title: Code worked yesterday, wokeup to this error "name 'y' is not defined". Euler integration equation. Python3
body: <p>Context:
  Integrate using Eulers method with the formulas and plot:</p>
  <ol>
  <li>dy/dt = ry * (1 - y/ymax)</li>
  <li>y(t+h) = y(t) + h*(dy/dt)</li>
  </ol>
  <p>The code in question:</p>
  <pre><code>## Establish global requirements
  import numpy as np
  import matplotlib.pyplot as plt
  
  ## Create variables
  y0 = 1                   # initialize variable
  y[0] = y0                # set start point
  h = 0.01                 # step size
  t = np.arange(1,101,h)   # time range from 0 to 100
  r = 0.14                 # given r value
  ymax = 50                # give ymax
  
  for i in range(0,len(t)-1):                              # loop from 0 to 100
      y[i+1] = y[i] + h*((r * y[i]) * (1 - (y[i]/ymax)))   # given formula, iterating through all t values to find y values 
      
  plt.plot(t,y)                                     # plot time vs integrated values
  plt.xlabel('time')                                # x-axis label
  plt.ylabel('y')                                   # y-axis label
  plt.title('Integrating using Eulers Method')      # title
  plt.show()                                        # remove random line of code output
  </code></pre>
  <p>and this is the error I keep getting, despite the code working yesterday</p>
  <pre><code>---------------------------------------------------------------------------
  NameError                                 Traceback (most recent call last)
  &lt;ipython-input-38-fe1b96e580a4&gt; in &lt;module&gt;
        5 ## Create variables
        6 y0 = 1                   # initialize variable
  ----&gt; 7 y[0] = y0                # set start point
        8 h = 0.01                 # step size
        9 t = np.arange(1,101,h)   # time range from 0 to 100
  
  NameError: name 'y' is not defined
  </code></pre>
  <p>Any help would be seriously appreciated. Thank you.</p>