Note: This is taken from the Chicken Wiki, where a more recent version could be available.
Sparse vectors are vectors that grow as large as they need to. That is, they can be indexed by arbitrarily large nonnegative integers. The implementation allows for arbitrarily large gaps by arranging the entries in a tree.
If you have the numbers extension installed then this extension will be built with an additional variant that supports bignum indices:
[procedure] (make-sparse-vector [DEFAULT]) -> sparse-vector [procedure] (sparse-vector? x) -> boolean [procedure] (sparse-vector-ref sparse-vector k) -> value [procedure] (sparse-vector-set! sparse-vector k value) [procedure] (sparse-vector->list sparse-vector) -> list
make-sparse-vector, sparse-vector-ref and sparse-vector-set! are analogous to make-vector, vector-ref and vector-set!, except that the indices passed to sparse-vector-ref and sparse-vector-set! can be arbitrarily large.
The optional DEFAULT argument to the make-sparse-vector procedure specifies the default value for indices whose elements have not been set. If the element at index i has not been set, (sparse-vector-ref i) returns DEFAULT or #f, if DEFAULT was not specified when creating the vector.
sparse-vector->list is for debugging: It returns a list of the consecutive elements in a sparse vector from 0 to the highest element that has been set. Note that the list will also include all the #f elements for the unset elements.
Richard Kelsey and Jonathan Rees
Copyright (c) 1993-2006 Richard Kelsey and Jonathan Rees All rights reserved. Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met: 1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. 2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution. 3. The name of the authors may not be used to endorse or promote products derived from this software without specific prior written permission. THIS SOFTWARE IS PROVIDED BY THE AUTHORS ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.