[R] indexing into a data.frame using another data.frame that also contains values for replacement

[Berend Hasselman]

Alice Wines wrote:
> 
> Hello all,
> 
>      I have a quandry I have been scratching my head about for a
> while. I've searched the manual and the web and have not been able to
> find an acceptable result, so I am hoping for some help.
> 
>      I have two data frames and I want to index into the first using
> the second, and replace the specific values I have indexed with more
> values from the second data.frame. I can do this using a loop, but I
> wanted a quicker solution with no loops involved.
> 
> Although my data set is much larger than this, a small example of what
> I am trying to do is as follows:
> 
> df1 <- data.frame(rows=c("A","B","C", "B", "C", "A"),
> columns=c("21_2", "22_2", "23_2", "21_2", "22_2", "23_2"),
> values=c(3.3, 2.5, 67.2, 44.3, 53, 66))
> df2 <- data.frame(matrix(rep(NA, length(df1$values)),nrow=3, ncol=3))
> names(df2) <- c("21_2", "22_2", "23_2")
> row.names(df2) <- c("A", "B", "C")
> 
>> df1
>   rows columns values
> 1    A    21_2    3.3
> 2    B    22_2    2.5
> 3    C    23_2   67.2
> 4    B    21_2   44.3
> 5    C    22_2   53.0
> 6    A    23_2   66.0
> 
> .......
> 
> What I want to see is the following:
> 
>>df3
>     21_2   22_2  23_2
> A   3.3    NA    66.0
> B   44.3  2.5    NA
> C   NA   53.0   67.2
> 

How about this (I have converted df2 into a matrix)

df2 <- matrix(rep(NA, length(df1$values)),nrow=3, ncol=3)
colnames(df2) <- c("21_2", "22_2", "23_2")
rownames(df2) <- c("A", "B", "C")

df2[cbind(df1$rows,df1$columns)] <- df1$values

# convert df2 to a data.frame

Berend--
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