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[hdalgarno]

(I am using F90/95 with the FTN95 Silverfrost compiler)

Is it possible to test for an undefined variable, that is intentionally undefined unless some event has occured?

For example, lets say I define X to be an integer but give it no initial value. During the course of the program X will be given a value ONLY if an event Y occurs. Later, I would like to initiate one set of commands if event Y has occured, and another set if it has not occured. I'd like to know whether I can test X to ask if it is undefined, or if it has a value, and use the outcome of this test to select which set of commands to initiate.

I realise that I could use a flag that is set when event Y occurs but I have limited flexibility to introduce new variables to this particular application, so an ability to test the existing variable X would be the preferred first solution.

I'd really appreciate any advice on this

[LitusSaxonicum]

There is no bit pattern in an INTEGER that reflects an undefined number, as all possible bit patterns represent positive and negative values including zero. There may be a bit pattern for a floating point (REAL) number that reflects that undefined status, but the test could be quite difficult - I've certainly never tried to use it.

What about using a LOGICAL variable? This only has two states, .TRUE. or .FALSE. and they are held in FTN95 to be the same bit patterns as 0 or 1 if the byte(s) are part of an INTEGER. That gives some bit patterns that neither mean .TRUE. or .FALSE. You could start your program by setting a LOGICAL variable the value 'FALSE., say in BLOCK DATA, or if you don't like that, in a routine called at startup. Later in your program, you could set the variable to .TRUE. if event Y occurs. Then you could test .TRUE. or .FALSE. like this:

[hdalgarno]

Hi Eddie

Thanks for your repsonse...I'm coming round to the idea that there is no other option than to test event Y by adding a flag. I'm surprised that Fortran doesn't have some sort of functionality to test for undefined variables.

What I have is something analagous to this:

integer :: X, Y
logical :: event = .false.

IF (event) then
X = 1
ENDIF

Y = X + 2

and I have to wait for runtime for it to fall over before it would tell me that X is undefined.

Whereas what I want is something like this:

integer :: X, Y
logical :: test, event

event = .false.

IF (event) then
X = 1
ENDIF

test = 'does X have a value?'

IF (test) then
Y = X + 2
ELSE
Y = X + 3
ENDIF

It seems sensible to me that Fortran should be able to do this, since it can detect a variable is undefined at runtime, but it appears it's not possible

[PaulLaidler]

FTN95 offers two predefined states. The compiler option /ZEROISE presets all variables to zero (I don't recall the details here) whilst the option /UNDEF presets all reals and integers to a certain bit pattern and flags a runtime error when the value is used before being set. Naturally there is a runtime overhead when using /UNDEF. /UNDEF is part of /CHECKMATE.

Basically you can do your own initialisation as long as you use values that can never occur naturally in your model. This is tricky for integers (/UNDEF uses 0x80808080 I think). For reals the ASCII standard defines NAN (not a number). I would have to look up if FTN95 supports this in any way.

[LitusSaxonicum]

... but X and Y, being INTEGERs are never undefined, as there is no pattern of bits that represents undefined for an integer. It's easy to prove this for an INTEGER*(1), because you can write out all the patterns, and see that they go from -254 ... 0 ... +255. There is a pattern of bits that represents undefined for a real - but difficult to test for it.

If you use a flag, then all you need to do is set it during the event. A flag of LOGICAL type is best because the test is simplest, but an INTEGER allows you to count the events.

Eddie

[JohnCampbell]

hdalgarno,

FTN95 can indicate a variable is undefined by setting it to an initial value that represents that it is undefined. ( not sure what this value is, but it can occur)
To achieve what you want, there must be a value of X which implies it is definately undefined. Often 0, -1 or 9999999 are values that suit that purpose and it is probably better to choose your own.
I would recommend that you initialise X to a value that is not sensible for it's use, so testing if X is set to this non-sensible value implies it's use is not required.
Alternatively you can use a logical variable, such as Use_X, which indicates the same, in a more elegant form.
It is often easier to have "X = -1" and test "if (X /= -1) then ..."

For real variables, I also have declared
REAL*8, PARAMETER :: DUMMY = -.99898 ! special coordinate for undefined coordinates
and test " IF (Y == dummy) ... "

Over many years of use I am yet to get this value being a valid number and cause a problem.
You just have to put up with the WARNING - 179: Comparing floating point quantities for equality may give misleading results

John

[hdalgarno]