How can i get up to 7 significant figures of my result in FORTRAN90?

StackOverflow https://stackoverflow.com/questions/10323357

  •  03-06-2021
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Question 

INTEGER, PARAMETER :: SINGLE=SELECTED_REAL_KIND(7)
REAL(KIND=SINGLE) :: K
REAL::X
...
K=X
WRITE(*,*) K

When i write 5 or 6 inside SELECTED_REAL_KIND, it outputs six significant figures but when i try 7, it outputs more than 7. Can you tell me why? Where i got it wrong? Assigning is problematic?

With 5 and 6 output-> 1.39256

With 7 output-> 1.3925623893

Okey I'm asking a simple question. How can i just output 7 significant digits of this number 1.3925623893 ?

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Solution

From gfortran's documentation:

SELECTED_REAL_KIND(P,R) returns the kind value of a real data type with decimal precision of at least P digits

The at least is important. Floating point numbers are most of the time stored using 4 bytes (single precision) or 8 bytes (double precision) and that's it. That's going from asking 6 to 7 significant figures actually adds even more figures: you switched from single to double precision.

What you really want is have a real large enough to store the precision you want, and you already did that correctly, then display only 7 significant figures using a format like:

write(*,'(F12.7)') K
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