So the solution is : tapply(content, list(factor1, factor2), mean)
An example of what it does :
> my.data
name item vote
1 Ricardo Coke 20
2 Ricardo Fanta 60
3 Ricardo Pepsi 100
4 Marie Pepsi 40
5 Marie Coke 60
6 Julia Fanta 60
7 Julia Coke 100
> attach(my.data)
> tapply(vote, list(name, item), mean) -> tastes
> tastes
Coke Fanta Pepsi
Julia 100 60 NA
Marie 60 NA 40
Ricardo 20 60 100
And then, you can compute the distance between people if you want :
> dist(tastes, diag = T)
Julia Marie Ricardo
Julia 0.00000
Marie 69.28203 0.00000
Ricardo 97.97959 88.31761 0.00000
That's it !
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