Typecast a hex string to single

Question

Michael 2022-8-17

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评论： Michael 2022-8-19

I have strings of hex values that I need to typecast to single precision values. I don't actually care about the actual single precision value, I am just trying to package the binary into a single datatype for transmission.

I want something like

MyHexString =  '7f8e2d38';
out = typecast(MyHexString,'single'); %This doesn't work because typecast needs number

But the typecast.m function requires a numeric value. All I want to do is to produce the single precision number equivalent of the binary data held in my hex string.

FYI: I was using

out = typecast(uint32(hex2dec(MyHexString)),'single');

This worked most of the time but is occasionally produced incorrect results because the hex2dec function produces a double and there is some loss on the subsequent uint32 cast. At least that's what I think is wrong.

How to I package hex into a single datatype?

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KSSV 2022-8-18

How about using hex2num.

James Tursa 2022-8-18

编辑：James Tursa 2022-8-18

"... occasionally produced incorrect results ..."

Can you post some specific examples where this happens? How can there be a loss converting to uint32?

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Answer 1

Jan 2022-8-18

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https://ww2.mathworks.cn/matlabcentral/answers/1781210-typecast-a-hex-string-to-single#answer_1028560

编辑：Jan 2022-8-18

在 MATLAB Online 中打开

str = '7f8e2d38';
vec = uint8(sscanf(str, '%2x'));
num = typecast(vec, 'single')
num = single
    4.1379e-05
num = typecast(flip(vec), 'single')   % Maybe MSB?
num = single
    NaN

NaN due to initial 7f

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Michael 2022-8-18

在 MATLAB Online 中打开

@Jan Thanks for helping with this! I don't quite understand why converting to a set of uint8s and then doing type cast works, but converting to a uint32 doesn't. Maybe something to do with the 'single' typecast operation? If you want to respond as to why this works (for posterity's sake) I'd appreciate it. Regardless, here is the script that does the conversion per the sscanf method you proposed. Thanks so much.

doubleNumIn = 1660811774.24557;
fullHexStringIn  = num2hex(doubleNumIn);
topHexStringIn   = fullHexStringIn(1:8);
botHexStringIn   = fullHexStringIn(9:16);
topVecIn         = uint8(sscanf(topHexStringIn,'%2x'));
botVecIn         = uint8(sscanf(botHexStringIn,'%2x'));
topSingleIn      = typecast(topVecIn,'single');
botSingleIn      = typecast(botVecIn,'single');
singleComplex    = topSingleIn+1i*botSingleIn;
%singleComplex IS THEN TRANSMITTED VIA COMPLEX SINGLE UDP MESSAGE. 
topSingleOut     = real(singleComplex);
botSingleOut     = imag(singleComplex);
topVecOut        = typecast(topSingleOut,'uint8');
botVecOut        = typecast(botSingleOut,'uint8');
topHexVecOut     = dec2hex(topVecOut)';
botHexVecOut     = dec2hex(botVecOut)';
topHexStringOut  = topHexVecOut(:)';
botHexStringOut  = botHexVecOut(:)';
fullHexStringOut = [topHexStringOut, botHexStringOut];
doubleNumOut     = hex2num(fullHexStringOut);
%Check input and output is correct
doubleNumIn == doubleNumOut
ans = logical
   1

Michael 2022-8-18

在 MATLAB Online 中打开

@Jan Okay. Thanks. After some more testing, I am still seeing an error with the conversion that I have traced back to this section of the conversion:

topSingleIn      = typecast(topVecIn,'single');
botSingleIn      = typecast(botVecIn,'single');
singleComplex    = topSingleIn+1i*botSingleIn;

When one of these values (topSingleIn or botSingleIn) becomes NaN during the typecast operation it will force the other in the singleComplex representation to be listed as NaN as well. See below.

