[R] Expanding matrix into dummies

Dimitri Liakhovitski dimitri.liakhovitski at gmail.com
Wed Dec 23 15:07:18 CET 2015

```Thank you very much, Marc and Bert - this is great!

On Tue, Dec 22, 2015 at 7:14 PM, Bert Gunter <bgunter.4567 at gmail.com> wrote:
> ... 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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