[R] "for" loop assistance -

[Steven Ranney]

All –

I have an example data frame

x	lc1	id
43.38812035	85	ga1
47.55710661	85	ga1
47.55710661	85	ga2
47.55710661	85	ga2
51.99211429	85	ga3
51.99211429	85	ga3
51.99211429	95	ga1
54.78449958	95	ga1
54.78449958	95	ga2
54.78449958	95	ga2
56.70201864	95	ga3
56.70201864	95	ga3
56.70201864	105	ga1
59.66361903	105	ga1
59.66361903	105	ga2
61.69573564	105	ga2
61.69573564	105	ga3
61.69573564	105	ga3
63.77469479	115	ga1
63.77469479	115	ga1
64.83191994	115	ga2
64.83191994	115	ga2
64.83191994	115	ga3
66.98222118	115	ga3
66.98222118	125	ga1
66.98222118	125	ga1
66.98222118	125	ga2
66.98222118	125	ga2
66.98222118	125	ga3
66.98222118	125	ga3

and I’m trying to extract means for every lc1 and id level so that I
would have a data frame that looks like

x.m	lc1	id
x.1	085	ga1
x.2	085	ga2
x.3	085	ga3
x.4	095	ga1
x.5	095	ga2
x.6	095	ga3
x.7	105	ga1
x.8	105	ga2
x.9	105	ga3
x.10	115	ga1
x.11	115	ga2
x.12	115	ga3
x.13	125	ga1
x.14	125	ga2
x.15	125	ga3

Now, I can use brute force to get the second data frame by

write.table(tapply(x[lc1=="085"], id[lc1=="085"], mean), "file.xls", sep=",")
write.table(tapply(x[lc1=="095"], id[lc1=="095"], mean), “file.xls",
sep=",", append=T)
write.table(tapply(x[lc1=="105"], id[lc1=="105"], mean), “file.xls",
sep=",", append=T)
etc…

and add the values for the lc1 column in the .xls file until I’ve
worked my way through every level for lc1 then read the file back into
R, but this would require a great deal of my time.  (I have 72 levels
for lc1 and 346 levels for id totalling over 20,000 lines.)

I am confident that there is a simple, more elegant solution available
to me that I am overlooking.  I am sure that I could use a “for” loop,
but as someone that is new to R programming, I am unsure of how to go
about creating the for loop to build the second data frame.  I've
tried to modify existing for loops that I already have but have been
unsuccessful.

Do you have any suggestions?

Thank you –

Steven Ranney