---
title: Detecting all neighbors within range
author: 
- name: Aaron Lun
  affiliation: Cancer Research UK Cambridge Institute, Cambridge, United Kingdom
date: "Revised: 28 September 2018"
output:
  BiocStyle::html_document:
    toc_float: true
package: BiocNeighbors 
vignette: >
  %\VignetteIndexEntry{3. Detecting neighbors within range}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}    
bibliography: ref.bib  
---

```{r, echo=FALSE, results="hide", message=FALSE}
require(knitr)
opts_chunk$set(error=FALSE, message=FALSE, warning=FALSE)
library(BiocNeighbors)
```

# Identifying all neighbors within range

Another application of the KMKNN or VP tree algorithms is to identify all neighboring points within a certain (Euclidean) distance of the current point.
We first mock up some data:

```{r}
nobs <- 10000
ndim <- 20
data <- matrix(runif(nobs*ndim), ncol=ndim)
```

We apply the `findNeighbors()` function to `data`:

```{r}
fout <- findNeighbors(data, threshold=1)
head(fout$index)
head(fout$distance)
```

Each entry of the `index` list corresponds to a point in `data` and contains the row indices in `data` that are within `threshold`.
For example, the 3rd point in `data` has the following neighbors:

```{r}
fout$index[[3]]
```

... with the following distances to those neighbors:

```{r}
fout$distance[[3]]
```

Note that, for this function, the reported neighbors are _not_ sorted by distance.
The order of the output is completely arbitrary and will vary depending on the random seed.
However, the identity of the neighbors is fully deterministic.

# Querying another data set for neighbors 

The `queryNeighbors()` function is also provided for identifying all points within a certain distance of a query point.
Given a query data set:

```{r}
nquery <- 1000
ndim <- 20
query <- matrix(runif(nquery*ndim), ncol=ndim)
```

... we apply the `queryNeighbors()` function:

```{r}
qout <- queryNeighbors(data, query, threshold=1)
length(qout$index)
```

... where each entry of `qout$index` corresponds to a row of `query` and contains its neighbors in `data`.
Again, the order of the output is arbitrary but the identity of the neighbors is deterministic.

# Further options

Most of the options described for `findKNN()` are also applicable here.
For example:

- `subset` to identify neighbors for a subset of points.
- `get.distance` to avoid retrieving distances when unnecessary.
- `BPPARAM` to parallelize the calculations across multiple workers.
- `raw.index` to return the raw indices from a precomputed index.

Note that the argument for a precomputed index is `precomputed`:

```{r}
pre <- buildIndex(data, BNPARAM=KmknnParam())
fout.pre <- findNeighbors(BNINDEX=pre, threshold=1)
qout.pre <- queryNeighbors(BNINDEX=pre, query=query, threshold=1)
```

Users are referred to the documentation of each function for specific details.

# Session information

```{r}
sessionInfo()
```

