Matlab: save an int8 vector in a binary file

Question

Hello I would like to get some help with something please:

I'm trying to make readable a int8 vector (which was actually retrieved from an already binary .mat file via a MySQL database).

I'd like to do something like:

fileId = fopen(fileName, 'w');
fwrite(fileId, vector,'uint8');
fclose(fileId);
load(fileName);

But Matlab throws an error "File may be corrupt".

The idea behind this, is to use a central database to stock our .mat files.

vector is a [ 306624x1 int8] and looks like:

vector = [77;65;84 ...

Thanks for any help

Edit

I'm continuing with this task, the solution that @Rotem gave me works perfectly. I have millions of .mat files, which are really slow to read from a hard drive.

So, I have changed to a database and in order to retrieve these files I do mysql queries (this is much faster).

I recover each file as a vector of bytes, then I write in a local hard drive and reloaded using load().

@Rotem helped me with that, but now I'm facing the problem of space disc. There is a way to perform this conversion directly without passing through fwrite - load?

Thanks for the lights!

You don't want fwrite, you want save if your goal is a MAT file. — dasdingonesin
– dasdingonesin, Commented Aug 23, 2016 at 12:18
Are you trying to create a database of binary representations of .mat files? You are using fwrite when your problem says you are reading in data? — zglin
– zglin, Commented Aug 23, 2016 at 12:31
Sorry, I wasn't clear enough. The problem is not the reading. My starting point is an uint8-vector which has the bytes of a binary (.mat) file (each component of the vector is a byte). What I'm trying to do is to rebuild the file from this sequence of bytes. That's why my try was to re-write this vector in a binary .mat file and trying to load it to recover the inicial data on the file. Thanks for your comments. — pacode
– pacode, Commented Aug 23, 2016 at 13:01

Rotem · Accepted Answer · 2016-08-23 16:56:54Z

1

I can't say I got the motivation behind it...

Check the following code sample:

%Create sample mat file with saved matrix A.
A = magic(3);
save('A.mat', 'A');
clear A; %Note: A is cleared from Matlab Workspace.

%Read A.mat to uint8 vector:
f = fopen('A.mat', 'r');
vector = fread(f, '*uint8'); %Use '*uint8' to keep vector in class uint8.
fclose(f);

%Write vector to B.mat as binary file:
f = fopen('B.mat', 'w');
fwrite(f, vector);
fclose(f);

%Load B.mat as mat file:
load('B.mat');

%Display A matrix (loaded from B.mat).
A

A =

8 1 6
3 5 7
4 9 2

answered Aug 23, 2016 at 16:56

Rotem

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3 Comments

pacode Over a year ago

Thanks for your help, thanks to you I realized that a typecast was missing in my code: int8 -> uint8.

pacode Over a year ago

Thanks for your help @Rotem. Do you know a method to read the data directly from the vector instead? (I edited my question explaning better my issue). Thanks for you help.

Rotem Over a year ago

I don't know of such method. Can't you simply delete MAT file after loading?

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