I am trying to take one's complement of 0 to get 1 but I get 4294967295. Here is 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++.
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11s complement depends on the number of bits in the thing you're finding the ones complement of. The defining feature of one's complement is that you have two numbers that are both zero. The negation operation in 1s complement is equivalent to the bitwise not operator as you've observed. I think you have a mistaken idea about how 1s complement works.Omnifarious– Omnifarious2010-08-11 20:00:48 +00:00Commented Aug 11, 2010 at 20:00
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6 Answers
Why do you expect 1? Bit-wise complement flips all the bits.
00000000000000000000000000000000 = 0
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bitwise NOT
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11111111111111111111111111111111 = 4294967295
Perhaps you are thinking of a logical NOT. In C++ this is written as !x.
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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.
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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.
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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...
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~ 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.
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You can use
unsigned int y= !x;
to get y = 1;
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