[R] Expanding matrix into dummies

[Bert Gunter]

... Perhaps worth noting is that the row indices can be created
directly without row():

result[cbind(rep.int(seq_len(6),5), as.vector(x))] <- 1

but the downside is that you have to know that a matrix is a vector
"stored" in column major order. I find this arcane detail quite handy,
though.

Cheers,
Bert


Bert Gunter

"The trouble with having an open mind is that people keep coming along
and sticking things into it."
-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )


On Tue, Dec 22, 2015 at 3:03 PM, Marc Schwartz <marc_schwartz at me.com> wrote:
>
>> On Dec 22, 2015, at 4:42 PM, Dimitri Liakhovitski <dimitri.liakhovitski at gmail.com> wrote:
>>
>> # I have a matrix x:
>>
>> k <- 20
>> N <- 5
>> set.seed(123)
>> x <- matrix(c(sample(1:k, N, replace = F),
>>              sample(1:k, N, replace = F),
>>              sample(1:k, N, replace = F),
>>              sample(1:k, N, replace = F),
>>              sample(1:k, N, replace = F),
>>              sample(1:k, N, replace = F)), byrow = T, ncol = 5)
>> colnames(x) <- paste0("column", 1:5)
>> (x)
>>
>> # I want to reshape it into a matrix 'result' with k columns (not N).
>> # 'result' should contain the same number of rows as x, and it should have
>> # in each row 1s in those columns that correspond to the entries in x
>> in the same row.
>> # For example, the first row of 'result' should contain 1s in columns
>> 6, 8, 15, 16, and 17, etc.
>> # The remaining entries should be zeros.
>>
>> result <- matrix(rep(0, nrow(x) * k), nrow = nrow(x))
>> colnames(result) <- paste0("item", 1:k)
>>
>> # I can see how to do it by looping through rows of result.
>> # But I need to do it fast (not using a loop).
>> # I feel like I should subset 'result' with 'x', but I am not sure how.
>>
>> Thank you very much!
>
>
> Something like this might work for you:
>
> result <- matrix(0, nrow(x), k)
>
>
>> result
>      [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10] [,11] [,12]
> [1,]    0    0    0    0    0    0    0    0    0     0     0     0
> [2,]    0    0    0    0    0    0    0    0    0     0     0     0
> [3,]    0    0    0    0    0    0    0    0    0     0     0     0
> [4,]    0    0    0    0    0    0    0    0    0     0     0     0
> [5,]    0    0    0    0    0    0    0    0    0     0     0     0
> [6,]    0    0    0    0    0    0    0    0    0     0     0     0
>      [,13] [,14] [,15] [,16] [,17] [,18] [,19] [,20]
> [1,]     0     0     0     0     0     0     0     0
> [2,]     0     0     0     0     0     0     0     0
> [3,]     0     0     0     0     0     0     0     0
> [4,]     0     0     0     0     0     0     0     0
> [5,]     0     0     0     0     0     0     0     0
> [6,]     0     0     0     0     0     0     0     0
>
>
> result[cbind(as.vector(row(x)), as.vector(x))] <- 1
>
>
>> result
>      [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10] [,11] [,12]
> [1,]    0    0    0    0    0    1    0    1    0     0     0     0
> [2,]    1    0    0    0    0    0    0    1    0     1     1     0
> [3,]    0    1    0    0    0    0    0    0    1     1     0     0
> [4,]    1    0    0    0    1    1    0    0    0     0     0     0
> [5,]    0    0    0    0    0    0    0    0    0     0     1     1
> [6,]    0    0    1    0    1    0    0    0    0     0     1     0
>      [,13] [,14] [,15] [,16] [,17] [,18] [,19] [,20]
> [1,]     0     0     1     1     1     0     0     0
> [2,]     0     0     0     0     1     0     0     0
> [3,]     1     0     0     0     0     0     0     1
> [4,]     0     0     0     1     0     1     0     0
> [5,]     0     1     0     0     1     1     0     0
> [6,]     0     0     1     0     0     0     1     0
>
>
> See ?row
>
> Regards,
>
> Marc Schwartz
>
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