# nnwhich.pp3

##### Nearest neighbours in three dimensions

Finds the nearest neighbour of each point in a three-dimensional point pattern.

##### Usage

```
## S3 method for class 'pp3':
nnwhich(X, \dots, k=1)
```

##### Arguments

- X
- Three-dimensional point pattern
(object of class
`"pp3"`

). - ...
- Ignored.
- k
- Integer, or integer vector. The algorithm will compute the distance to the
`k`

th nearest neighbour.

##### Details

For each point in the given three-dimensional
point pattern, this function finds
its nearest neighbour (the nearest other point of the pattern).
By default it returns a vector giving, for each point,
the index of the point's
nearest neighbour. If `k`

is specified, the algorithm finds
each point's `k`

th nearest neighbour.

The function `nnwhich`

is generic. This is the method
for the class `"pp3"`

.

If there are no points in the pattern,
a numeric vector of length zero is returned.
If there is only one point,
then the nearest neighbour is undefined, and a value of `NA`

is returned. In general if the number of points is less than or equal
to `k`

, then a vector of `NA`

's is returned.

To evaluate the *distance* between a point and its nearest
neighbour, use `nndist`

.

To find the nearest neighbours from one point pattern
to another point pattern, use `nncross`

.

##### Value

- Numeric vector or matrix giving, for each point,
the index of its nearest neighbour (or
`k`

th nearest neighbour).If

`k = 1`

(the default), the return value is a numeric vector`v`

giving the indices of the nearest neighbours (the nearest neighbout of the`i`

th point is the`j`

th point where`j = v[i]`

). If`k`

is a single integer, then the return value is a numeric vector giving the indices of the`k`

th nearest neighbours.If

`k`

is a vector, then the return value is a matrix`m`

such that`m[i,j]`

is the index of the`k[j]`

th nearest neighbour for the`i`

th data point.

##### Warnings

A value of `NA`

is returned if there is only one point
in the point pattern.

##### See Also

##### Examples

```
X <- runifpoint3(30)
m <- nnwhich(X)
m2 <- nnwhich(X, k=2)
```

*Documentation reproduced from package spatstat, version 1.21-0, License: GPL (>= 2)*