[NotOverUnderated]

I am using OpenFOAM 7 to program a custom function object. Somewhere in the code, I needed to compute the absolute value of a scalar.

The code never worked as expected, and I had to spend approximately 2 hours in order to find the bug which turns out to be:

abs doesn't return the absolute value. for example abs(5.222) will always return 5.

* I am wondering why abs function in OpenFOAM behaves like that? what is the purpose? (unless I am missing something, I find it misleading).

* How to compute the absolute value of a scalar in OpenFOAM? (I know I can do: value = (value < 0)? -value: value) but I would like to know if there is a function to do that)

BTW, I needed to use abs from cmath which returns the expected value.

Could you please explain that?

Thank you
d.f

Update:

To compute the absolute value of a scalar in OpenFOAM use mag function, e.g mag(-7.556) will return 7.556.

[obscureed]

This is a very valid question (though old and unanswered). Can someone point to the documentation of abs(), please, and an explanation of why it does not calculate the absolute value?

[Tobermory]

There is no abs() function in OpenFOAM ... it's a C++ function that returns the absolute value of an integer (eg see https://cplusplus.com/reference/cstdlib/abs/). If called with a double, abs() will covert the double to an int and then return its absolute value. So all working as intended. As the OP points out - mag was the function that he wanted.

[obscureed]

Ah, OK, thanks. Is there any way to warn users if they are making this mistake? -- As the OP mentioned, it's unexpected and difficult to detect.

From the C++ documentation, abs(float) will work as expected if <cmath> is included. What would this look like in a coded runtime function?

[Tobermory]

Originally Posted by obscureed

Is there any way to warn users if they are making this mistake?

I am afraid not. Again - this has nothing to do with OpenFOAM. You'd have the same issue if you were writing your own standalone C++ program ... My suggestion, if you are coding, is to check out any maths function that you are using in the OpenFOAM DOxygen literature. I am often doing that, for functions like pow() etc.