Approximate Nearest Neighbors

ANN is a C++ library which computes approximate nearest neighbors, by David Mount and Sunil Arya.

ANN is a library written in the C++ programming language to support both exact and approximate nearest neighbor searching in spaces of various dimensions. It was implemented by David M. Mount of the University of Maryland, and Sunil Arya of the Hong Kong University of Science and Technology. ANN stands for "Approximate Nearest Neighbors". ANN is also a testbed containing programs and procedures for generating data sets, collecting and analyzing statistics on the performance of nearest neighbor algorithms and data structures, and visualizing the geometric structure of these data structures.

In the nearest neighbor problem a set P of data points in d-dimensional space is given. These points are preprocessed into a data structure, so that given any query point q, the nearest (or generally k nearest) points of P to q can be reported efficiently. ANN is designed for data sets that are small enough that the search structure can be stored in main memory (in contrast to approaches from databases that assume that the data resides in secondary storage). The distance between two points can be defined in many ways. ANN assumes that distances are measured using any class of distance functions called Minkowski metrics. These include the well-known Euclidean distance, Manhattan distance, and max distance.


ANN is available in a C++ version.

Related Data and Programs:

ANN_TEST, a C++ program which can be used with the ANN library to perform approximate nearest neighbor calculations.

ANN_TO_FIG, a C++ program which can convert the record of a search carried out by ANN_TEST into a graphical image in the FIG format.


  1. David Mount,
    Department of Computer Science,
    University of Maryland,
    College Park, MD 20742 USA,
  2. Sunil Arya,
    Department of Computer Science,
    The Hong Kong University of Science and Technology,
    Clear Water Bay, Kowloon, Hong Kong,


  1. Sunil Arya, David Mount, Nathan Netanyahu, Ruth Silverman, Angela Wu,
    An Optimal Algorithm for Approximate Nearest Neighbor Searching in Fixed Dimensions,
    Journal of the ACM,
    Volume 45, Number 6, November 1998, pages 891-923.
  2. David Mount,
    The ANN Programming Manual.
  3., the web site.

Source Code:

Examples and Tests:

You can go up one level to the C++ source codes.

Last revised on 19 March 2008.