fortran90 - Fortran behavior of a tiny program -


i new in fortran90 (30 minutes ago...) , have program:

program example1     implicit none     real (kind=8) :: x,y,z     x = 3.d0     y = 2.d-1     z = x + y      print *, "y = ", y     print *, "x = ", x     print *, "z = ", z  end program example1

but when run with:

gfortran example1.f90 ./a.out

the output is:

y =   0.20000000000000001      x =    3.0000000000000000      z =    3.2000000000000002

why not 3.2000000000000000 ??? doing wrong? why y has 1 in last digit?? , why z has 2 in last digit?? sorry if dumb question, dont understand doing wrong...

thanks!!

there's absolutely nothing wrong program. issue has real's inability represent decimals precisely, without error. problem numbers not composed of negative powers of 2 must represented approximately. that's why there small error in 16-th decimal place. more information representation of reals take @ article on wikipedia. another great article on same subject.

if replace 0.2 0.25, problem go away, because 0.25 2 ^ -2.


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