Weird output for bitwise NOT
I am trying to take one's complement of 0 to get 1 but I get 4294967295. Here is开发者_开发知识库 what I have done:
unsigned int x = 0;
unsigned int y= ~x;
cout << y;
My output is 4294967295 but I expect 1, why is this so? By the way, I am doing this in C++.
Why do you expect 1? Bit-wise complement flips all the bits.
00000000000000000000000000000000 = 0
|
bitwise NOT
|
v
11111111111111111111111111111111 = 4294967295
Perhaps you are thinking of a logical NOT. In C++ this is written as !x
.
You have to look at this in binary to understand exactly what is happening.
unsigned int x = 0
, is 00000000 00000000 00000000 00000000
in memory.
The ~x
statement flips all bits, meaning the above turns into:
11111111 11111111 11111111 11111111
which converts to 4294967295 in decimal form.
XOR will allow you flip only certain bits. If you only want to flip the least significant bit, use x ^ 1
instead.
Where did you get the expectation of 1 from?
Your understanding of bitwise operations clearly shows is lacking, it would be prudent to work through them first before posting in here...
you're not confusing with a ! which is a logical NOT, are you?
a ~ bitwise complement or a bitwise NOT operation flips all the bits from 1 to 0 and vice versa depending on where in the bitmask is set, so for example, a 1 is
00000000 00000000 00000000 00000001
doing a ~ bitwise NOT on that flips it to
11111111 11111111 11111111 11111110
which gives you the maximum value less 1 of the integer datatype on a 32bit system.
Here is a worthy linky to this which shows you how to do bit-twiddling here.
An integer is more than just 1 bit (it's 4 bytes, or 32 bits). By notting it, you'r flipping everything, so in this case 00000... becomes 11111...
~ flips all of the bits in the input. Your input is an unsigned int, which has 32 bits, all of which are 0. Flipping each of those 0-bits gives you 32 1-bits instead, which is binary for that large number.
If you only want to flip the least significant bit, you can use y = x ^ 1
- that is, use XOR instead.
You can use
unsigned int y= !x;
to get y = 1;
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