4 ms·

I must be missing something here. Wouldn't an unsigned int be unable to represent -1 since the sign bit is part of the value?

SAI_Peregrinus 3y ago
C implicitly converts integer types as needed. The specific conversion rules are rather convoluted, see section 6.3 of the ISO C standard for the details. Before C23 this was implementation defined, but since C23 now mandates two's complement representation of signed integers and wrapping for unsigned overflow (as before) the `-1` is converted to UINT_MAX. Then the `%d` format specifier performs a conversion from `unsigned int` back to `int` for printing, resulting in `-1`. Implicit conversions between types of the same rank are guaranteed to "round-trip", you get back the value you started with when converting back to the starting type. If they'd specified an unsigned format specifier for printing then they wouldn't get `-1` printed.
- gpderetta 3y ago
  I'm pretty sure that this was well defined and portable well before C23. Although the representation was implementation defined, IIRC signed to unsigned has always been guaranteed to be as-if in two-complement.
  - SAI_Peregrinus 3y ago
    You're probably correct, but I didn't have the earlier standards as quickly to hand to cite. The storage representation being twos complement is new, I didn't mean to imply that the conversion was as well.
free_bip 3y ago
The -1 here is hiding the fact that -1 is really just 0xffffffff due to two's complement (architecture dependant of course) And printing it as %d is technically a misuse of printf, since %d means print as a signed integer. If you did %u (print as unsigned integer) instead you'd see the value is really 4,294,967,295 (again, platform dependant)
- robertlagrant 3y ago
  I knew someone would say this :) But my point is that how can I assign a negative number to an unsigned int in the first place?
  - drekipus 3y ago
    I would argue that you're not. But I use python in my day job
    - robertlagrant 3y ago
      Hah yes, me too. Python has the opposite problem. C changes the type of a supplied value to fit the declared variable type. Python changes the type of the variable to fit its value's (or reference's) type. So in C if I assign a char literal to an int, I get an int. If I do the same in Python, I get a string. In a strongly typed language, which I'm defining as "needs the left and right hand side of an assignment expression to match somehow", I'd get a type error.
deleted 3y ago
[deleted]

SAI_Peregrinus 3y ago

C implicitly converts integer types as needed. The specific conversion rules are rather convoluted, see section 6.3 of the ISO C standard for the details. Before C23 this was implementation defined, but since C23 now mandates two's complement representation of signed integers and wrapping for unsigned overflow (as before) the `-1` is converted to UINT_MAX. Then the `%d` format specifier performs a conversion from `unsigned int` back to `int` for printing, resulting in `-1`. Implicit conversions between types of the same rank are guaranteed to "round-trip", you get back the value you started with when converting back to the starting type. If they'd specified an unsigned format specifier for printing then they wouldn't get `-1` printed.

gpderetta 3y ago
I'm pretty sure that this was well defined and portable well before C23. Although the representation was implementation defined, IIRC signed to unsigned has always been guaranteed to be as-if in two-complement.
- SAI_Peregrinus 3y ago
  You're probably correct, but I didn't have the earlier standards as quickly to hand to cite. The storage representation being twos complement is new, I didn't mean to imply that the conversion was as well.

free_bip 3y ago

The -1 here is hiding the fact that -1 is really just 0xffffffff due to two's complement (architecture dependant of course) And printing it as %d is technically a misuse of printf, since %d means print as a signed integer. If you did %u (print as unsigned integer) instead you'd see the value is really 4,294,967,295 (again, platform dependant)

robertlagrant 3y ago
I knew someone would say this :) But my point is that how can I assign a negative number to an unsigned int in the first place?
- drekipus 3y ago
  I would argue that you're not. But I use python in my day job
  - robertlagrant 3y ago
    Hah yes, me too. Python has the opposite problem. C changes the type of a supplied value to fit the declared variable type. Python changes the type of the variable to fit its value's (or reference's) type. So in C if I assign a char literal to an int, I get an int. If I do the same in Python, I get a string. In a strongly typed language, which I'm defining as "needs the left and right hand side of an assignment expression to match somehow", I'd get a type error.

deleted 3y ago

[deleted]