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I am using a Fortran code that writes REAL type variables in h5files, so it uses this code 

call h5dcreate_f(file_id,varn,H5T_NATIVE_REAL, filespace,dset_id, hdferr)

and of course other h5 functions like geth5dims.

But I am confused as to why when I open the h5 file using either R or HDFview, REAL variables have up to 20+ digits of precision instead of the 7 that I would expect from a fortran REAL. Example: In fortran when I print out a variable I have 0.1662154 but in my h5file the same variable is 0.166215423415234653524...

Why all these extra digits, are these digits just noise? It's important because I will do further calculations that are sensitive to rounding errors.

Using the write statement with FORMAT(F30.25) in Fortran I get 0.0006564661744050681591034 but in R I get 0.00065646617440506816, if in Fortran I only use write(,) var or print *,var instead I get only 6.565466E-4

Vladimir F Героям слава
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Herman Toothrot
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  • could you show the exact value (no "..." ), My guess would be they are upcasting to double, which is fine for your precision so long as you use the same type when you read it back. – agentp Sep 04 '16 at 13:52
  • @agentp you gave me a good lead but that's not quite the answer, what it does, it actually copies all decimals up to 22 digitas, at least that's what I can see from R, and when I print the value in fortran I see it has more than 25 decimal digits, so that function is writing as many as X digits but I don't know how many. – Herman Toothrot Sep 05 '16 at 14:46
  • when you print anything in fortran the number of digits you see is determined by the *type* and format specification. That has nothing to do with whatever is in your hd5. You should show the print/write statement that is producing 25 digits. Maybe you should include the `r` tag. – agentp Sep 05 '16 at 21:13
  • @agentp I have added more info. – Herman Toothrot Sep 06 '16 at 07:23
  • You don't have to use any **EDIT** or **UPDATE** like in discussiin threads. Just edit the question to be as clear and complete as possible. Add information to any place not just at the end. Anyone can click on the date of the edit below the question to see the complete edit history. – Vladimir F Героям слава Sep 06 '16 at 07:59
  • see what you get if you write your original value with that format. – agentp Sep 06 '16 at 11:49
  • @agentp I don't think I can do that, it is a built in function of the hdf5 library. I could try save it with double, is that what you mean? – Herman Toothrot Sep 07 '16 at 15:48
  • I mean don't even save to hd5 at all just print `varn` or whatever it is that you "think" is *exactly* `0.1662154` with `f30.25` – agentp Sep 07 '16 at 17:05
  • @agentp those are the values I have report in the last sentence of my question – Herman Toothrot Sep 07 '16 at 21:01

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