Practice with elemental function

[CRquantum]

@Beliavsky

So cannot safely write a = merge(sqrt(b), 0.0, b >= 0) with b real, but you can write a = (b >= 0 ? sqrt(b) : 0.0).

Could you please elaborate a little bit more, about why “cannot safely write a = merge(sqrt(b), 0.0, b >= 0) with b real”?
and why it is safe to write a = (b >= 0 ? sqrt(b) : 0.0)?
Thanks!

PS.
I know a topic about merge() is below for example,

Is there another function in Fortran like `merge` that does not evaluate its arguments?

The merge function does not evaluate its arguments, unless they are needed. Here is a simple test program, that passes: program test_merge implicit none integer :: i, j i = merge(f1(), f2(), .true.) if (j /= 1) error stop if (i /= 10) error stop i = merge(f1(), f2(), .false.) if (j /= 2) error stop if (i /= 20) error stop contains integer function f1() result(r) j = 1 r = 10 end function integer function f2() result(r) j = 2 r = 20 end function end program A…