MalteseUnderdog - 9 months ago 64

R Question

I have a

`data.frame`

`x a 1`

x b 2

x c 3

y a 3

y b 3

y c 2

I want this in matrix form so I can feed it to heatmap to make a plot. The result should look something like:

`a b c`

x 1 2 3

y 3 3 2

I have tried

`cast`

Answer

There are many ways to do this. This answer starts with my favorite ways, but also collects various ways from answers to similar questions scattered around this site.

```
tmp <- data.frame(x=gl(2,3, labels=letters[24:25]),
y=gl(3,1,6, labels=letters[1:3]),
z=c(1,2,3,3,3,2))
```

Using reshape2:

```
library(reshape2)
acast(tmp, x~y, value.var="z")
```

Using matrix indexing:

```
with(tmp, {
out <- matrix(nrow=nlevels(x), ncol=nlevels(y),
dimnames=list(levels(x), levels(y)))
out[cbind(x, y)] <- z
out
})
```

Using `xtabs`

:

```
xtabs(z~x+y, data=tmp)
```

You can also use `reshape`

, as suggested here: Convert table into matrix by column names, though you have to do a little manipulation afterwards to remove an extra columns and get the names right (not shown).

```
> reshape(tmp, idvar="x", timevar="y", direction="wide")
x z.a z.b z.c
1 x 1 2 3
4 y 3 3 2
```

There's also `sparseMatrix`

within the `Matrix`

package, as seen here: R - convert BIG table into matrix by column names

```
> with(tmp, sparseMatrix(i = as.numeric(x), j=as.numeric(y), x=z,
+ dimnames=list(levels(x), levels(y))))
2 x 3 sparse Matrix of class "dgCMatrix"
a b c
x 1 2 3
y 3 3 2
```

The `daply`

function from the `plyr`

library could also be used, as here: http://stackoverflow.com/a/7020101/210673

```
> library(plyr)
> daply(tmp, .(x, y), function(x) x$z)
y
x a b c
x 1 2 3
y 3 3 2
```

`dcast`

from reshape2 also works, as here: Reshape data for values in one column, but you get a data.frame with a column for the `x`

value.

```
> dcast(tmp, x~y, value.var="z")
x a b c
1 x 1 2 3
2 y 3 3 2
```

Similarly, `spread`

from "tidyr" would also work for such a transformation:

```
library(tidyr)
spread(tmp, y, z)
# x a b c
# 1 x 1 2 3
# 2 y 3 3 2
```