Subtlty of C, literal integer constants are signed, so s is converted to
signed before the % and so the whole abs is signed.
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
>>>
>>>
>>>
>>>
>>>
