!        PHSPshl
!
! program to plot phase space trajectories for the simple
!   harmonic oscillator.
! this program will either print out the values of position, x
!   and velocity, v, or plot them.
!
input prompt "Input 0 for print, 1 for plot:": noptp
input prompt "Input amplitude: ": A
input prompt "Input time step and max. time:": tstep, tmax
if noptp = 1 then
     clear
     let xmin = -1.33333*A
     let xmax = 1.33333*A
     let ymin = -1.1*A
     let ymax = 1.1*A
     call axes(xmin,xmax,ymin,ymax)
end if
if noptp = 0 then
     print "time","position","velocity"
end if
for t = 0 to tmax step tstep
     let x = A*cos(t)
     let v = A                        !Replace with correct functional
     pause .1                         !  form.
     if noptp = 1 then 
          plot x,v;
     else
          print t,x,v
     end if
next t
end
!
!
sub axes(xmin,xmax,ymin,ymax)
     let xdel=xmax-xmin
     let xb=xdel/10000
     let ydel=ymax-ymin
     let yb=ydel/10000
     set window xmin-xb,xmax+xb,ymin-yb,ymax+yb
     plot 0,0;
     plot 0,ymax;
     plot 0,ymin
     plot xmin,0;
     plot xmax,0
     let xtstep=xdel/8
     let xtsize=ydel/60
     for x=0 to xmax step xtstep            !tick marks on x-axis
          plot x,xtsize;
          plot x,0
     next x
     for x = 0 to xmin step -xtstep
          plot x,xtsize;
          plot x,0
     next x
     let ytstep=ydel/6
     let ytsize=xdel/100
     for y=0 to ymax step ytstep            !tick marks on y-axis
          plot 0,y;
          plot ytsize,y
     next y
     for y=0 to ymin step -ytstep
          plot 0,y;
          plot ytsize,y
     next y
end sub