[R] replace matrix values with names from a dataframe

[Spencer Graves]

	  Does the following help:

 > set.seed(1)
 > Lvls <- factor(letters[1:4])
 > A <- array(sample(4, 6, replace=TRUE), dim=c(2,3))
 > A[] <- levels(Lvls)[A]
 > A
      [,1] [,2] [,3]
[1,] "b"  "c"  "a"
[2,] "b"  "d"  "d"
 >

	  If not, PLEASE do read the posting guide! 
"http://www.R-project.org/posting-guide.html" and submit another 
question (if the process of working the posting guide does not itself 
provide enlightenment).

	  spencer graves

jhainm at fas.harvard.edu wrote:

> Hi,
> 
> I am looking for a way to replace matrix values with names from a dataframe.
> 
> Let me do this by example: I have a dataframe:
> 
> 
>>data
> 
>   city.name
> 1    munich
> 2     paris
> 3     tokio
> 4    london
> 5    boston
> 
> each city name corresponds to only one index number (there is only one
> observation for each city). After doing some matching I end up with a matrix
> that looks something like this:
> 
> 
>>X
> 
>        [,1] [,2]
>   [1,]    2    4
>   [2,]    5    1
>   [3,]    5    3
>   [4,]   12  217
>   [5,]   16   13
> 
> Here the numbers in the matrix are the index numbers from my original dataset,
> each row is a matched pair (so e.g. the first row tells me that obs. number 2
> (i.e. Paris) was matched to obs number 4 (i.e. London)).
> 
> Now I am looking for a quick way to transform the index numbers back to city
> names, so that at the end I have a matrix that looks something like this:
> 
> 
>>X.transformed
> 
>          [,1]     [,2]
>   [1,]  paris   london
>   [2,] boston   munich
>   [3,] boston    tokio
>   [4,]     12      217
>   [5,]     16       13
> 
> etc. So instead of the index number, the matrix should contain the names that
> corresponds to it. In my real data, I have many many names and replacing each
> value by hand would take too long. Any help is highly appreciated.
> 
> Thank you.
> 
> Regards,
> Jens
> 
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