title: Python program errors
body: <p>I'm writing a program that needs to find all pairs of initial conditions (x0,y0) where the x goes extinct first and plot them.the equations are: </p>
  
  <blockquote>
    <p>j=x-y+100 and
    i=x+y-100 </p>
  </blockquote>
  
  <p>and the constraints are from (0,200) in both the x and y directions. </p>
  
  <p>my code looks like this:</p>
  
  <pre><code>for x in range (0,201):
  for y in range (0,201):
      c=0
      i=x
      j=y
  while (x &gt; 0 and y &gt; 0):
      i=x-y+100
      j=x+y-100
      c=c+1
  if i==0:
      plot(i,j)
  if c==50:
      break
  </code></pre>
  
  <p>and my error says:</p>
  
  <pre><code>    Traceback (most recent call last):                i=x-y+100
    File "", line 1, in &lt;module&gt;
  
    File "/tmp/tmpqTzdAj/___code___.py", line 3, in &lt;module&gt;
      exec compile(u'for x in range (_sage_const_0 ,_sage_const_201 ):\n    for y in range (_sage_const_0 ,_sage_const_201 ):\n        c=_sage_const_0 \n        i=x\n        j=y\n        while (x &gt; _sage_const_0  and y &gt; _sage_const_0 ):\n            i=x-y+_sage_const_100 \n            j=x+y-_sage_const_100 \n            c=c+_sage_const_1 \n        if i==_sage_const_0 :\n            plot([i],[j])\n        if c==_sage_const_50 :\n            break
    File "", line 11, in &lt;module&gt;
  
    File "/app/sage/sage-6.1.1/local/lib/python2.7/site-packages/sage/misc/decorators.py", line 692, in wrapper
      return func(*args, **kwds)
    File "/app/sage/sage-6.1.1/local/lib/python2.7/site-packages/sage/misc/decorators.py", line 537, in wrapper
      return func(*args, **options)
    File "/app/sage/sage-6.1.1/local/lib/python2.7/site-packages/sage/plot/plot.py", line 1134, in plot
      G = _plot(funcs, *args, **kwds)
    File "/app/sage/sage-6.1.1/local/lib/python2.7/site-packages/sage/plot/plot.py", line 1236, in _plot
      funcs, ranges = setup_for_eval_on_grid(funcs, [xrange], options['plot_points'])
    File "/app/sage/sage-6.1.1/local/lib/python2.7/site-packages/sage/plot/misc.py", line 132, in setup_for_eval_on_grid
      range_steps = [abs(range[1] - range[0])/(p-1) for range, p in zip(ranges, plot_points)]
  IndexError: list index out of range
  </code></pre>