Unsigned Casting C

[CJPowell27]

hey guys, I've been trying to figure out signed and unsigned integer casting in C lately and I'm kinda stuck.  If i cast a number like -3 as unsigned, what would it turn out to be? I'm trying to evaluate whether (unsigned) -3 > 35 is true or false and I just can't figure out the casting. Any help is appreciated, thanks.

[Hunter259]

10 minutes ago, CJPowell27 said:

hey guys, I've been trying to figure out signed and unsigned integer casting in C lately and I'm kinda stuck.  If i cast a number like -3 as unsigned, what would it turn out to be? I'm trying to evaluate whether (unsigned) -3 > 35 is true or false and I just can't figure out the casting. Any help is appreciated, thanks.

http://stackoverflow.com/questions/50605/signed-to-unsigned-conversion-in-c-is-it-always-safe

Mattering on platform that number can be absolutely massive. So that -3 would be like subtracting 2 from the largest unsigned int available on the platform.

[zMeul]

unsigned int does not have negative numbers

I haven't test it, but converting -3 into unsigned int will be 3

 

is -3 bigger than 35? false

is 3 bigger than 35? false

[Hunter259]

3 minutes ago, zMeul said:

unsigned int does not have negative numbers

I haven't test it, but converting -3 into unsigned int will be 3

 

is -3 bigger than 35? false

is 3 bigger than 35? false

No. It uses 2's compliment to compute the positive number resulting in what is effectively UINT_MAX +1 - Your_Number.

[zMeul]

1 minute ago, Hunter259 said:

No. It uses 2's compliment to compute the positive number resulting in what is effectively UINT_MAX +1 - Your_Number.

sorry, I have't tested it and don't a C compiler at hand

[JoostinOnline]

36 minutes ago, CJPowell27 said:

hey guys, I've been trying to figure out signed and unsigned integer casting in C lately and I'm kinda stuck.  If i cast a number like -3 as unsigned, what would it turn out to be? I'm trying to evaluate whether (unsigned) -3 > 35 is true or false and I just can't figure out the casting. Any help is appreciated, thanks.

Compile a program with this code 

int32_t number = - 3;

printf("unsigned %i equals %u", number,  (uint32_t) number);

 

 

That should tell you what it is. You're forcing it to treat the signed integer as an unsigned integer. You can change number to whatever you want, as long as it's not more than 32 bits. 

[Mira Yurizaki]

10 hours ago, CJPowell27 said:

hey guys, I've been trying to figure out signed and unsigned integer casting in C lately and I'm kinda stuck.  If i cast a number like -3 as unsigned, what would it turn out to be? I'm trying to evaluate whether (unsigned) -3 > 35 is true or false and I just can't figure out the casting. Any help is appreciated, thanks.

If you type cast an signed value to unsigned, it becomes the data type's max value plus the value of the signed number. For example, if -3 was an 8-bit data type, its unsigned equivalent would be 252. So (unsigned) -3 > 35 would be true.

 

I would suggest reading this topic because this is how negative numbers for integers are coded: https://en.wikipedia.org/wiki/Two's_complement

[Unimportant]

It is worth noting that this signed-unsigned conversion can happen implicitly, for example:

int x = -3;
unsigned int y = 35;

if (x > y)
{
	//will be true!
}

The C standard states that x will implicitly be converted to unsigned before the comparison is made. This is the reason many compilers warn about signed-unsigned comparisons.

 

Also semi-related is the fact that unsigned overflow is defined, but signed overflow is undefined behavior:

unsigned int x = UINT_MAX;
++x;	//Well defined, x == 0

int y = INT_MAX;
++y; 	//Undefined!