[R] Formula in a data-frame

[Raoni Rodrigues]

Hello all,

I'm new in R, and I have a data-frame like this (dput information below):

Specie           Fooditem Occurrence Volume
1  Schizodon            vegetal          1   0.05
2  Schizodon           sediment          1   0.60
3  Schizodon            vegetal          1   0.15
4  Schizodon               alga          1   0.05
5  Schizodon           sediment          1   0.90
6  Schizodon           sediment          1   0.30
7  Schizodon           sediment          1   0.90
8   Astyanax terrestrial_insect          1   0.10
9   Astyanax            vegetal          1   0.85
10  Astyanax   aquatical_insect          1   0.05
11  Astyanax            vegetal          1   0.90
12  Astyanax          un_insect          1   0.85


for each specie, I have to calculate a food item importance index, that is:

Fi x Vi / Sum (Fi x Vi)

Fi  = percentual frequency of occurrence of a food item
Vi = percentual volume of a food item

So, using ddply (plyr) function, I was able to calculate the total
frequency of occurrence and total volume of each food item, using:

Frequency = ddply (dieta, c('Specie','Fooditem') , summarise,
Frequency = sum (Occurrence))

Volume = ddply (dieta, c('Specie','Fooditem') , summarise, Volume =
sum (Volume))

and calculate total frequency and total volume for a given specie:

TFrequency = ddply (Frequency, 'Specie' , summarise, TF = sum (Frequency))

TVolume = ddply (dieta, c('Specie') , summarise, Volume = sum (Volume))

but once they have different length, I could not use together to
create a percentage needed in my formula.

Any suggestions?

Thanks in advanced for help and attention,

Raoni

dput (diet)

structure(list(Specie = structure(c(2L, 2L, 2L, 2L, 2L, 2L, 2L,
1L, 1L, 1L, 1L, 1L), .Label = c("Astyanax", "Schizodon"), class = "factor"),
    Fooditem = structure(c(6L, 3L, 6L, 1L, 3L, 3L, 3L, 4L, 6L,
    2L, 6L, 5L), .Label = c("alga", "aquatical_insect", "sediment",
    "terrestrial_insect", "un_insect", "vegetal"), class = "factor"),
    Occurrence = c(1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,
    1L), Volume = c(0.05, 0.6, 0.15, 0.05, 0.9, 0.3, 0.9, 0.1,
    0.85, 0.05, 0.9, 0.85)), .Names = c("Specie", "Fooditem",
"Occurrence", "Volume"), class = "data.frame", row.names = c(NA,
-12L))

sessionInfo()
R version 2.15.1 (2012-06-22)
Platform: i386-pc-mingw32/i386 (32-bit)
Windows XP
-- 
Raoni Rosa Rodrigues
Research Associate of Fish Transposition Center CTPeixes
Universidade Federal de Minas Gerais - UFMG
Brasil
rodrigues.raoni at gmail.com