问题
When I give to a variable such value: e = 17|-15;
, I get -15 as an answer after compiling.I can't understand what arithmetic c++ uses. How does it perform a bit-wise OR operation on negative decimals?
回答1:
It's just doing the operation on the binary representations of your numbers. In your case, that appears to be two's complement.
17 -> 00010001
-15 -> 11110001
As you can see, the bitwise OR
of those two numbers is still -15
.
In your comments above, you indicated that you tried this with the two's complement representations, but you must have done something wrong. Here's the step by step:
15 -> 00001111 // 15 decimal is 00001111 binary
-15 -> ~00001111 + 1 // negation in two's complement is equvalent to ~x + 1
-15 -> 11110000 + 1 // do the complement
-15 -> 11110001 // add the 1
回答2:
It does OR operations on negative numbers the same way it does so on positive numbers. The numbers are almost certainly represented in two's-complement form, which gives you these values:
17 = 0000000000010001 -15 = 1111111111110001
As you can see, all the bits of 17 are already set in −15, so the result of combining them is again −15.
回答3:
A bitwise or with a negative number works JUST like a bitwise or with a positive number. The bits in one number are ored with the bits in the other number. How your processor represents negative numbers is a different matter. Most use something called "two's complement", which is essentially "invert the number and add 1".
So, if we have, for simplicity, 8 bit numbers:
15 is 00001111
Inverted we get 11110000
Add one 11110001
17 is 00010001
Ored together 11110001
回答4:
17 = b00010001
-15 = b11110001 <--- 2s complement
| -15 = b11110001
回答5:
The operator |
is a "bitwise OR
" operator, meaning that every bit in the target is computed as the OR
-combination of the corresponding bits in the two operands. This means, that a bit in the result is 1
if any of the two bits in the numbers at the same positions are 1
, otherwise 0
.
Clearly, the result depends on the binary representation of the numbers which again depends on the platform.
Almost all platforms use the Two's complement, which can be thought as a circle of unsigned numbers, in which negative numbers are just in the opposite direction than positive numbers and "wrap around" the circle.
Unsigned integers:
Signed integers:
The calculation of your example is as follows.
17: 00000000 00000000 00000000 00010001
-15: 11111111 11111111 11111111 11110001
------------------------------------------
-15: 11111111 11111111 11111111 11110001
回答6:
you have to looks at how the bits work
Basically, if either number has a 1
in a particular spot, than the result will also have a 1
-15 : 11110001 (two's complement)
17 : 00010001
-15 | 17 : 11110001
as you can see, the result is the same as -15
来源:https://stackoverflow.com/questions/14326900/how-does-c-do-bitwise-or-operations-on-negative-numbers