title: pandas get the row-wise minimum value of two or more columns
body: <p>How can I reference the minimum value of two dataframes as part of a pandas dataframe equation? I tried using the python <code>min()</code> function which did not work. I'm sorry if this is well-documented somewhere but I have not been able to find a working solution for this problem. I am looking for something along the lines of this:</p>
  <pre><code>data['eff'] = pd.DataFrame([data['flow_h'], data['flow_c']]).min() *Cp* (data[' Thi'] - data[' Tci'])
  </code></pre>
  <p>I also tried to use pandas <code>min()</code> function, which is also not working.</p>
  <pre><code>min_flow = pd.DataFrame([data['flow_h'], data['flow_c']]).min()
  
  InvalidIndexError: Reindexing only valid with uniquely valued Index objects
  </code></pre>
  <p>I was confused by this error. The data columns are just numbers and a name, I wasn't sure where the index comes into play.</p>
  <pre class="lang-py prettyprint-override"><code>import pandas as pd
  import numpy as np
  
  np.random.seed(365)
  rows = 10
  flow = {'flow_c': [np.random.randint(100) for _ in range(rows)],
          'flow_d': [np.random.randint(100) for _ in range(rows)],
          'flow_h': [np.random.randint(100) for _ in range(rows)]}
  data = pd.DataFrame(flow)
  
  # display(data)
     flow_c  flow_d  flow_h
  0      82      36      43
  1      52      48      12
  2      33      28      77
  3      91      99      11
  4      44      95      27
  5       5      94      64
  6      98       3      88
  7      73      39      92
  8      26      39      62
  9      56      74      50
  </code></pre>