title: Proportional Tables by Group
body: <p>I have a data.frame structured as follows:</p>
  
  <pre><code>location               gender        15.19     20.30      31.40      41.64      65.
  New York                Female          2         41         13         19        1
  New York                  Male          1         23         15         17        2
  San Francisco           Female          1         27         14         14        3
  San Francisco             Male          4         24         14         10        1
  Mexico City             Female          1         40         26         11        3
  Mexico City               Male          4         23         35          8        3
  Paris                   Female          2         12         10          6        0
  Paris                     Male          1         20         13         11        1
  </code></pre>
  
  <p>...and need to turn it into a proportion table with each cell representing its proportion of the two rows for its given city. <a href="https://stackoverflow.com/questions/24576515/relative-frequencies-proportions-with-dplyr">This</a> is one solution but is there an easier way to do it for multiple columns (and just transform them without making new columns?)</p>
  
  <p>EDIT
  Correct output would give each cell as a proportion of ALL cells within that city, so that all cells that share the location 'New York' would add up to 1, as would all cells that share location 'San Francisco', etc. i.e. :</p>
  
  <pre><code> location             gender        15.19     20.30      31.40      41.64        65.
   New York             Female          .01       .31        .1         .14        .01
   New York               Male          .01       .17       .11         .13        .02
  </code></pre>