[R] Merge dataframes

[Uwe Ligges]

On 07.10.2011 15:34, jdanielnd wrote:
> Hello,
>
> I am having some problems to use the 'merge' function. I'm not sure if I got
> its working right.
>
> What I want to do is:
>
> 1) Suppose I have a dataframe like:
>
>          height           width
> 1        1.1                2.3
> 2        2.1                2.5
> 3        1.8                1.9
> 4        1.6                2.1
> 5        1.8                2.4
>
> 2) And I generate a second dataframe sampled from this one, like:
>
>          height           width
> 1        1.1                2.3
> 3        1.8                1.9
> 5        1.8                2.4
>
> 3) Next, I add a new variable from this dataframe:
>
>          height            width         color
> 1        1.1                2.3            red
> 3        1.8                1.9            red
> 5        1.8                2.4            blue
>
> 4) So, I want to merge those dataframes, so that the new variable, color, is
> binded to the first dataframe. Of course some cases won't have value for it,
> since I generated this variable in a smaller dataframe. In those cases I
> want the value to be NA. The result dataframe should be:
>
>          height            width         color
> 1        1.1                2.3            red
> 2        2.1                2.5            NA
> 3        1.8                1.9            red
> 4        1.6                2.1            NA
> 5        1.8                2.4            blue
>
> I have written some codes, but they're not working properly. The new
> variable has its values mixed up, and they do not correspond to its
> row.names.
>
> # Generate the first dataframe
> data1<- data.frame(height=rnorm(20,3,0.2),width=rnorm(20,2,0.5))
> # Sample a smaller dataframe from data1
> data2<- data1[sample(1:20,15,replace=F),]
> # Generate the new variable
> color<- sample(c("red","blue"),15,replace=T)
> # Bind the new variable to data2
> data2<- cbind(data2, color)
> # Merge the data1 and data2$color by row.names, and force it to has the same
> values that data1. Next it generates a new dataframe where column 1 is the
> row.name, and then sort it by the row.name from data1.
> data.frame(merge(data1,data2$color, by=0,
> all.x=T),row.names=1)[row.names(data1),]
>
> I'm not sure what am I doing wrong. Can anyone see where the mistake is?

Just let merge do the work and prepend the rownames:

cbind(names = rownames(data1), merge(data1, data2, all.x = TRUE, 
sort=FALSE))


Uwe Ligges


>
> Thank you!
>
> Cheers,
>
> Joao D.
>
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