title: pandas dataframe convert column type to string or categorical
body: <p>How do I convert a single column of a pandas dataframe to type string? In the df of housing data below I need to convert zipcode to string so that when I run linear regression, zipcode is treated as categorical and not numeric. Thanks!</p>
  
  <pre><code>df = pd.DataFrame({'zipcode': {17384: 98125, 2680: 98107, 722: 98005, 18754: 98109, 14554: 98155}, 'bathrooms': {17384: 1.5, 2680: 0.75, 722: 3.25, 18754: 1.0, 14554: 2.5}, 'sqft_lot': {17384: 1650, 2680: 3700, 722: 51836, 18754: 2640, 14554: 9603}, 'bedrooms': {17384: 2, 2680: 2, 722: 4, 18754: 2, 14554: 4}, 'sqft_living': {17384: 1430, 2680: 1440, 722: 4670, 18754: 1130, 14554: 3180}, 'floors': {17384: 3.0, 2680: 1.0, 722: 2.0, 18754: 1.0, 14554: 2.0}})
  print (df)
         bathrooms  bedrooms  floors  sqft_living  sqft_lot  zipcode
  722         3.25         4     2.0         4670     51836    98005
  2680        0.75         2     1.0         1440      3700    98107
  14554       2.50         4     2.0         3180      9603    98155
  17384       1.50         2     3.0         1430      1650    98125
  18754       1.00         2     1.0         1130      2640    98109
  </code></pre>