title: product bought by how many unique users?
body: <p>i have a dataset like this</p>
  
  <pre><code>users                                     b           c       d      product
  8fa683e59c02c04cb781ac689686db07    ggstart 1.46276E+12 00:00.0 55107008    
  335644267c1d5f04eaea7bc6f51b1861    ggstart 1.46276E+12 00:00.0 55107008    
  ca3071aad676bc963795a2b09635cdf0    ggstop  1.46277E+12 00:00.0 55107008    
  17412dec7d3d02c9b0b1c3d1c3571c5c    ggstop  1.46276E+12 00:00.0 10655437    
  f81167c854f1a0c86cab6188f9995824    ggstart 1.46276E+12 00:00.1 55107008    
  17412dec7d3d02c9b0b1c3d1c3571c5c    ggstart 1.46276E+12 00:00.1 10655437    
  a2659df45c8d05f326225fa5b1063ac9    ggstart 1.46276E+12 00:00.1 30900473    
  b8bbef76f8dfee2fe190a283cd5a19a7    ggstart 1.46276E+12 00:00.1 18121481    
  e8ebfc3f39512eda3aa0702b13ffed63    ggstart 1.46276E+12 00:00.1 18121481    
  988e4873861347113519fbee6dd1c3b0    ggstart 1.46276E+12 00:00.2 55107008    
  583361d66ad8b0827cd08d3a5d64af89    ggstop  1.46276E+12 00:00.2 55107008    
  </code></pre>
  
  <p>users, b,c,product are columns. </p>
  
  <p>i have to determine  each product is bought by how many unique users. there are thousands of lines like this.
  Keep in mind:</p>
  
  <pre><code>there can be many users buying the same product,
  each customer have more than one product bought
  </code></pre>
  
  <p>first i tried to make matrix of unique users and unique PRODUCT. BUT i have only 16 product and users are like 5000.
  is there any approach i can find </p>