I'm getting started in Python programming. I'm reading a basic tutorial, but this point is not very clear to me. I would appreciate any help you can give me.
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5This has nothing to do with pyton but with how negative numbers are represented in binary (see wikipedia/google for 2's complement). – Oct 12 '10 at 16:14
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@skaffman: *This has nothing to do with python* – SilentGhost Oct 12 '10 at 16:28
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@skaffman: title is just fine – SilentGhost Oct 12 '10 at 17:08
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1Does every language use the ~ operator? If not, then I don't see why the Python tag is bad. – AHungerArtist Oct 12 '10 at 17:12
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~3 means 'invert' 3. With two's complement on natural number datatypes, this becomes -4, as the binary representation is inverted (all bits are flipped).
Chris Dennett
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2Just to show it in hex:3 = 0x00000003 ~3 = 0xFFFFFFFC and the numbers are signed therefore it is negative. – Vinzenz Oct 12 '10 at 16:14
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Because signed integers are usually stored using two's complement, which means that the bitwise inverse of an integer is equal to its algebraic inverse minus one.
Michael Borgwardt
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~3 means "change all the 1s to 0s and 0s to 1s", so if 3 in binary is 0000000000000011, then ~3 is 1111111111111100. since the first bit of ~3 is a 1, its a negative number. to find out which negative number, in 2s comliment, you invert all bits and add 1, so inverted we are back to 3, then added 1 we get 4.
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It's the invert operator, and returns the bitwise inverse of the number you give it.
MStodd
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It's not just Python, it's the integer numeric representation of almost all modern computers: two's complement. By the definition of two's complement, you get a negative number by complementing the positive number and adding one. In your example, you complemented with ~ but did not add one, so you got the negative of your number minus one.
Mark Ransom
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