-2 < 1 = false. Why?

Hy! Sorry for my bad engl开发者_如何学JAVAish, anyway the questions is:

I have this code in objective-c:

unsigned int a = 1; 
int b = -2
if (b < a);

I expect true and instead the result of the if(b < a)is false, why?

C automatically converts -2 into an unsigned int in the comparison. The result is that the comparison is actually (4294967294 < 1), which it is not.

You are comparing signed to unsigned. The signed value is promoted to unsigned, which results in a large number (0xFFFFFFFD I think) which is definitely bigger than 1

The int b is promoted to an unsigned temporary variable in order to do the comparison. This means that it ends up being greater than a.

See here for the rules: http://msdn.microsoft.com/en-us/library/3t4w2bkb(VS.80).aspx

Drop the "unsigned".

If you look at the binary representation of -2 and then use that binary value as an unsigned int, then b>a

Hope that helps!

You can't compare signed and unsigned numbers like that. Most likely the unsigned gets promoted to a signed value resulting in either undefined behaviour or a really big number (depending on how the negative value was stored).