title: How can I eliminate "unsupported operand type(s) for -: 'str' and 'str'" error as follows?
body: <p>When I run the following code,</p>
  
  <pre><code>def difference(dataset, interval=1):
      diff = list()
      for i in range(interval, len(dataset)):
          value = dataset[i] - dataset[i - interval]
          diff.append(value)
      return numpy.array(diff)
  
  series = read_csv('Houston_weather.csv', header=None)
  X = series.values
  days_in_year = 365
  differenced = difference(X, days_in_year)
  </code></pre>
  
  <p>I get the following error, is there a way to eliminate it? I am working on time series of weather.</p>
  
  <pre><code>---------------------------------------------------------------------------
  TypeError                                 Traceback (most recent call last)
  &lt;ipython-input-70-a0a67d687691&gt; in &lt;module&gt;
        4 X = series.values
        5 days_in_year = 365
  ----&gt; 6 differenced = difference(X, days_in_year)
  
  &lt;ipython-input-64-b7e841922d29&gt; in difference(dataset, interval)
        3     diff = list()
        4     for i in range(interval, len(dataset)):
  ----&gt; 5         value = dataset[i] - dataset[i - interval]
        6         diff.append(value)
        7     return numpy.array(diff)
  
  TypeError: unsupported operand type(s) for -: 'str' and 'str'
  </code></pre>
  
  <p>My data frame is:
  <a href="https://i.stack.imgur.com/3Q5zn.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/3Q5zn.png" alt="enter image description here"></a></p>