Operations on a big data frame

Hi,
May be this helps:
dat1<- read.table(text="
?? P1_prom Nom
1 -6.17 Pt_00187
2 -6.17 Pt_00187
3 -6.17 Pt_00187
4 -6.17 Pt_00187
5 -6.17 Pt_00187
6 -6.17 Pt_01418
7 -5.77 Pt_01418
8 -5.37 Pt_01418
9 -4.97 Pt_01418
10 -4.57 Pt_01418
",sep="",header=TRUE,stringsAsFactors=FALSE) 

library(zoo)
?dat1$PT_promMean<-rollmean(dat1$P1_prom,5,fill=NA,align="left")
?dat1
#?? P1_prom????? Nom PT_promMean
#1??? -6.17 Pt_00187?????? -6.17
#2??? -6.17 Pt_00187?????? -6.17
#3??? -6.17 Pt_00187?????? -6.09
#4??? -6.17 Pt_00187?????? -5.93
#5??? -6.17 Pt_00187?????? -5.69
#6??? -6.17 Pt_01418?????? -5.37
#7??? -5.77 Pt_01418????????? NA
#8??? -5.37 Pt_01418????????? NA
#9??? -4.97 Pt_01418????????? NA
#10?? -4.57 Pt_01418????????? NA
A.K.


Hello all, 

I have a big data frame that looks like this: 
? ? ? ? P1_prom	Nom 
1	-6.17	Pt_00187 
2	-6.17	Pt_00187 
3	-6.17	Pt_00187 
4	-6.17	Pt_00187 
5	-6.17	Pt_00187 
6	-6.17	Pt_01418 
7	-5.77	Pt_01418 
8	-5.37	Pt_01418 
9	-4.97	Pt_01418 
10	-4.57	Pt_01418 
- 
- 
- 
25000 

where Nom represents a point in a map, and P1_prom represents 
the value of an operation we perfomed on each point (note that we 
performed 5 repetitions for each point, hence, each point has 5 values). 
What I am trying to do, with no success, is to create a new column, 
in which each row corresponds to the mean value of P1_prom for each 
point. So basically what I need the program to do is to write in the 
first row of the new column the average of the first five values of 
P1_prom, in the second row the average of the next five values, and so 
on. 
Could anybody guide me on how to do this. 
Thank you very much, 
Veronica