Note however, that this behavior has changed in 0.4 and you need to be more
explicit in converting UInts to Ints:
julia> arr = hex2bytes("14fb9c03")
4-element Array{UInt8,1}:
0x14
0xfb
0x9c
0x03
julia> f(x) = abs((x % 3) - 3)
f (generic function with 1 method)
julia> function test_abs(bytes_input::Array{Uint8})
input_len::Uint = sizeof(bytes_input)
a::Int = f(input_len)
println("a: ", a, " input len: ", input_len)
end
test_abs (generic function with 1 method)
julia> test_abs(arr)
ERROR: InexactError()
in test_abs at ./none:3
On Sun, Sep 6, 2015 at 7:53 AM, Yichao Yu <yyc1...@gmail.com> wrote:
> On Sat, Sep 5, 2015 at 11:57 PM, <ele...@gmail.com> wrote:
> >
> > Subtlty of C, literal integer constants are signed, so s is converted to
> > signed before the % and so the whole abs is signed.
>
> And in any case abs in c is `int abs(int);` so the argument is always
> converted to signed integer before passing to `abs`.
>
> >
> >
> > On Sunday, September 6, 2015 at 1:23:46 PM UTC+10, Corey Moncure wrote:
> >>
> >> I see... It's actually a Uint, not a Uint8 that is passed to f(), but
> the
> >> result is the same. I'm running into a lot of traps like this.
> Consider
> >> the following analogous C code:
> >>
> >> int f () {
> >> unsigned int s = 4;
> >> int a;
> >> a = abs((s % 3) - 3);
> >> printf("a: %d", a);
> >> return a;
> >> }
> >>
> >> The C compiler has no trouble evaluating the abs(...) the way it's
> >> intended even though 's' is clearly typed as an unsigned int. What's
> going
> >> on in Julia? It's like in Julia world we aren't allowed to ask what 2
> - 3
> >> evaluates to if 2 is an unsigned type, regardless of what type we would
> >> store the result in?
> >>
> >>
> >>
> >> On Saturday, September 5, 2015 at 10:57:01 PM UTC-4, Seth wrote:
> >>>
> >>> You're passing a UInt8 to f() which results in a mod of
> >>> 0x0000000000000001, from which 3 is subtracted resulting in
> >>> 0xfffffffffffffffe return value. Casting that to an Int, you get -2.
> >>>
> >>> Try f(UInt8(4)) to see this in action.
> >>>
> >>> In 0.4, your test_abs() example results in an InexactError.
> >>>
> >>> On Saturday, September 5, 2015 at 7:31:35 PM UTC-7, Corey Moncure
> wrote:
> >>>>
> >>>> Can someone help me understand what's going on here?
> >>>> Maybe I've been sitting at my desk too long?
> >>>>
> >>>> julia> arr = hex2bytes("14fb9c03")
> >>>> 4-element Array{Uint8,1}:
> >>>> 0x14
> >>>> 0xfb
> >>>> 0x9c
> >>>> 0x03
> >>>>
> >>>> julia> f(x) = abs((x % 3) - 3)
> >>>> f (generic function with 1 method)
> >>>>
> >>>> julia> function test_abs(bytes_input::Array{Uint8})
> >>>> input_len::Uint = sizeof(bytes_input)
> >>>> a::Int = f(input_len)
> >>>> println("a: ", a, " input len: ", input_len)
> >>>> end
> >>>> test_abs (generic function with 1 method)
> >>>>
> >>>> julia> test_abs(arr)
> >>>> a: -2 input len: 4 <-- minus 2????
> >>>>
> >>>> julia> sizeof(arr)
> >>>> 4
> >>>>
> >>>> julia> f(4)
> >>>> 2 <-- expected
> >>>>
> >>>>
> >>>>
> >>>>
> >
>
