R help - Jul 2008 - build matrix with the content of one column of a data frame in function of two factors

Hello,

First, thanks for your help (and sorry for my english !)

I have a data frame in which each row represents a vote (in percent, only
20,40, 60,80,100) of one person on one content, with three columns : name
(the name of the voters), content_id, vote :

str(votesredac)
'data.frame':	1000 obs. of  3 variables:
 $ name      : chr  "Guillemette Faure" "Guillemette Faure"
"Guillemette
Faure" "Pascal Rich\xe9" ...
 $ content_id: num  385241 384926 384635 383266 384814 ...
 $ value     : num  100 100 100 20 100 100 20 100 100 100 ...

I want to build a matrix with one column for each content and one line for
each user, containing the votes. This matrix can content NAs (each person
hasn't voted on all contents). If each row and column was labelled, would be
better.


Thanks again !
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Seems that the following makes what I want :

attach(votesredac)
tapply(value, list(name, content_id), mean)

Only thing is, I don't need to make a mean - there is only one or no value.



VinceD wrote:
> 
> Hello,
> 
> First, thanks for your help (and sorry for my english !)
> 
> I have a data frame in which each row represents a vote (in percent, only
> 20,40, 60,80,100) of one person on one content, with three columns : name
> (the name of the voters), content_id, vote :
> 
> str(votesredac)
> 'data.frame':	1000 obs. of  3 variables:
>  $ name      : chr  "Guillemette Faure" "Guillemette
Faure" "Guillemette
> Faure" "Pascal Rich\xe9" ...
>  $ content_id: num  385241 384926 384635 383266 384814 ...
>  $ value     : num  100 100 100 20 100 100 20 100 100 100 ...
> 
> I want to build a matrix with one column for each content and one line for
> each user, containing the votes. This matrix can content NAs (each person
> hasn't voted on all contents). If each row and column was labelled,
would
> be better.
> 
> 
> Thanks again !
> 
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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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