops, but I'm no expert on Matlab.

---

Original answer:

Well, the do while construct is essentially an infinite loop that is terminated if the <conditions> are true.

conv = .false.
do while ( conv .eqv. .true. )
  ! ... do stuff
enddo

corresponds to

conv = .false.
do
  if ( conv .eqv. .true. ) exit
  ! ... do stuff
enddo

This should be doable in Matlab as well. The Matlab syntax is (taken from here):

while expression
   statements
end

As a side-note: I would define a simple do-loop as a finite loop, though...

do i=1,10
  ! ... do stuff
enddo ! i

This construct is not equivalent to the do while-loop.