fortran - Result of GAMMA underflows its kind

Question

I would like to calculate gamma(-170.1) using the program below:

program arithmetic  
! program to do a calculation  
real(8) :: x  
x = GAMMA(-170.1)  
print *, x  
end program  

but I get the error:

test.f95:4.10:

x = GAMMA(-170.1) 1 Error: Result of GAMMA underflows its kind at (1)

when I compile with gfortran. According to Maple gamma(-170.1) = 5.191963205*10^(-172) which I think should be within the range of the exponent of the variable x as I've defined it.

Answer 1

The below modification of your program should work. Remember that in Fortran the RHS is evaluated before assigning to the LHS, and that floating point literals are of default kind, that is single precision. Thus, making the argument to GAMMA double precision the compiler chooses the double precision GAMMA.

program arithmetic  
! program to do a calculation  
integer, parameter :: dp = kind(1.0d0)
real(dp) :: x  
x = GAMMA(-170.1_dp)  
print *, x  
end program