Read binary file 3 bytes at a time

2015 6 10
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更新时间：2015 6 11
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Hi, I am new and I discovered that the fread by default reads a binary file 1 byte at a time, how can I read the file 3 bytes at a time. Thanks

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James Tursa
James Tursa 2015-6-10
编辑：James Tursa 2015-6-10

0 个投票

Try something like this. It reads in the entire file as uint8, groups them in 3 bytes, inserts an extra 0 byte in front of the group of 3, and then reinterprets the bytes as 4 byte unsigned integers. If you don't get the result you want, we may have endian issues to deal with and may need to swap bytes.
x = fread(fid,inf,'*uint8'); % read as uint8
x = reshape(x,3,[]); % groups of 3 in columns
[m,n] = size(x); % get size
x = [zeros(1,n,'uint8'); x]; % insert row of 0's at top to get 4 bytes per number
x = typecast(x(:),'uint32'); % reinterpret bytes as 4-byte unsigned integers

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matlab gives me this error as soon as it executes the first line
Error using fread Invalid precision.
James Tursa
James Tursa 2015-6-10
This is a typo that I am ALWAYS making and will ALWAYS make throughout my life ... unit8 should be uint8.
ahha ok, It was also my fault, I copied and pasted it without checking. Ok now it goes through but it stops at the last line with this error:
Error using typecast The first input argument must be a vector.
James Tursa
James Tursa 2015-6-10
Sorry, you grabbed it before my last edit using x(:) in the last line.

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Guillaume
Guillaume 2015-6-10

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A much simpler solution would have been to read the file with a precision of 'ubit24' (or 'bit24' if the integer are signed), which is basically a 3 byte integer:
x = fread(fid, 'ubit24'); %that's it. all done

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James Tursa
James Tursa 2015-6-11
Good one ... I learned something today about formats available for fread.

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Walter Roberson
Walter Roberson 2015-6-10

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fread(fid, 3)

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This reads only the first 3 values
I want to read the full 3 bytes
James Tursa
James Tursa 2015-6-10
编辑：James Tursa 2015-6-10
Do you mean you have a file of many bytes, and each group of 3 bytes in the file should be read in as an individual number? If so, what type of number? An integer of some sort?
Domenico Bellantoni
编辑：Domenico Bellantoni 2015-6-10
@James , yes! Just like that! They are unsigned integer.for each line of the file there is an integer number represented using 3 bytes and I want to read it

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