FIX2DEC fails for negative binary string

2018 3 22
4 个回答
更新时间：2018 3 23
4 次查看（30 天）

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Dear Sir/Madam,
When fix2dec('10011.10110') is used with negative binary strings the following error occurs.
Any help fixing would be much appreciated.
Error using bitcmp
ASSUMEDTYPE must be an integer type name.
Error in twos2dec (line 47)
x(xneg) = -( bitcmp(x(xneg), nbits(xneg)) + 1 );
Error in fix2dec (line 56)
x = twos2dec(t) .* 2.^-frac;
https://au.mathworks.com/matlabcentral/fileexchange/38889-two-s-complement-binary-strings?focused=5250320&tab=function
Regards Joe

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回答（4 个）

Walter Roberson
Walter Roberson 2018-3-22

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Negative binary strings do not exist. To represent negative in binary, you have to use an encoding scheme. The three common encoding schemes are
  1. separated sign, where a bit is reserved for saying whether the value is positive of negative. IEEE 754 format uses separated sign
  2. one's complement, where each bit of the number is directly inverted. This is not common in practice but it is used in some contexts
  3. two's complement, in which you invert the bits and then add one. This is common for integer representation.
For one's complement and two's complement if you are using a fixed-point representation with a separator between integer and fraction, then you need to decide whether the fraction is also complemented or not (calculations work out easier if it is complemented.)
Joe Miller
Joe Miller 2018-3-22

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Dear Walter, Thank you for the reply. Does anyone know how to fix the fix2dec() function? Regards Joe

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Walter Roberson
Walter Roberson 2018-3-22
There is no fix; the function is not broken. You are passing it invalid input.
It would be legitimate to ask for an enhancement to process encodings of negative values, but first you need to define exactly what the desired new encoding is.

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Joe Miller
Joe Miller 2018-3-22
编辑：Walter Roberson 2018-3-22

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Dear Walter,
I am just trying to get the example working.
In the header of "fix2dec()" the example is given with output.
% Example:
% >> fix2dec(['10011.10110'; ' 11.1 '; ' 0.101110 '])
%
% ans =
%
% -12.3125
% -0.5000
% 0.7188
Both the first and second part of the example fails.
Regards Joe

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Walter Roberson
Walter Roberson 2018-3-22
The bitcmp() functionality used was valid only until R2014a, and was removed in R2014b. There is no replacement MATLAB call for it.
The n bit two's complement for a value is 2^n-1 minus the value.

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Joe Miller
Joe Miller 2018-3-23

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Dear Walter, Thank you. I see now that the function bitcmp() does not operate the same in newer versions of matlab. Is there a function to be able to convert twos complement binary to decimal. The format is two complement fix32_29 "sii.fffffffffffffffffffffffffffff". Regards Joe

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Walter Roberson
Walter Roberson 2018-3-23
Please do not create a new Answer for each response. Just click on the "Comment on this Answer" link.
Walter Roberson
Walter Roberson 2018-3-23
The two's complement for an integer, V, that is n bits long is ((2^n-1)-V) . There is no MATLAB call for this except in the cases where n is 8 or 16 or 32 or 64.

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