Conversion of OR logical operator from C++ to Fortran

Question

What would be the conversion of following C++ logical operator into Fortran 90 (.f90)? If ( vx is present or vy is present). Here vx and vy are components of velocity

if(vx || vy){
vT=sqrt(vx*vx + vy*vy);
}

I have tried following

if(vx .or. vy) then
vT = sqrt(vx*vx + vy*vy)
end if

but I am getting error:

operands of logical operator `.or.` at (1) are REAL(8)/REAL(8).

Can anyone guide me here?

Answer 1

The C++ version is implicitly comparing vx and vy with zero.

In Fortran, you have to do so explicitly ¹ :

if (vx /= 0 .or. vy /= 0) then

Since the if statement looks like a performance optimization, it might be worth questioning whether it's needed altogether or could be replaced with an unconditional assignment to vT (that would set vT to zero if both vx and vy are zero).

¹ I hope I got this right; haven't programmed in Fortran for many years.

Answer 2

In the present case it is not relevant, but in general it should be noted that Fortran logical operations are not short-circuited. So, for example, the following C++ code

if (a == 0 || 10 / a == 1)
{
   ...
}

is not equivalent to

if (a == 0 .or. 10 / a == 1) then
    ...
end if

in Fortran. A compiler may decide to evaluate the second term first and then... oops. It should be written using two nested if s.