Contents of file named thedoubler.f90

program thedoubler
   implicit none
   integer, parameter :: nmax = 10000
   real :: ampl(nmax),beg,dt
   integer :: npts,nerr,ip

   ! Read in the data file
   call rsac1('data.sac',ampl,npts,beg,dt,nmax,nerr)

   ! loop over the data points, doubling each
   do ip=1,npts
      ampl(ip) = ampl(ip) * 2.0
   enddo
      
   ! set a user character header 
   call setkhv('KUSER0','DOUBLED!',nerr)

   ! write the altered file back to 
   !    disk under a different name.
   call wsac0('doubled.sac',ampl,ampl,nerr)
end program thedoubler