rnnd_query {rnndescent}

Query an index for approximate nearest neighbors

Description

Takes a nearest neighbor index produced by rnnd_build() and uses it to find the nearest neighbors of a query set of observations, using a back-tracking search with the search size determined by the method of Iwasaki and Miyazaki (2018). For further control over the search effort, the total number of distance calculations can also be bounded, similar to the method of Harwood and Drummond (2016).

Usage

rnnd_query(
  index,
  query,
  k = 30,
  epsilon = 0.1,
  max_search_fraction = 1,
  init = NULL,
  n_threads = 0,
  verbose = FALSE,
  obs = "R"
)

Arguments

index

A nearest neighbor index produced by rnnd_build().

query

Matrix of n query items, with observations in the rows and features in the columns. Optionally, the data may be passed with the observations in the columns, by setting obs = "C", which should be more efficient. Possible formats are base::data.frame(), base::matrix() or Matrix::sparseMatrix(). Sparse matrices should be in dgCMatrix format. Dataframes will be converted to numerical matrix format internally, so if your data columns are logical and intended to be used with the specialized binary metrics, you should convert it to a logical matrix first (otherwise you will get the slower dense numerical version). Sparse and non-sparse data cannot be mixed, so if the data used to build index was sparse, the query data must also be sparse. and vice versa.

k

Number of nearest neighbors to return.

epsilon

Controls trade-off between accuracy and search cost, as described by Iwasaki and Miyazaki (2018). Setting epsilon to a positive value specifies a distance tolerance on whether to explore the neighbors of candidate points. The larger the value, the more neighbors will be searched. A value of 0.1 allows query-candidate distances to be 10% larger than the current most-distant neighbor of the query point, 0.2 means 20%, and so on. Suggested values are between 0-0.5, although this value is highly dependent on the distribution of distances in the dataset (higher dimensional data should choose a smaller cutoff). Too large a value of epsilon will result in the query search approaching brute force comparison. Use this parameter in conjunction with max_search_fraction to prevent excessive run time. Default is 0.1. If you set verbose = TRUE, statistics of the number of distance calculations will be logged which can help you tune epsilon.

max_search_fraction

Maximum fraction of the reference data to search. This is a value between 0 (search none of the reference data) and 1 (search all of the data if necessary). This works in conjunction with epsilon and will terminate the search early if the specified fraction of the reference data has been searched. Default is 1.

init

An optional matrix of k initial nearest neighbors for each query point.

n_threads

Number of threads to use.

verbose

If TRUE, log information to the console.

obs

set to "C" to indicate that the input data orientation stores each observation as a column. The default "R" means that observations are stored in each row. Storing the data by row is usually more convenient, but internally your data will be converted to column storage. Passing it already column-oriented will save some memory and (a small amount of) CPU usage.

Value

the approximate nearest neighbor index, a list containing:

• idx an n by k matrix containing the nearest neighbor indices.

• dist an n by k matrix containing the nearest neighbor distances.

References

Harwood, B., & Drummond, T. (2016). Fanng: Fast approximate nearest neighbour graphs. In Proceedings of the IEEE Conference on Computer Vision and Pattern Recognition (pp. 5713-5722).

Iwasaki, M., & Miyazaki, D. (2018). Optimization of indexing based on k-nearest neighbor graph for proximity search in high-dimensional data. arXiv preprint arXiv:1810.07355. https://arxiv.org/abs/1810.07355

See Also

rnnd_query()

Examples

iris_even <- iris[seq_len(nrow(iris)) %% 2 == 0, ]
iris_odd <- iris[seq_len(nrow(iris)) %% 2 == 1, ]

iris_even_index <- rnnd_build(iris_even, k = 4)
iris_odd_nbrs <- rnnd_query(index = iris_even_index, query = iris_odd, k = 4)