underoverflow, Fortran

underoverflow

!gfortran, gcc version 5.4.0 20160609

program epsilon
implicit none
integer, parameter :: DP=8, SP=4
real(DP) :: eps_d, x_d, xa, x, under_d, over_d
real(SP) :: eps_s, x_s, under_s, over_s
integer :: i, j
read *,x_d,x_s
!x_d=4.0_DP
!x_s=4.0_SP

print *,"x_d=",x_d
print *,"x_s=",x_s

eps_d=1.0_DP
eps_s=1.0_SP
do while (1.0_SP+eps_s .gt. 1.0_SP)
  eps_s=eps_s/2
end do
do while (1.0_DP+eps_d .gt. 1.0_DP)
  eps_d=eps_d/2
end do
print *,"single precision epsilon=",eps_s
print *,"double precision epsilon=",eps_d

x_d=x_d+eps_d
x_s=x_s+eps_s
print *,"x_d+eps_d=",x_d
print *,"x_s+eps_s=",x_s
!xa=5.5_DP
!x=1.0-xa+(xa*xa)/2.0-(xa*xa*xa)/6.0+(xa*xa*xa*xa)/24.0-(xa*xa*xa*xa*xa)/120.0+  &
!  (xa*xa*xa*xa*xa*xa)/720.0-(xa*xa*xa*xa*xa*xa*xa)/5040.0+ &
!   (xa*xa*xa*xa*xa*xa*xa*xa)/40320.0-(xa*xa*xa*xa*xa*xa*xa*xa*xa)/362880.0+ &
!    (xa*xa*xa*xa*xa*xa*xa*xa*xa*xa)/3628800.0
!
!print *,x
!x=1.0+xa+(xa*xa)/2.0+(xa*xa*xa)/6.0+(xa*xa*xa*xa)/24.0+(xa*xa*xa*xa*xa)/120.0+  &
!  (xa*xa*xa*xa*xa*xa)/720.0+(xa*xa*xa*xa*xa*xa*xa)/5040.0+ &
!   (xa*xa*xa*xa*xa*xa*xa*xa)/40320.0+(xa*xa*xa*xa*xa*xa*xa*xa*xa)/362880.0+ &
!    (xa*xa*xa*xa*xa*xa*xa*xa*xa*xa)/3628800.0
!print *,1.0/x,exp(-xa)
under_s=1.0_SP
under_d=1.0_DP
over_s=1.0_SP
over_d=1.0_DP
do i=1,2000
under_s=under_s/2.0_SP
over_s=over_s*2.0_SP
if(i.lt.1022) then
under_d=under_d/2.0_DP
over_d=over_d*2.0_DP
else
under_d=under_d/1.1_DP
over_d=over_d*1.01_DP
end if
 print *,"single precision=",i,under_s,over_s
 print *,"double precision=",i,under_d,over_d
end do
end program epsilon


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