Read older MATFILE in the current version.

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dipak sanap 2023-4-26

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评论： Steven Lord 2023-4-27

test_read.mat

Hi, I have very old matlab files. Current Matlab dose not support the old format. I have tried reading it in Python and R. Python does read it but the output data is not correct. Is there a way to read these files and retreive the data. I have attached a test file for the reference. Any help would be great!

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Walter Roberson 2023-4-26

https://www.mathworks.com/matlabcentral/fileexchange/22675-read-vaxd-and-vaxg-files-in-r2008b-and-later

https://www.mathworks.com/matlabcentral/answers/13901-converting-vax-float-to-matlab-float

dpb 2023-4-26

在 MATLAB Online 中打开

Table 1-8, Level 4 MAT-File Matrix Header Format
Field Description
type The type flag contains an integer whose decimal digits encode storage information. If the
integer is represented as MOPT where M is the thousands digit, O is the hundreds digit, P is the
tens digit, and T is the ones digit, then:
M indicates the numeric format of binary numbers on the machine that wrote the file. Use this
table to determine the number to use for your machine:
0 IEEE Little Endian (PC, 386, 486, DEC Risc)
1 IEEE Big Endian (Macintosh, SPARC®, Apollo, SGI, HP
9000/300, other Motorola® systems)
2 VAX D-float
3 VAX G-float
4 Cray
...

Looking at the given file (had to do this locally, couldn't seem to fopen the attachment)...

>> fn='test_read.mat';
>> fid=fopen(fn,'r');
>> type=fread(fid,1,'int4')
>> type =
        3000
>> fid=fclose(fid);

That would indeed, seem to be a MATLAB 4 VAX G-float array....

There's a link to the description of .mat files V4, V5 available at https://www.mathworks.com/matlabcentral/answers/93868-is-there-documentation-on-the-structure-of-mat-files-in-matlab#answer_103219

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Answer 1

DGM 2023-4-26

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https://ww2.mathworks.cn/matlabcentral/answers/1953539-read-older-matfile-in-the-current-version#answer_1223884

编辑：DGM 2023-4-26

在 MATLAB Online 中打开

FWIW, I was able to use Walter's first link to read the file.

myfilename = 'test_read.mat';
fid = fopen(myfilename, 'r', 'ieee-le');
% for sake of simplicity, i'm going to just presume M0PT is always 3000
% always VAX-G, double-precision, numeric
fseek(fid,4,-1);
% get the matrix size
nrows = fread(fid, 1, 'uint32');
ncols = fread(fid, 1, 'uint32');
% i'm going to just ignore the possibility that the matrix is complex-valued
fseek(fid,4,0);
% read the original variable name
namelen = fread(fid, 1, 'uint32');
varname = fread(fid, namelen-1, '*char')
fseek(fid,1,0); % name is followed by a null character
% read the matrix
A = freadVAXG(fid, [nrows ncols], 'double');
% i assume there's only one header in the stream, so just close the file
fclose(fid);
% reshape the data and test it against Steven's recovered version
A = reshape(A,nrows,[]);
load new_test_read.mat
immse(A,E) % they match

Of course, I'm assuming that other files are similar. (real-valued numeric double in VAX-G format, with only a single matrix in the file)

The details of the MAT-file format are specified here:

https://www.mathworks.com/help/pdf_doc/matlab/matfile_format.pdf

See page 1-25 for the V4 format

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DGM 2023-4-26

编辑：DGM 2023-4-26

在 MATLAB Online 中打开

Is namelen zero or actually empty?

Is this for the same mat-file as before?

I guess if possible, we could also just assume that the variable name is a fixed length.

myfilename = 'test_read.mat';
fid = fopen(myfilename, 'r', 'ieee-le');
% for sake of simplicity, i'm going to just presume M0PT is always 3000
% always VAX-G, double-precision, numeric
fseek(fid,4,-1);
% get the matrix size
nrows = fread(fid, 1, 'uint32');
ncols = fread(fid, 1, 'uint32');
% i'm going to just ignore the possibility that the matrix is complex-valued
% i'm going to assume that the variable name is always a single character
fseek(fid,10,0);
% read the matrix
A = freadVAXG(fid, [nrows ncols], 'double');
% i assume there's only one header in the stream, so just close the file
fclose(fid);
% reshape the data and test it against Steven's recovered version
A = reshape(A,nrows,[]);
load new_test_read.mat
immse(A,E) % they match

That said, i'm not sure why it wouldn't read namelen, so I can't be certain that it would read anything else correctly anyway.

DGM 2023-4-26

编辑：DGM 2023-4-26

在 MATLAB Online 中打开

I really don't know how fread() could return empty. If i'm missing some sort of platform-dependent behavior or something, maybe someone can tell me what I'm doing wrong.

myfilename = 'test_read.mat';
fid = fopen(myfilename, 'r', 'ieee-le');
% for sake of simplicity, i'm going to just presume M0PT is always 3000
% always VAX-G, double-precision, numeric
fseek(fid,4,-1);
% get the matrix size
nrows = fread(fid, 1, 'uint32')
nrows = 64
ncols = fread(fid, 1, 'uint32')
ncols = 64
% i'm going to just ignore the possibility that the matrix is complex-valued
fseek(fid,4,0);
% read the original variable name
namelen = fread(fid, 1, 'uint32')
namelen = 2
varname = fread(fid, namelen-1, '*char')
varname = 'E'
fseek(fid,1,0); % name is followed by a null character
% read the matrix
A = freadVAXG(fid, [nrows ncols], 'double');
% i assume there's only one header in the stream, so just close the file
fclose(fid);
% reshape the data and test it against Steven's recovered version
A = reshape(A,nrows,[]);
load new_test_read.mat
immse(A,E) % they match
ans = 0

dipak sanap 2023-4-27

@DGM, it worked, Idk, I just restarted the Matlab and moved everything to clear directory. Thank you so much! I just have a final question. How can I make it work for files with different size arrays? Like just row or vector or matrix?

DGM 2023-4-27

As I understand it, the V4 format only supports 2D arrays, and the size of the arrays should be fetched from the header (nrows x ncols). Unless I'm missing something, the given example should already support variations in array sizes (so long as the other assumptions don't change).

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Answer 2

Steven Lord 2023-4-26

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new_test_read.mat

I tried loading the original data using the oldest release of MATLAB I can run and was able to read it. It must have been created by a very old release (pre-v4 perhaps?). I resaved it as a newer MAT-file, attached here.

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Cris LaPierre 2023-4-27

I found I could open the file in R2007b, and R2008a, but not R2008b.

I couldn't launch anything earlier than that on my computer.

Steven Lord 2023-4-27

Actually now that I think about it I didn't use the oldest release I believe can run on my machine (which is different from the oldest release whose files I could access.) I went as far back as release R11 (even though that's not remotely close to a supported release on my OS) because I know I can run that release with only a little difficulty.

Since I work on functions that have existed in some cases since Cleve's original MATLAB (predating MathWorks!) sometimes it's useful to perform "archaeology" to figure out when a particular behavior was introduced and that can entail reading or running code from very old releases.

Is there a TMW service to translate old files that have been orphaned by dropping support for early releases, Steven? That would seem to be a valuable service that a 'bot could handle like the "convert pdf to Excel" services that abound.

I don't know if there is such a service, though it's an interesting idea.

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Read older MATFILE in the current version.

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Read older MATFILE in the current version.

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