fwrite sometimes uses 5 bytes to write a 4-byte single
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For me, the following code
fid = fopen('test.dat','wt');
x = single(1.28);
fwrite(fid,x,'single');
bytes = ftell(fid)
produces the result
bytes =
5
instead of the expected result
bytes =
4
which I get for most other values of x. The value of 1.28 is one example, but not an isolated case. In general, about 1 to 2 percent of values take 5 bytes instead of 4. For example,
fid = fopen('test.dat','wt');
x = single(rand(10000,1));
fwrite(fid,x,'single');
bytes = ftell(fid)
produces the result
bytes =
40127
Does anyone know what is going on here and why?
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Walter Roberson
il 27 Lug 2011
If you want to use fwrite() you should not be opening the file as a text file ('wt'). Opening as a text file tells the file system to convert the line terminator.
>> typecast(single(1.28),'uint8')
ans =
10 215 163 63
10 happens to correspond to newline, so that is going to be converted to the byte sequence 13 10 215 163 63 (CR LF then some bytes)
When you are working with binary, leave the 't' off of 'rt' and 'wt'.
2 Commenti
Jan
il 27 Lug 2011
@Walter: The next text-mode file access problem. I hope this strange feature (I do not claim that it is never useful) will disappear in the next 50 years...
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