[R] Correlation matrix for data in long format

[Serguei Kaniovski]

Hello,

I cannot figure out how to use "tapply" to compute the correlation matrix 
in the variable "x" between the states? The data is in long format, e.g.:

state,year,x
Alabama,2001,0.45
Alabama,2002,0.47
Alabama,2003,0.48
Alabama,2004,0.44
Arizona,2001,0.34
Arizona,2002,0.32
Arizona,2003,0.38
Arizona,2004,0.36

Thank you in advance for your help,
Serguei Kaniovski