title: How to conditionally set empty column values based on previous columns
body: <p>I want to insert values for col 3 as follows:</p>
  
  <pre><code>A+ if col 1 is a and col 2 is positive
  A- if col 1 is a and col 2 is negative
  B+ if col 1 is B and col 2 is positive
  B- if col 1 is B and col 2 is negative
  </code></pre>
  
  <p>Data:</p>
  
  <pre><code>    COL1  COL2  COL3
  0     a    -2
  1     a     4
  2     b     10
  3     b     -8
  4     a     10
  5     b     5
  </code></pre>
  
  <p>Therefore I want a resulting df of:</p>
  
  <pre><code>     COL1  COL2  COL3
  0     a    -2      A-
  1     a     4      A+
  2     b     10     B+
  3     b     -8     B-
  4     a     10     A+
  5     b     5      B+
  </code></pre>