[Octave-bug-tracker] [bug #61968] pow2 with two arguments is not Matlab compatible

[Markus Mützel]

Follow-up Comment #17, bug #61968 (project octave):

`std::ldexpf` not existing in in GCC nor clang might be a compiler bug, a
documentation issue, or an (intentional) deviation from the standard. I didn't
find any further information on that.
But honestly, I don't know why anyone would even need to use `std::ldexpf`.
Instead, it would be clearer and more in the spirit of C++ to just use
`std::ldexp` (and potentially cast the input argument to `float` if that is
really what one would like to do).

@Kai: Why do you prefer the C functions `ldexp` and `ldexpf` over the (easier
to use) C++ function `std::ldexp`? After all, Octave is mainly written in C++.
And your proposed implementation of `Fexp2` is also written in C++ (with some
functions mixed in from C for some reason I'd like to understand).

> pow2(f,e) with two arguments calls ldexp.  This is documented Matlab
behavior (comment #0): discussions forbidden 😅

As I'm understanding their documentation, they are describing the function by
comparing it to `ldexp`. They don't *actually* write that the function is
implemented with `ldexp`...
But that's just a minor detail.
It might still be interesting to check how much of a performance difference
there is between using `std::exp2` or `std::ldexp` for different compiler or
STL implementations.

> Somehow in Octave the C symbols leak (intentionally?) into the C++
"octave::" namespace.  I got hard errors using std::exp2, etc.

Tbh, I don't understand what you are trying to say...


I was thinking the only difference between `elem` and `xelem` is that the
latter doesn't check for out-of-range indices. By that, it's slightly more
efficient than `elem`. If you are sure that you'll not exceed the range of the
array, you could use `xelem`. Is there really a difference with respect to
"unique data"?

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