CODEWITHSUNDEEP
fortranvariable-assignment
scana
scana

Reputation: 111

Real value after assignment different from the real expression

I'm translating a code from Fortran. I get a weird behavior during assignment. I know that adding the code could be very helpful but I can't add the exact code (I'm not authorized) and I was not successful to replicate it.

The lines are the following (qk was predefined and qk1 was not):

   print*,"qk",qk     
   print*,"qk1",qk1
   QK1=QK
   print*,"qk",qk
   print*,"qk1",qk1

and I get these values printed:

    qk   21909779.000000000     
    qk1   6.44842193E+32
    qk   21909779.000000000     
    qk1   21909780.0

The point is that I would expect to get qk1 equal to qk... why are they different? When I try to replicate it, obviously I get the same values printed.

Since I didn't add the code I do not expect a precise answer... does anyone have any idea about what to check?

Upvotes: 1

Views: 323

Answers (1)

Ian Bush
Ian Bush

Reputation: 7432

The reason is that as indicated in the comments qk is single precision, qk1 double, and at the values required the spacing between single precision reals is 2:

Program one_out

  Use, Intrinsic :: iso_fortran_env, Only : real32, real64

  Implicit None

  Real( real64 ) :: qk64
  Real( real32 ) :: qk32

  qk64 = 21909779.0_real64
  qk32 = qk64

  Write( *, * ) 'qk64 = ', qk64
  Write( *, * ) 'qk32 = ', qk32

  Write( *, * ) '64 spacing ', Spacing( qk64 )
  Write( *, * ) '32 spacing ', Spacing( qk32 )

End Program one_out

ian@eris:~/work/stack$ gfortran -Wall -Wextra -fcheck=all -std=f2008 one_out.f90 
one_out.f90:11:9:

   qk32 = qk64
         1
Warning: Possible change of value in conversion from REAL(8) to REAL(4) at (1) [-Wconversion]
ian@eris:~/work/stack$ ./a.out
 qk64 =    21909779.000000000     
 qk32 =    21909780.0    
 64 spacing    3.7252902984619141E-009
 32 spacing    2.00000000

The most important lesson here is always, always use Implicit None - and the second is that compiler warning are useful, turn them on and work out what they mean!

Upvotes: 6

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