fortran90 - Is Fortran unable to do the addition between 865398.78 and -865398.78? Why the answer is -0.03?

Question

In the code below I am adding together 865398.78 and -865398.78. I expect to get 0, but instead I get -0.03.

Source Code:

program main
real(8) :: x

open(10,file="test.txt")
read(10,*)x
print *,"x=",x
x=x+865398.78
print *,"x+865398.78=",x
end program

Result:

x=  -865398.780000000     
x+865398.78= -3.000000002793968E-002

Am I wrong with the usage of "read" codes or something else?

Answer 1

The number 865398.78 is represented in single precision in your code. Single precision can handle about 7 significant digits, while your number has 8. You can make it double precision by writing

x=x+865398.78_8