rpf_knn {rnndescent}

Find nearest neighbors using a random projection forest

Description

Returns the approximate k-nearest neighbor graph of a dataset by searching multiple random projection trees, a variant of k-d trees originated by Dasgupta and Freund (2008).

Usage

rpf_knn(
  data,
  k,
  metric = "euclidean",
  use_alt_metric = TRUE,
  n_trees = NULL,
  leaf_size = NULL,
  max_tree_depth = 200,
  include_self = TRUE,
  ret_forest = FALSE,
  margin = "auto",
  n_threads = 0,
  verbose = FALSE,
  obs = "R"
)

Arguments

data

Matrix of n items to generate neighbors for, with observations in the rows and features in the columns. Optionally, input can be passed with 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).

k

Number of nearest neighbors to return. Optional if init is specified.

metric

Type of distance calculation to use. One of:

• "braycurtis"

• "canberra"

• "chebyshev"

• "correlation" (1 minus the Pearson correlation)

• "cosine"

• "dice"

• "euclidean"

• "hamming"

• "hellinger"

• "jaccard"

• "jensenshannon"

• "kulsinski"

• "sqeuclidean" (squared Euclidean)

• "manhattan"

• "rogerstanimoto"

• "russellrao"

• "sokalmichener"

• "sokalsneath"

• "spearmanr" (1 minus the Spearman rank correlation)

• "symmetrickl" (symmetric Kullback-Leibler divergence)

• "tsss" (Triangle Area Similarity-Sector Area Similarity or TS-SS metric)

• "yule"

For non-sparse data, the following variants are available with preprocessing: this trades memory for a potential speed up during the distance calculation. Some minor numerical differences should be expected compared to the non-preprocessed versions:

• "cosine-preprocess": cosine with preprocessing.

• "correlation-preprocess": correlation with preprocessing.

For non-sparse binary data passed as a logical matrix, the following metrics have specialized variants which should be substantially faster than the non-binary variants (in other cases the logical data will be treated as a dense numeric vector of 0s and 1s):

• "dice"

• "hamming"

• "jaccard"

• "kulsinski"

• "matching"

• "rogerstanimoto"

• "russellrao"

• "sokalmichener"

• "sokalsneath"

• "yule"

Note that if margin = "explicit", the metric is only used to determine whether an "angular" or "Euclidean" distance is used to measure the distance between split points in the tree.

use_alt_metric

If TRUE, use faster metrics that maintain the ordering of distances internally (e.g. squared Euclidean distances if using metric = "euclidean"), then apply a correction at the end. Probably the only reason to set this to FALSE is if you suspect that some sort of numeric issue is occurring with your data in the alternative code path.

n_trees

The number of trees to use in the RP forest. A larger number will give more accurate results at the cost of a longer computation time. The default of NULL means that the number is chosen based on the number of observations in data.

leaf_size

The maximum number of items that can appear in a leaf. The default of NULL means that the number of leaves is chosen based on the number of requested neighbors k.

max_tree_depth

The maximum depth of the tree to build (default = 200). If the maximum tree depth is exceeded then the leaf size of a tree may exceed leaf_size which can result in a large number of neighbor distances being calculated. If verbose = TRUE a message will be logged to indicate that the leaf size is large. However, increasing the max_tree_depth may not help: it may be that there is something unusual about the distribution of your data set under your chose metric that makes a tree-based initialization inappropriate.

include_self

If TRUE (the default) then an item is considered to be a neighbor of itself. Hence the first nearest neighbor in the results will be the item itself. This is a convention that many nearest neighbor methods and software adopt, so if you want to use the resulting knn graph from this function in downstream applications or compare with other methods, you should probably keep this set to TRUE. However, if you are planning on using the result of this as initialization to another nearest neighbor method (e.g. nnd_knn()), then set this to FALSE.

ret_forest

If TRUE also return a search forest which can be used for future querying (via rpf_knn_query()) and filtering (via rpf_filter()). By default this is FALSE. Setting this to TRUE will change the output list to be nested (see the Value section below).

margin

A character string specifying the method used to assign points to one side of the hyperplane or the other. Possible values are:

• "explicit" categorizes all distance metrics as either Euclidean or Angular (Euclidean after normalization), explicitly calculates a hyperplane and offset, and then calculates the margin based on the dot product with the hyperplane.

• "implicit" calculates the distance from a point to each of the points defining the normal vector. The margin is calculated by comparing the two distances: the point is assigned to the side of the hyperplane that the normal vector point with the closest distance belongs to.

• "auto" (the default) picks the margin method depending on whether a binary-specific metric such as "bhammming" is chosen, in which case "implicit" is used, and "explicit" otherwise: binary-specific metrics involve storing the data in a way that isn't very efficient for the "explicit" method and the binary-specific metric is usually a lot faster than the generic equivalent such that the cost of two distance calculations for the margin method is still faster.

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 graph as 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.

• forest (if ret_forest = TRUE) the RP forest that generated the neighbor graph, which can be used to query new data.

k neighbors per observation are not guaranteed to be found. Missing data is represented with an index of 0 and a distance of NA.

References

Dasgupta, S., & Freund, Y. (2008, May). Random projection trees and low dimensional manifolds. In Proceedings of the fortieth annual ACM symposium on Theory of computing (pp. 537-546). doi:10.1145/1374376.1374452.

See Also

rpf_filter(), nnd_knn()

Examples

# Find 4 (approximate) nearest neighbors using Euclidean distance
# If you pass a data frame, non-numeric columns are removed
iris_nn <- rpf_knn(iris, k = 4, metric = "euclidean", leaf_size = 3)

# If you want to initialize another method (e.g. nearest neighbor descent)
# with the result of the RP forest, then it's more efficient to skip
# evaluating whether an item is a neighbor of itself by setting
# `include_self = FALSE`:
iris_rp <- rpf_knn(iris, k = 4, n_trees = 3, include_self = FALSE)
# for future querying you may want to also return the RP forest:
iris_rpf <- rpf_knn(iris,
  k = 4, n_trees = 3, include_self = FALSE,
  ret_forest = TRUE
)