On Sat, Sep 5, 2015 at 11:57 PM, <[email protected]> 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
>>>>
>>>>
>>>>
>>>>
>
