title: Pandas dataframe lookup a value in different dataframe and assign a value
body: <p>I have 2 different dataframes.
  The first one looks like:</p>
  
  <pre><code>     joint  label     x    z      y    pt
  0        1    NaN  50.4  0.0  -8.40    10
  1        2  shell  52.2  0.0  -8.40    20
  2        3  shell  54.0  0.0  -8.40    30
  3        4  shell  55.8  0.0  -8.40    40
  4        5  shell  57.6  0.0  -8.40    50
  </code></pre>
  
  <p>and my second dataframe looks like:</p>
  
  <pre><code>     member  joint1  joint2        joint1_pt        joint2_pt
  0         1       1       2                0                0
  1         2       2       3                0                0
  2         3       3       4                0                0
  3         4       4       5                0                0
  </code></pre>
  
  <p>I would like take use the <strong><em>pt value</em></strong> that corresponds on a specific jointe and use it on the second dataframe so it will look like the following:</p>
  
  <pre><code>     member  joint1  joint2        joint1_pt        joint2_pt
  0         1       1       2                10              20
  1         2       2       3                20              30
  2         3       3       4                30              40
  3         4       4       5                40              50
  </code></pre>
  
  <p>can you please help me with an example/idear on how should i approach this?
  Thank you in advance!!</p>