>> topSingleIn
topSingleIn =
  single
  -7.3895907e-32
>> botSingleIn
botSingleIn =
  single
   NaN
   
>> topSingleIn+1i*botSingleIn
ans =
  single
        NaN +       NaNi
>> typecast(topSingleIn,'uint8')
ans =
  1×4 uint8 row vector
    65   216   191   139
>> typecast(real(topSingleIn+1i*botSingleIn),'uint8')
ans =
  1×4 uint8 row vector
   214   107   207   255
    

The summation of the single values to a complex number forces the real or imaginary part to be NaN if the other is NaN. This is root cause of the problem. Not sure how to get around this. Any ideas?

For some context: I need these single values to be combined as a single precision complext number so that I can concatinate it to a vector I am sending via UDP. This is how I am timestamping each vector that is transmitted over UDP. My other data is complex single precision data, which is why I going through all this work to get my timestamps into this odd format.

Thanks.

Michael 2022-8-19

在 MATLAB Online 中打开

@Walter Roberson After thinking a bit, I came to realize that millisecond precision is okay for my purposes. So my solution now is to split the digits of the millisecond version of the posix time into parts, both of which are limited by flintmax('single'). A basic example is shown below. I think because I limit the values to no greated than flintmax('single'), I should never have NaN values transmitted. The maximum transmitted value will be 167,772,169,999,999 (~1.7e14) which is far more than current posixtime in milliseconds (~1.7e12). The problem I was having previously is that I was trying to transmit microsecond time which is ~1.7e15, which is greater than 1.7e14.

currTime = posixtime(datetime('now'));
millisecondsIn = round(currTime*10^3)
millisecondsIn = 1.6609e+12
digitsplit = 7;
topDigits = floor(millisecondsIn*10^-digitsplit);
botDigits = millisecondsIn - topDigits*10^digitsplit;
topDigitsSingle  = single(topDigits);
botDigitsSingle  = single(botDigits);
singleComplex    = complex(topDigitsSingle, botDigitsSingle);
%singleComplex IS THEN TRANSMITTED VIA COMPLEX SINGLE UDP MESSAGE. 
%Max transmitted value is based on flintmax('single') = 16777216
%   Real can transmit up to 16777216
%   Imag can transmit up to  9999999
%Range of transmitted values is -167772169999999 <= ms <= 167772169999999
%This is sufficient to transmit millisecond precision posix timestamps
%
topDoubleOut     = double(real(singleComplex));
botDoubleOut     = double(imag(singleComplex));
millisecondsOut  = topDoubleOut*10^digitsplit + botDoubleOut
millisecondsOut = 1.6609e+12
millisecondsOut == millisecondsIn
ans = logical
   1

Walter Roberson 2022-8-19

在 MATLAB Online 中打开

You are transmitting in binary, right? Rather than formatting the single precision data as text?

Because if so then it does not matter that the printable version of a series of bytes might be NaN, as long as the byte sequence is exactly recoverable when transmitted in binary.

format long g
s = '7f8e2d38';
d = uint8(sscanf(s, '%2x').')
d = 1×4
   127   142    45    56
df = typecast(fliplr(d), 'single')
df = single
   NaN
typecast(df, 'uint8')
ans = 1×4
    56    45   142   127
filename = tempname();
fid = fopen(filename, 'w')
fid = 
     3
fwrite(fid, df, 'single')
ans = 
     1
fclose(fid)
ans = 
     0
ls('-l', filename)
-rw-r--r-- 1 mluser worker 4 Aug 19 18:21 /tmp/tp3dab8982_f12e_4817_9513_f7aa0e567ad9
fid = fopen(filename, 'r')
fid = 
     3
dfr = fread(fid, '*single')
dfr = single
   NaN
fclose(fid)
ans = 
     0
typecast(dfr, 'uint8')
ans = 1×4
    56    45   142   127

Michael 2022-8-19

Yes. That's what I was originally doing i.e. transmitting the raw binary. I believe the main issues I was having is that I was converting the double to a hex and then splitting up that hex. When one part of the resulting split hex string would result in a NaN value, using () +1i() in matlab would then force the other part of the number to be NaN as well. I got around that by using complext(real, imag). The code I posted above that splits the digits rather than the hex seems to do what I need related to transmitting time values. I believe that the transmission of the raw binary would work as well. Thanks for all your help!

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