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Help!!! how to search for some xx xx xx xx(hex) in a dat file very fast!!!

Asked by Eric Jiang on 29 Jun 2019
Latest activity Commented on by Eric Jiang on 3 Jul 2019
Help!!! I have a dat file, about 40MB,
I want to search for xx xx xx xx (hex),
I can do it using for or while loop, but it's too slow because of 40 million Bytes !
how to speed up,thanks!


What format is the file?

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3 Answers

Answer by Guillaume
on 30 Jun 2019
Edited by Guillaume
on 30 Jun 2019
 Accepted Answer

Unlike per isakson, I'm assuming that you're looking for a byte pattern (given in hexadecimal format) in a binary file. If you're looking for a pattern of hexadecimal characters in a text file see per's answer.
hexpattern = ['41'; 'AB'; 'FF'; '7E']; %you haven't specified how this is stored. Taking a guess
filetosearch = 'C:\somewhere\somefolder\somefile.dat'; %doesn't have to have .dat extension
%read file
fid = fopen(filetosearch, 'r');
assert(fid > 0, 'Failed to open file. Most likely the wrong path was specified');
filecontent = fread(fid, [1 Inf], '*uint8'); %read all bytes at once
%pattern search
patternvalues = hex2dec(hexpattern);
patternlocation = strfind(filecontent, patternvalues); %despite its name strfind also works for numbers
sprintf('Hex pattern was found at byte(s) %s', strjoin(compose('%d', patternlocation), ', '));
edited as I got per isakson and dpb mixed up


thanks,you got what i meant。because the pc I use is a little bit old,when i try to read all bytes at once, it came out a out of memory error, how to solve this problem? i dont understand, the dat file is 40MB, howcome it would be out of memory?
Did you use the code in my answer? You shouldn't get an out of memory error with it for a 40 MB file.
What does
thank you!!!, you code is very usefull, but another problem came out, when i use strfind,the out of memory error happened, i guess the algrithm of strfind needs lots of memory to do the job, how to solve this problem?

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Answer by per isakson
on 30 Jun 2019
Edited by per isakson
on 30 Jun 2019

Your question is very vaque and leaves room for interpretation.
I assume that dat-file is an ordinary text file. I cannot guess in what form you want the hex-strings, which are found.
However, I made a little test
  • created a 10MB text file, cssm.txt
  • created a script, cssm.m
txt = fileread( 'cssm.txt' );
cac = regexp( txt, '([0-9A-F]{2} ){3}[0-9A-F]{2}', 'match' );
  • ran cssm
Elapsed time is 0.133106 seconds.
Elapsed time is 0.357219 seconds.
  • and peeked at the result
>> cac{[1,2,3601]}
ans =
'01 23 45 67'
ans =
'89 AB CD EF'
ans =
'01 23 45 67'
I doubt that you can do it significantly faster with plain Matlab on a standard desktop PC
Triggered by Guillaume's answer: To get the locations of the hex-strings replace
cac = regexp( txt, '([0-9A-F]{2} ){3}[0-9A-F]{2}', 'match' );
[cac,loc] = regexp( txt, '([0-9A-F]{2} ){3}[0-9A-F]{2}', 'match', 'start' );
and peek
>> loc([1,2,3601])
ans =
33 2793 9936083


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Answer by dpb
on 30 Jun 2019
Edited by dpb
on 30 Jun 2019

If it's performance you're looking for, pass the job off to a grep utility...there are any number of freeware versions available for Windows if not one already installed on your system...
Altho seem to now recall there may be a FEX submission in mex form...I didn't search to see if really is, but suggest probably worth doing so...


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