title: R conditional sum in data frame depending on word in a column
body: <p>I have a data frame containing words and numeric entries. I want to sum all the entries for which the row entry in the word now is identical. </p>
  
  <pre><code>District name   Population   Child birth rate
  A               30,000       .7
  A               20,000       .5
  B               10,000       .09
  B               15,000       .6
  C               80,000       .007
  </code></pre>
  
  <p>I want to sum up the population and child birth rates on the district level.
  I tried using lapply and sum, but I can't figure it out.</p>
  
  <p>The result to dput(head(mydata) is: </p>
  
  <pre><code>structure(list(District = structure(c(5L, 5L, 5L, 5L, 5L, 5L), .Label =         c("Charlottenburg-Wilmersdorf", 
  "Friedrichshain-Kreuzberg", "Lichtenberg", "Marzahn-Hellersdorf", 
  "Mitte", "Neukoelln", "Pankow", "Reinickendorf", "Spandau", "Steglitz-Zehlendorf", 
  "Tempelhof-Schoeneberg", "Treptow-Koepenick"), class = "factor"), 
  Population = c(81205L, 70911L, 5629L, 12328L, 78290L, 84789L
  ), Overall.crime = c(27864L, 13181L, 943L, 4515L, 15673L, 
  16350L), Robbery = c(315L, 195L, 20L, 79L, 232L, 261L), Mugging = c(183L, 
  81L, 9L, 54L, 111L, 118L), Assault = c(2016L, 1046L, 51L, 
  468L, 1679L, 1718L), Molestation.Stalking = c(480L, 429L, 
  16L, 114L, 567L, 601L), Theft = c(13587L, 4961L, 396L, 2019L, 
  6725L, 6954L), Car.Theft = c(185L, 149L, 10L, 28L, 159L, 
  159L), Bycicle.Theft = c(1444L, 561L, 95L, 123L, 588L, 595L
  ), Burglary = c(557L, 297L, 37L, 87L, 397L, 528L), Arson = c(36L, 
  51L, 7L, 15L, 28L, 56L), Property.Damage = c(2113L, 871L, 
  64L, 260L, 1257L, 1172L), Drug.Offenses = c(781L, 538L, 24L, 
  87L, 604L, 492L)), .Names = c("District", "Population", "Overall.crime", 
  "Robbery", "Mugging", "Assault", "Molestation.Stalking", "Theft", 
  "Car.Theft", "Bycicle.Theft", "Burglary", "Arson", "Property.Damage", 
  "Drug.Offenses"), row.names = c(NA, 6L), class = "data.frame")
  </code></pre>
  
  <p>I had spared you all those German names before, but I guess that was stupid since the problem is within the data... </p>
  
  <p>Using ddply gives me following error:</p>
  
  <pre><code>Error in df$Population : object of type 'closure' is not subsettable
  </code></pre>
  
  <p>Thank you for any help!            </p>