File1 - Data20100131.csv
stock1,10.2
stock2,9.5
stock3,34.7

File2 - Data20100228.csv
stock1,10.5
stock2,13.5
stock3,23.1

File3 - Data20100331.csv
stock1,11.1
stock2,9.2
stock3,44.6

##</code><code>##

{
  20100228 => 13.5,
  20100331 => 9.2,
}