Two Compiler Errors

I am chasing down a few remaining errors left in my code. The code works but on occasion generates an overflow error somewhere.

Whilst checking everything out, using the maximum error reporting within gfortran, I get the following two errors:


 4219 |             if (z00 == 0. .or. z01 == 0. .or. z11 == 0. .or. z10 == 0.) then
      |                                                            1
Error: Equality comparison for REAL(8) at (1) [-Werror=compare-reals]

 3741 |         zmlog = min(zmlog, 2.)
      |                           1
Error: GNU Extension: Different type kinds at (1) [-Werror]

All variables defined in these statement are explicitly defined as double precision.
These two errors (warnings) appear in several locations.

What am I doing wrong and what should I do to resolve these?

Your literal constants are default real kind. For the first statement, it does not matter as the default kind value 0. is exactly representable in double precision.

The second error is obvious from the error message.

Ah, okay. So, the declaration of 0. and 2. is real and not double.

So, declaring all literals as doubles would remove the error; got you!

I didn’t spot that, so the clarity of the second error was lost on me; I thought x. would declare as a double (for some reason).

No, you’re wrong about what the error message says. If states that kind of zmlog is not the same as the kind of 2., which is true if you declared zmlog as double precision.

Simple add a kind suffix to your literal constants.

Yes, I realise that; thank you. Your first comment led me to question what I had interpreted the line to state and have solved that one (and similar).

I can’t get the first one to clear, though. I tried this to no avail:

if (z00 == 0d0 .or. z01 == 0d0 .or. z11 == 0d0 .or. z10 == 0d0) then

z00 et al are declared as double precision and they all derive their individual values via another variable, also declared as double precision. However, this latter one (zkap()) appears in a read, where the data type being read in might well be defined as a real?

read (52, *)(zkap(idense, itemp), idense = 1, nrval)

I’m assuming I should define the format of the read, if I want to clear the warning?

This is there as a reminder that exact equality of floating point values is very rarely true, and you should make sure you understand that if you’re going to do it. For example, what do you think the following program will print? T or F?

program floating_point_math
  print *, 0.1 + 0.2 == 0.3
end program

Ok, now run it and see.

1 Like

Due to binary representation, and assuming these are 8-bit real values, it’d come out as false.

My issue is that I am dealing with a large program that I did not originally write. It was written in F77 and I have, slowly, moved it across to F90. However, there are places where I now find the logic runs the risk of failing, yet the code works - for very large sequences (50000 steps, taking a day or two to process).

Thank you for the help, btw.

What those kinds of comparisons generally end up doing is serving as a proxy for “Did some other branch in the code get taken”. I.e.

program roundabout
  implicit none
  real :: x
  real, parameter :: magic_value = 42.0

  x = magic_value
  if (the_stars_align) then
    x = something_else()
  end if

  if (x == magic_value) then
    print *, "The stars didn't align"
    print *, "The stars aligned"
  end if
end program

because there’s virtually zero chance that something_else will return the magic_value. Most projects just end up defaulting to the convention that the magic value happens to be zero, because they didn’t really think about it (or at least in that way). Now if you see something like

program has_a_bug
  if (sum(things) == 100.0) then
  end if
end program

It’s pretty much guaranteed to be a bug, because again, the chances of that summation ever being exactly 100 are pretty much nil.

1 Like

You’re suffering from too strict of a set of compiler options. You’re using -Werror to turn warnings into errors. This is a good thing for development, but gfortran is trying to be too helpful with a warning. In general, it is a bad idea to test for equality of floating point numbers, and that is the source of the warning. There are exceptions to the rule. Testing against 0 (or any small integer value) is one of them as it is exactly representable in double precision; gfortran however does not recognize this exception and complains.

This is the recommended approach, rather than adding D exponents. If you do this with a parameter constant, then when you later decide to use one of the other supported KINDs, it would just involve changing one parameter value, rather than numerous D exponents scattered throughout the code.

Would you mind expanding on this for me?

Just that you can define something like dp and use if as the suffix for all the constants, and then
if you wanted to change to another kind all you have to do is change the definition of dp (of course that is oversimplifying as your computations could call a function that only takes certain kinds, etc …). As @kargl noted comparing to zero is generally OK and so on and that the compiler is being “too helpful”. The second example IF below would probably remove your messages, but (probably) would have performance hit compared to your original if called intensively, but it shows everything done in terms of the kind “dp” which is generally a good idea.

 integer,parameter :: dp=kind(0.0d0)
real(kind=dp) :: z00, z01, z02, z03, tol=2*epsilon(0.0_dp)

if (z00 == z03 .or. any([z01,z02] == 0.0_dp) )then

! use of functions could be slower, but should not produce warnings

if (abs(z00 - z03) < tol .or. any(abs([z01,z02]) < tol) )then
   write(*,*)'OK NOW',z00,z03,z00-z03,tol
   write(*,*)'BAD NOW',z00,z03,z00-z03,tol


not saying this is a great solution, it is just for demonstration purposes.

Thank you for the clarity. Curious that you have defined dp as an integer, yet then appear to define it in terms of a double?

However, more relevant is that I think I need to find out what the code is trying to achieve and remove these lines altogether. The model already takes considerable time to run and I am trying to make it more efficient. Hahahaha!

All kinds are integer values. The KIND() function returns the integer value representing the kind possessed by the argument. The value of the argument is irrelevant; it is just returning the kind for that type of argument. You may see them by predefined names like INT8 or REAL64 but those are just integer constants.

In floating point computation, the problem you have with comparing reals is how small does the value need to become to be considered zero ?

You might consider replacing the test by using a logical function IS_ZERO. This could be used as:
if ( is_zero (z00) .or. is_zero (z01) .or. is_zero (z11) .or. is_zero (z10) ) then


logical function IS_ZERO ( value )
 real(8) :: value
 real(8) :: typical_value = 1.0
 is_zero = abs ( value ) <= tiny ( typical_value )
end function IS_ZERO

The advantage of this test is that it removes all the compiler warnings.
The problem remains : what is a typical value ?

In reality, most often, we have initialised a real variable ( as Z00 = 0.) and then later we simply want to test if it has been changed. Zero is a significant number for any real kind, so the z00 == 0. is a safe test.
Perhaps a change could be to define “real(8) :: zero = 0”
Z00 = zero
… ( computation)
then use “if ( Z00 == zero ) …”, Although this would produce more compiler warnings !!

Be aware that most real numbers are not exact.

! Try the following in gfortran
  real(4) :: one_4 = 0.1_4
  real(8) :: one_8 = 0.1_8
  real(8) :: one_x
  one_x = one_4
  if ( one_4 /= one_8 ) write (*,*) "one_4 is not one_8", one_4, one_8, one_x
  if ( one_4 == one_x ) write (*,*) "one_4 is one_x", one_4, one_x, one_8
  if ( one_4 == one_8 ) write (*,*) "unexpected one_4 is one_8"
  if ( one_4 /= one_x ) write (*,*) "unexpected one_4 is not one_x"

Then change to the following and see the result. ( this would not have happened in most F77 compilers, eg LF77 )
real(4) :: one_4 = 0.1
real(8) :: one_8 = 0.1

1 Like