Function for certain values in rows

I have a paneldata which looks like:

(Only th开发者_如何转开发e substantially cutting for my question)

Persno 122 122 122 333 333 333 333 333 444 444 
Income 1500 1500 2000 2000 2100 2500 2500 1500 2000 2200
year 1 2 3 1 2 3 4 5 1 2

I need a command or function to recognize the different Person. For all rows with the same Person I would like to give out the average income.

Thank you very much.

My favorite tool to solve problems like this is ddply, in the plyr package.

 library(plyr)
 p = pdata.frame(data.frame(year=rep(c(1,2,3),3), persno = c(1,1,1,2,2,2,3,3,3), income=c(1500,1500,2000,2000,2100,2500,2500,1500,2000)))
 dply(p, .(persno), summarize, mean.income = mean(income))

which gives us the output

  persno mean.income
  1      1    1666.667
  2      2    2200.000
  3      3    2000.000