How can I convert a negative integer to a binary string, in other words, how can I find Two's Complement in MATLAB?

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MathWorks Support Team 2013-6-26

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评论： Walter Roberson 2020-10-2

DEC2BIN converts non-negative decimal integers to a binary string. I want to create binary strings, with a leading sign bit, from any (negative or positive) integer.

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Answer 1

MathWorks Support Team 2013-6-26

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https://ww2.mathworks.cn/matlabcentral/answers/98649-how-can-i-convert-a-negative-integer-to-a-binary-string-in-other-words-how-can-i-find-two-s-comple#answer_107997

This can be accomplished using the TYPECAST function. For example, to find a binary string for 'n' with respect to 8-bit two's complement, you can use the command,

dec2bin(typecast(int8(n),'uint8'))

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Walter Roberson 2015-10-29

在 MATLAB Online 中打开

You are incorrect. The binary equivalent of 10.2 involves an infinite repeating decimal, the same way that the decimal representation of 1/7 involves an infinite repeating decimal.

There is no one representation in binary for negative values or for fractions. There are several competing standards for faking it. One of the standards is known as IEEE 754 Double Precision. It is that standard which MATLAB uses to represent numbers in binary. dec2bin(typecast(10.2,'uint64'),64) gives you the string representation of the binary used for 10.2

As to how to get back to the 10.2... with difficulty. Which is why people do not use this method.

Actually checking again I find it doesn't quite work. Instead you need

x = -10.2;
z2 = reshape(dec2bin(typecast(x,'uint8'),8).',1,[])

and you recover with

typecast(uint8(bin2dec(reshape(z2,8,[]).')),'double')

If you want to get back exactly what you started with, you need to use this (or equivalent.) Or, alternately, you could define your own format with precision sufficient for your needs. Be sure to take into account how you represent negatives and how you represent a potentially large range of values.

Walter Roberson 2020-10-1

在 MATLAB Online 中打开

You cannot get back the correct number if you convert -5.82 to fewer bits.

value = -5.82;
bits32 = dec2bin(typecast(single(value),'uint32'))
retrieved = typecast(uint32(bin2dec(bits32)),'single');
value - retrieved

ans =
  single
    1.716614e-07

The best retrieved value you can get differs from the original by about 3 parts per billion.

Walter Roberson 2020-10-2

在 MATLAB Online 中打开

Sure there are other ways to generate 32 bit representations of double precision numbers, but think about this for a moment:

There are at most 2^32 different 32 bit numbers.

In double precision, for any given floating point mantissa, there are 52 bits of fraction, so thare are at least 2^52 different double precision numbers. For example 1 + (0:2^52-1)/2^53 constructs 2^52 different double precision floating point numbers with the same mantissa:

>> num2hex(1)
ans =
    '3ff0000000000000'
>> num2hex(2-eps)
ans =
    '3fffffffffffffff'
>> log2(eps)
ans =
   -52

By the Pigeon Hole principle, it is obvious that you cannot come even close to representing all possible double precision numbers in 32 bits. No matter what encoding you use for the 32 bits, there are at most 2^32 different states, and that is not enough to encode 2^52 different values.

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