fwrite function round numbers

Question

I would like to store data in to binary file (precision 'long' - manufacturer's requirement) but after reading file in another software, numbers are rounded. I figured out that when I try to read that file back in MATLAB, numbers are also rounded. Small example:

>> number=-3.913;
>> fid_out=fopen('test','wb');
>> fwrite(fid_out,number,'long');
>> fclose all;
>> fid_in=fopen('test','rb');
>> fread(fid_in,'long')

ans =

    -4

I am sorry, I am not so familiar with binary number type. The reason why I want to use 'long' is external software data decoder. Only information which I got is this:

Some clue?

Answer 1

You got rounded number because you are writing and reading using long, which is a 32 bit integer. To make it work, you need replace long by floating types like double, float etc.

number=-3.913;
fid_out=fopen('test','wb');
fwrite(fid_out,number,'double');
fclose all;
fid_in=fopen('test','rb');
fread(fid_in,'double')

You will get:

ans =

    -3.9130

---

Edit: If you have to use long, you can first multiple the numbers by a big scale number like 1000 to make it a long and at the same time don't lose the accuracy. Then when loading the data back, you can get the original values by further dividing them by the same scale.