# by.ppp

##### Apply a Function to a Point Pattern Broken Down by Factor

Splits a point pattern into sub-patterns, and applies the function to each sub-pattern.

##### Usage

```
# S3 method for ppp
by(data, INDICES=marks(data), FUN, ...)
```

##### Arguments

- data
Point pattern (object of class

`"ppp"`

).- INDICES
Grouping variable. Either a factor, a pixel image with factor values, or a tessellation.

- FUN
Function to be applied to subsets of

`data`

.- …
Additional arguments to

`FUN`

.

##### Details

This is a method for the generic function `by`

for point patterns (class `"ppp"`

).

The point pattern `data`

is first divided into subsets
according to `INDICES`

. Then the function `FUN`

is applied to each subset. The results of each computation are
returned in a list.

The argument `INDICES`

may be

a factor, of length equal to the number of points in

`data`

. The levels of`INDICES`

determine the destination of each point in`data`

. The`i`

th point of`data`

will be placed in the sub-pattern`split.ppp(data)$l`

where`l = f[i]`

.a pixel image (object of class

`"im"`

) with factor values. The pixel value of`INDICES`

at each point of`data`

will be used as the classifying variable.a tessellation (object of class

`"tess"`

). Each point of`data`

will be classified according to the tile of the tessellation into which it falls.

If `INDICES`

is missing, then `data`

must be a multitype point pattern
(a marked point pattern whose marks vector is a factor).
Then the effect is that the points of each type
are separated into different point patterns.

##### Value

A list (also of class `"anylist"`

or `"solist"`

as
appropriate) containing the results returned
from `FUN`

for each of the subpatterns.

##### See Also

##### Examples

```
# NOT RUN {
# multitype point pattern, broken down by type
data(amacrine)
by(amacrine, FUN=density)
by(amacrine, FUN=function(x) { min(nndist(x)) } )
# how to pass additional arguments to FUN
by(amacrine, FUN=clarkevans, correction=c("Donnelly","cdf"))
# point pattern broken down by tessellation
data(swedishpines)
tes <- quadrats(swedishpines, 5, 5)
B <- by(swedishpines, tes, clarkevans, correction="Donnelly")
unlist(lapply(B, as.numeric))
# }
```

*Documentation reproduced from package spatstat, version 1.55-1, License: GPL (>= 2)*