How to read 8-byte floating point numbers from binary？use fread

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aijiangzhao aixuwu 2021-10-28

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1573388-how-to-read-8-byte-floating-point-numbers-from-binary-use-fread

编辑： Jan 2021-11-12

采纳的回答： Jan

fseek(fid,56,'bof');

x = fread(fid,1,'float64','b')

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aijiangzhao aixuwu 2021-10-29

This is correct, but a new question has arisen. For example, how can "DC 88 12 C0 C1 4F 7C A9" be replaced with "C0 12 88 DC A9 7C 4F C1"? Then read as double-precision floating point

Walter Roberson 2021-10-29

When you fopen you should specify the byte order as the third parameter to fopen, unless you are reading from something with mixed orders.

If you are reading from something with mixed orders but know the order for one particular binary fread then specify the byte order as a parameter to fread.

If you are reading a sequence of bytes at one time and regrouping them (such as if you were reading a structure with mixed datatype) then if necessary use swapbytes after any necessary typecast()

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采纳的回答

Jan 2021-10-30

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1573388-how-to-read-8-byte-floating-point-numbers-from-binary-use-fread#answer_820378

编辑：Jan 2021-11-12

在 MATLAB Online 中打开

With 'ieee-le' format the bytes are read in the order [8,7,6,5,4,3,2,1], with 'ieee-be' as [1,2,3,4,5,6,7,8]. There is no format to store doubles with the byte order [4,3,2,1,8,7,6,5]. Yo you have to convert the order manually by reading the bytes, mix them as wanted and use typecast for the conversion.

[EDITED] As code:

bytes = fread(fid, 8, '*uint8');
bytes = bytes([4,3,2,1,8,7,6,5]);
value = typecast(bytes, 'double')

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Walter Roberson 2021-10-31

在 MATLAB Online 中打开

If the data is in big-endian then either specify ieee-be or swapbytes()

format long g
T = -pi
T = 
         -3.14159265358979
fprintf('source data\n');
source data
t64 = typecast(T, 'uint64');
fprintf('%016x\n', t64(1));
c00921fb54442d18
t8 = typecast(T, 'uint8');
fprintf('%02x ', t8(1:8)); fprintf('\n');
18 2d 44 54 fb 21 09 c0 
fn = tempname;
fid = fopen(fn, 'w', 'ieee-be');
fwrite(fid, T, 'double');
fclose(fid);
fprintf('reading from file\n');
reading from file
dinfo = dir(fn);
fprintf('file is %d bytes long\n', dinfo.bytes);
file is 8 bytes long
fid = fopen(fn, 'r');
in_bytes = fread(fid, [1 inf], '*uint8');
fprintf('%02x ', in_bytes); fprintf('\n');
c0 09 21 fb 54 44 2d 18 
frewind(fid)
in_64 = fread(fid, [1 inf], '*double');
in_64
in_64 = 
     3.20737563067642e-192
swapbytes(in_64)
ans = 
         -3.14159265358979
fprintf('%016x\n', typecast(in_64, 'uint64'));
182d4454fb2109c0
frewind(fid)
in_64 = fread(fid, [1 inf], '*double', 'ieee-be');
in_64
in_64 = 
         -3.14159265358979
swapbytes(in_64)
ans = 
     3.20737563067642e-192
fprintf('%016x\n', typecast(in_64, 'uint64'));
c00921fb54442d18
fclose(fid);

Jan 2021-11-12

在 MATLAB Online 中打开

As I have suggested already, except for the changed byte order:

% fseek(fid, 56, 'bof');
% bytes = fread(fid, 8, '*uint8');
% Manually:
bytes = uint8(sscanf('C5 B9 FB 60 C1 4B FD CA', '%x')).';
value = typecast(fliplr(bytes), 'double')  % As your fread('float64')
value = -8.0410e+27
% [197, 185, 251, 96, 193, 75, 253, 202]
bytes = bytes([4,3,2,1,8,7,6,5]);
value = typecast(bytes, 'double')
value = -3.6689e+06