[R] Data frame manipulation - newbie question

[José Augusto M. de Andrade Junior]

Hi all,

Could someone please explain how can i efficientily query a data frame
with several factors, as shown below:

---------------------------------------------------------------------------------------------------------
Data frame: pt.knn
---------------------------------------------------------------------------------------------------------
row | k.idx   |   step.forwd  |  pt.num |   model |   prev  |  value
|  abs.error
1      200        0                  1             lm          09
10.5       1.5
2      200        0                  2             lm          11
10.5       1.5
3      201        1                  1             lm          10
12          2.0
4      201        1                  2             lm          12
12          2.0
5      202        2                  1             lm          12
12.1       0.1
6      202        2                  2             lm          12
12.1       0.1
7      200        0                  1             rlm         10.1
10.5       0.4
8      200        0                  2             rlm         10.3
10.5       0.2
9      201        1                  1             rlm         11.6
12          0.4
10    201        1                  2             rlm         11.4
12          0.6
11    202        2                  1             rlm         11.8
12.1       0.1
12    202        2                  2             rlm         11.9
12.1       0.2
----------------------------------------------------------------------------------------------------------

k.idx, step.forwd, pt.num and model columns are FACTORS.
prev, value, abs.error are numeric

I need to take the mean value of the numeric columns  (prev, value and
abs.error) for each k.idx and step.forwd and model. So: rows 1 and 2,
3 and 4, 5 and 6,7 and 8, 9 and 10, 11 and 12 must be grouped
together.

Next, i need to plot a boxplot of the mean(abs.error) of each model
for each k.idx.
I need to compare the abs.error of the two models for each step and
the mean overall abs.error of each model. And so on.

I read the manuals, but the examples there are too simple. I know how
to do this manipulation in a "brute force" manner, but i wish to learn
how to work the right way with R.

Could someone help me?
Thanks in advance.

José Augusto
Undergraduate student
University of São Paulo
Business Administration Faculty