[R] Help with factors

[Serguei Kaniovski]

Hallo All,

I have difficulties understanding how factors work in R. Suppose a have 
data in the panel form below. I would to compute a correlation coefficient 
(actually apply a different function of two time series) in the V variable 
between members of the two sexes in each city over time. How can this be 
done?

Thank you in advance,
Serguei

city, year, sex, V
1, 1975, 1, 25.3044
1, 1975, 0, 16.5711
1, 1976, 0, 16.6072
1, 1976, 1, 24.2841
1, 1977, 0, 14.8838
1, 1977, 1, 24.8124
1, 1978, 1, 23.0570
1, 1978, 0, 14.5627
1, 1979, 1, 21.2071
1, 1979, 0, 13.5277
2, 1975, 1, 62.4457
2, 1975, 0, 26.9745
2, 1976, 1, 67.3025
2, 1976, 0, 31.4600
2, 1977, 1, 53.0577
2, 1977, 0, 25.1941
2, 1978, 0, 23.3694
2, 1978, 1, 40.1452
2, 1979, 1, 44.5686
2, 1979, 0, 23.4042