converting c for loop to fortran do loop

Question

I am converting a cpp prog (from another author) to a Fortran prog, my C is not too strong. I came across for-loop constructs starting with

for (int n = 1; 1; ++n) {
...

I would have expected this to convert to a Fortran Do as per

Do n=1, 1, 2
...

... at least that is my guess based on my understanding of what ++n will do.

Is my translation correct? If so, the loop will cycle at most once, so what am I missing ???

I understand that in some ways c for-loops have a "do-while" aspect, and hence wrinkles porting to Fortran Do's.

Anyway ... a clarification would be much appreciated.

EDITED: after some prompt responses, and I think I see where this is going

First, the exact C code copy/paste but "trimming" a little, is

    for (int n = 1; 1; ++n) {
      const double coef = exp(-a2*(n*n)) * expx2 / (a2*(n*n) + y*y);
      prod2ax *= exp2ax;
      prodm2ax *= expm2ax;
      sum1 += coef;
      sum2 += coef * prodm2ax;
      sum4 += (coef * prodm2ax) * (a*n);
      sum3 += coef * prod2ax;
      sum5 += (coef * prod2ax) * (a*n);
      // test convergence via sum5, since this sum has the slowest decay
      if ((coef * prod2ax) * (a*n) < relerr * sum5) break;
    }

So yes, there is a "break" in the loop, which on the Fortran side is replaced with an "Exit".

I think the key seems to be from the answers below that the original code's author created the for (int n=1; 1 ; ++n ) precisely to create a an infinite loop, and I had not guessed that this for construct would create an infinite loop.

Anyway, I can certainly create an infinite loop with an "Exit" in Fortran (though I expect I might "do" it a bit more judiciously)

Many thanks to all.

It seems the Mr Gregory's response was the one that imediately lead to a solution for me, so I will mark his correct. As for the Fortran side, there are a number of alternatives such as:

  Do While
    :
    If( something ) Exit
  End Do

but being old fashioned I would probably use a construct with a "limit" such as

   Do i=1, MaxIter
    :
    If( something ) Exit
   End Do

For slightly fancier applications I might include a return flag in case it did not converge in MaxIter's etc.

Which Fortran? Fortran77, Fortran95, Fortran2005? And which C++? C++11? BTW, why do you convert it? Calling C++ code from Fortran might be simpler than converting it! — Basile Starynkevitch
– Basile Starynkevitch, Commented Sep 18, 2017 at 6:25

High Performance Mark · Accepted Answer · 2017-09-18 04:30:58Z

2

It's difficult to be definitive without seeing how the C++ program breaks out of that loop, but a straightforward Fortran equivalent would be

n = 1
do
    ! code, including an exit under some condition, presumably on the value of n
    n = n+1
end do

If the loop is terminated when n reaches a critical value then the equivalent might be

do n = 1, critical_value  ! no need to indicate step size if it is 1
    ! code
end do

answered Sep 18, 2017 at 4:30

High Performance Mark

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Comments

kmoser · Accepted Answer · 2017-09-18 23:05:38Z

1

Are you sure you wrote the C code correctly? Typically loops in C/C++ are done like this:

for (int n = 1; n < 10; ++n) {
    // ...
}

Note the "n < 10" test condition. Your code's test condition is simply 1, which will always evaluate to Boolean "true". This means the code will loop infinitely, unless there's a break inside the loop, which you haven't shown.

++n means "increment n".

So if the code you've shown is indeed correct, the FORTRAN equivalent would be:

n = 1
do
    [Body of the loop, which you haven't shown]
    n = n + 1
enddo

edited Sep 18, 2017 at 23:05

answered Sep 18, 2017 at 3:46

kmoser

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2 Comments

kmoser Over a year ago

Okay, then do while ( .TRUE. )?

kmoser Over a year ago

Point taken, and I've edited my response. The larger picture here is not so much how to do a FOR loop in FORTRAN, but that the C loop shown by the OP is not a typical C for() loop.

Carey Gregory · Accepted Answer · 2017-09-18 03:39:12Z

0

Here's what

for (int n = 1; 1; ++n)

does:

It sets n to 1, then loops infinitely, incrementing n by 1 at the end of each loop iteration. The loop will never terminate unless something inside the loop breaks out.

It's been a long time since I wrote Fortran but as I recall the do loop you translated it to is not correct.

answered Sep 18, 2017 at 3:39

Carey Gregory

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Comments

R Sahu · Accepted Answer · 2017-09-18 03:45:44Z

0

I don't think you can translate

for (int n = 1; 1; ++n)

to a FORTRAN DO loop. From what I recall, the notion of the generic conditional in C/C++ cannot be emulated in a FORTRAN DO loop.

The equivalent of

Do n=1, 1, 2

in C/C++ is

for ( int n = 1; n <= 1; n += 2 )

answered Sep 18, 2017 at 3:45

R Sahu

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Comments

Dúthomhas · Accepted Answer · 2017-09-18 06:20:15Z

0

A few notes in addition to CareyGregory’s answer.

++n means ‘increment n by one (before n is evaluated)’

In C and C++, a for loop has three clauses, much like in FORTRAN:

for (init; condition; increment)

The difference is that each of the clauses must be a complete expression, whereas in FORTRAN the clauses are just values. It is just a ‘short’ way of writing an equivalent while loop:

int n = 1;          │ for (int n = 1; 1; ++n)    │ n = 1
while (1)           │ {                          │ do 
{                   │   ...                      │   ...
  ...               │ }                          │   n = n + 1
  ++n;              │                            │ enddo
}                   │                            │

edited Sep 18, 2017 at 6:20

answered Sep 18, 2017 at 3:48

Dúthomhas

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