user3570187 - 1 year ago 100

R Question

I have data in the following form and i would like to create a matrix from this data.

`B<- c('nancy','bill','bob','badri','bill')`

c<- c('martial-arts','dance','sports','judo','judo')

df<- data.frame(B,C)

I want to create a matrix who belongs to which group and with user as row.names. Can anyone have any suggestions?

`user martial-arts dance sports judo`

nancy 1 0 0 0

bill 0 1 0 1

bob 0 0 1 0

badri 0 0 0 1

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Answer Source

Maybe something like this:

```
x <- c('nancy','bill','bob','badri','bill')
y <- c('martial-arts','dance','sports','judo','judo')
x0 <- unique(x); y0 <- unique(y)
mat <- matrix(0L, length(x0), length(y0), dimnames = list(x0, y0))
mat[cbind(match(x, x0), match(y, y0))] <- 1L
# martial-arts dance sports judo
#nancy 1 0 0 0
#bill 0 1 0 1
#bob 0 0 1 0
#badri 0 0 0 1
```

I have used matrix indexing:

`match(x, x0)`

gives row index;`match(y, y0)`

gives column index;`cbind(match(x, x0), match(y, y0))`

gives matrix index where 1 is.

If you have an idea that the resulting matrix has a lot more zeros than ones, you may construct a sparse matrix:

```
library(Matrix)
sparseMatrix(i = match(x, x0), j = match(y, y0), x = 1, dimnames = list(x0, y0))
#4 x 4 sparse Matrix of class "dgCMatrix"
# martial-arts dance sports judo
#nancy 1 . . .
#bill . 1 . 1
#bob . . 1 .
#badri . . . 1
```

I like @thelatemail's alternative:

```
xtabs(~ x + y)
xtabs(~ x + y, sparse = TRUE)
```

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