[R] coefficients lm of data.frame

[Alfredo Alessandrini]

Hi,

I've a data frame like this:

> as.data.frame(cbind(rnorm(1:12),rnorm(1:12)))
            V1          V2
1  -1.30849402 -0.52094136
2   0.96157302  0.76217871
3  -0.44223351 -1.72630871
4  -0.10432438 -1.04732942
5  -1.38748914  0.95877311
6  -0.63965975  0.65494811
7  -0.24058318  0.19496830
8  -0.11172988  1.01680655
9   0.08065333  0.22168589
10  0.25196536  0.84619914
11 -0.59536986 -0.08243074
12  1.09115054  0.49822977

I need to add two columns as result of the fitting of linear model
based on a preset numbers of row.

For example if I need to compute a lm each 4 rows, I get the
data.frame below, where intercept1 and coeff1 is obtained from V1 and
V2 of first 4 rows lm(V2 ~ V1), and so on...


      V1          V2 "intercept" "coeff"
1   0.6931694  0.05797771   intercept1   coeff1
2  -1.4069786  0.23983307   intercept1  coeff1
3  -1.4901708  0.45079601   intercept1   coeff1
4   0.2215696  1.87888983   intercept1   coeff1
5  -0.5828106  0.90376622   intercept2   coeff2
6  -0.7607985  0.71419938   intercept2   coeff2
7   0.1273495  0.06199312   intercept2   coeff2
8  -0.5612245  1.02223971   intercept2  coeff2
9  -0.1439178  0.92135354   intercept3   coeff3
10 -1.1011662  0.02894731   intercept3   coeff3
11 -0.4098710 -0.01231322   intercept3   coeff3
12  1.1511811 -0.63923140   intercept3   coeff3


Thanks in advance,

Alfredo