rpf_build {rnndescent}

Create a random projection forest nearest neighbor index

Description

Builds a "forest" of Random Projection Trees (Dasgupta and Freund, 2008), which can later be searched to find approximate nearest neighbors.

Usage

rpf_build(
  data,
  metric = "euclidean",
  use_alt_metric = TRUE,
  n_trees = NULL,
  leaf_size = 10,
  max_tree_depth = 200,
  margin = "auto",
  n_threads = 0,
  verbose = FALSE,
  obs = "R"
)

Arguments

data

Matrix of n items to generate the index 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).

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"). 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. Only applies if the implicit margin method is used.

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. This value should be chosen to match the expected number of neighbors you will want to retrieve when running queries (e.g. if you want find 50 nearest neighbors set leaf_size = 50) and should not be set to a value smaller than 10.

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.

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

a forest of random projection trees as a list. Each tree in the forest is a further list, but is not intended to be examined or manipulated by the user. As a normal R data type, it can be safely serialized and deserialized with base::saveRDS() and base::readRDS(). To use it for querying pass it as the forest parameter of rpf_knn_query(). The forest does not store any of the data passed into build the tree, so if you are going to search the forest, you will also need to store the data used to build it and provide it during the search.

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_knn_query()

Examples

# Build a forest of 10 trees from the odd rows
iris_odd <- iris[seq_len(nrow(iris)) %% 2 == 1, ]
iris_odd_forest <- rpf_build(iris_odd, n_trees = 10)

iris_even <- iris[seq_len(nrow(iris)) %% 2 == 0, ]
iris_even_nn <- rpf_knn_query(
  query = iris_even, reference = iris_odd,
  forest = iris_odd_forest, k = 15
)