title: Change values in Dataframe with function in R
body: <p>i have a data frame, which looks like this, but huge so I can't do anything manually:</p>
  
  <pre><code>   Bank  Country  KeyItem    Year    Value 
      A      AU     Income     2010     1000
      A      AU     Income     2011     1130
      A      AU     Income     2012     1160
      B      USA    Depth      2010     10000
  </code></pre>
  
  <p>What I  want to do is create a function where I can select the Bank, the Keyitem and from which year onwards and it returns a dataframe with the values as percentage of the first value . Like this:</p>
  
  <pre><code>   Bank  Country  KeyItem    Year    Value
      A      AU     Income     2010     100
      A      AU     Income     2011     113
      A      AU     Income     2012     116
  </code></pre>
  
  <p>Thank you in advance! </p>