Home > manopt > core > getLinesearch.m

getLinesearch

PURPOSE

Returns a hint for line-search algorithms.

SYNOPSIS

function t = getLinesearch(problem, x, d, storedb, key)

DESCRIPTION

``` Returns a hint for line-search algorithms.

function t = getLinesearch(problem, x, d)
function t = getLinesearch(problem, x, d, storedb)
function t = getLinesearch(problem, x, d, storedb, key)

For a line-search problem at x along the tangent direction d, computes
and returns t such that retracting t*d at x yields a good point around
where to look for a line-search solution. That is: t is a hint as to
"how far to look" along the line.

storedb is a StoreDB object, key is the StoreDB key to point x.

CROSS-REFERENCE INFORMATION

This function calls: This function is called by:
• linesearch_constant Forces a constant multiplier on the descent direction chosen by the algorithm.
• linesearch_hint Armijo line-search based on the line-search hint in the problem structure.

SOURCE CODE

```0001 function t = getLinesearch(problem, x, d, storedb, key)
0002 % Returns a hint for line-search algorithms.
0003 %
0004 % function t = getLinesearch(problem, x, d)
0005 % function t = getLinesearch(problem, x, d, storedb)
0006 % function t = getLinesearch(problem, x, d, storedb, key)
0007 %
0008 % For a line-search problem at x along the tangent direction d, computes
0009 % and returns t such that retracting t*d at x yields a good point around
0010 % where to look for a line-search solution. That is: t is a hint as to
0011 % "how far to look" along the line.
0012 %
0013 % storedb is a StoreDB object, key is the StoreDB key to point x.
0014 %
0016
0017 % This file is part of Manopt: www.manopt.org.
0018 % Original author: Nicolas Boumal, July 17, 2014.
0019 % Contributors:
0020 % Change log:
0021 %
0022 %   April 3, 2015 (NB):
0023 %       Works with the new StoreDB class system.
0024
0025     % Allow omission of the key, and even of storedb.
0026     if ~exist('key', 'var')
0027         if ~exist('storedb', 'var')
0028             storedb = StoreDB();
0029         end
0030         key = storedb.getNewKey();
0031     end
0032
0033
0034     if isfield(problem, 'linesearch')
0035     %% Compute the line-search hint function using linesearch.
0036
0037         % Check whether this function wants to deal with storedb or not.
0038         switch nargin(problem.linesearch)
0039             case 2
0040                 t = problem.linesearch(x, d);
0041             case 3
0042                 % Obtain, pass along, and save the store for x.
0043                 store = storedb.getWithShared(key);
0044                 [t, store] = problem.linesearch(x, d, store);
0045                 storedb.setWithShared(store, key);
0046             case 4
0047                 % Pass along the whole storedb (by reference), with key.
0048                 t = problem.linesearch(x, d, storedb, key);
0049             otherwise
0051                     'linesearch should accept 2, 3 or 4 inputs.');
0052                 throw(up);
0053         end
0054
0055     else
0056     %% Abandon computing the line-search function.
0057
0058         up = MException('manopt:getLinesearch:fail', ...
0059             ['The problem description is not explicit enough to ' ...
0060              'compute a line-search hint.']);
0061         throw(up);
0062
0063     end
0064
0065 end```

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