Thanks Bakul, I also understand now what Matt Harden was referring to with his comment. At a bit level both operations cause the same effect on the underlying bits of the variable but it's meaning changes depending on the type. All clear!
Cheers, Pablo On Wed, Jul 26, 2017 at 1:51 PM, Bakul Shah <[email protected]> wrote: > [Sorry, didn't see your original response] > > In the Go *language* (not just the compiler), when x is an unsigned int, > -x is treated as a "shorthand" for ^x+1. Another interpretation is that for > finite ints all arithmetic operations are modulo the word size. Thus -1 > modulo 2^64 is 0xffffffff. You might even say that a 2's complement > machine's native int type is an unsigned int! It is just that most > arithmetic operations set relevant condition codes (or equivalent) that > allows you (and the compiler) to interpret an int as a signed int or an > unsigned one. > > I would suggest reading up on how computers do arithmetic. Programming > languages are merely syntactic sugar on top of processor instruction sets > :-) > > On Jul 25, 2017, at 8:16 PM, Pablo Rozas Larraondo < > [email protected]> wrote: > > Ok, I might be understanding this differently then: > > The -1 * x operation cannot be defined for unsigned integers as -1 > overflows. Therefore, -x in the case of unsigned types is interpreted > differently by the compiler, resulting in a ^x + 1 operation. Is that a > correct assumption? > > By the way, yes, the title should say Bit instead of Byte, sorry about the > confusion. > > On Wed, Jul 26, 2017 at 11:35 AM, Matt Harden <[email protected]> > wrote: > >> Both statements are true for both signed and unsigned integers. >> >> On Mon, Jul 24, 2017, 04:11 Pablo Rozas Larraondo < >> [email protected]> wrote: >> >>> Thanks Bakul, I think I have a better understanding of what's going on >>> after reading your response. >>> >>> Is it correct to say that the Go compiler treats the prepended minus >>> sign differently depending on the variable being a signed or an unsigned >>> integer? >>> >>> By looking at this example: https://play.golang.org/p/feqQsuPkqk >>> >>> It seems to me that in the case of an unsigned integer it's treated as a >>> bitwise operation : -x == ^x + 1 >>> But for signed integers -x == -1 * x >>> >>> Cheers, >>> Pablo >>> >>> >>> >>> On Mon, Jul 24, 2017 at 5:36 AM, Bakul Shah <[email protected]> wrote: >>> >>>> This is a standard trick to find the least significant set bit in a >>>> word. Only works for 2's complement numbers! >>>> -x == ~x+1 >>>> For example: x = 00110000b (24), ~x+1 = 11001111+1 = 11010000. Adding >>>> them yields 00010000; thus only the least significant set bit remains set. >>>> >>>> Note that func LSB(x uint64) uint64 { return x&-x } works too. In your >>>> example you get an error because in Go literal constants are untyped. It is >>>> a pragmatic decision -- see https://blog.golang.org/constants >>>> >>>> On Jul 23, 2017, at 5:50 AM, Pablo Rozas Larraondo < >>>> [email protected]> wrote: >>>> >>>> I have seen Go code using this function to find out the least >>>> significant byte of unsigned integers: >>>> >>>> func LSB(ci uint64) uint64 { return uint64(ci) & -uint64(ci) } >>>> >>>> This function works fine but I wonder why, if call the same AND >>>> operation, it results in an error: "constant -X overflows uint64" >>>> >>>> Here is a playground example to illustrate this: >>>> https://play.golang.org/p/_0EYtlLnmG >>>> >>>> Does anyone know what changes when -uint64() is called in a return >>>> statement? How a negative uint should be interpreted? >>>> >>>> Thank you, >>>> Pablo >>>> >>>> -- >>>> You received this message because you are subscribed to the Google >>>> Groups "golang-nuts" group. >>>> To unsubscribe from this group and stop receiving emails from it, send >>>> an email to [email protected]. >>>> For more options, visit https://groups.google.com/d/optout. >>>> >>>> >>> >>> -- >>> You received this message because you are subscribed to the Google >>> Groups "golang-nuts" group. >>> To unsubscribe from this group and stop receiving emails from it, send >>> an email to [email protected]. >>> For more options, visit https://groups.google.com/d/optout. >>> >> > > -- > You received this message because you are subscribed to the Google Groups > "golang-nuts" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > For more options, visit https://groups.google.com/d/optout. > > > -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
