@Wladimirufc: You responded that the type is int. This data type (usually)
is 32 bits in length, and stores integers in the twos-complement number
system. See http://en.wikipedia.org/wiki/Two%27s_complement for an
explanation of this number system. In this number system, the bits
(numbered 0 to 31 from the right) have values 2^i (2 to the ith power) for
i = 0, 1, ..., 30, while bit 31 has value -2^31.
In your code, you set x to have a 1-bit in bit 31 and zeros elsewhere.
Thus, the value of x is -2^31, which is the value printed by the first
printf() statement. This value is negative, so the value is negated. In
twos-complement arithmetic, a value is negated by complementing the value
(switching 0s to 1s and 1s to 0s) and then adding 1. I.e., the statement x
= -x is functionally equivalent to x = (~x) + 1. For your value of x, the
resulting value is the same as the original value, so the second printf()
statement also prints the value -2^31.
I'm glad you asked this question because it is very important to understand
how quantities are stored in C and how the arithmetic works.
Dave
On Tuesday, August 21, 2012 11:25:11 AM UTC-5, wladimirufc wrote:
> Somebody could explain to me why this happen:
> x = (1<<31);
> printf("%d\n",x);
> if(x<0) x = -x;
> printf("%d\n",x);
>
> Output:
>
> -2147483648
> -2147483648
>
> Wladimir Araujo Tavares
> *Federal University of Ceará <http://lia.ufc.br/%7Ewladimir/>
> Homepage <http://lia.ufc.br/%7Ewladimir/> |
> Maratona<https://sites.google.com/site/quixadamaratona/>|
> *
>
>
>
--
You received this message because you are subscribed to the Google Groups
"Algorithm Geeks" group.
To view this discussion on the web visit
https://groups.google.com/d/msg/algogeeks/-/7a3Wer6PRRQJ.
To post to this group, send email to [email protected].
To unsubscribe from this group, send email to
[email protected].
For more options, visit this group at
http://groups.google.com/group/algogeeks?hl=en.
