DylanMuir / SimpleMap

Provide a simple map between integers and objects

SimpleMap

A matlab handle class for persistent storage of arbitrary data. Provides a mapping from a single subscript to arbitrary data.

Usage

Create an empty map

>> smObj = SimpleMap
smObj = 
SimpleMap containing 0 stored values, max index of 0.

Assign some data

>> smObj(1) = [1 2 3]; 
>> smObj(2) = 'a';
>> smObj(4) = {4 5 6}
smObj = 
SimpleMap containing 3 stored values, max index of 4.

Extract the data

>> a = smObj(1);
>> b = smObj(3)
b =
     []

Find out which indices are used in the map

>> vbIndices = spy(smObj)
vbIndices =
     1     1     0     1

>> find(smObj)
ans =
     1     2     4

SimpleMaps are a handle class

>> smObj = SimpleMap;	% Make a new empty SimpleMap
>> smObj2 = smObj;		% Copy the SimpleMap object
>> smObj(1) = 'Map 1';	% Assign data to the first map
>> smObj2(1)				% The data is also present in the copy
ans =
Map 1

The benefit of SimpleMap being a handle class, is to enable persistent data store within objects that are not themselves handle classes.

Empty the map

>> empty(smObj);
>> smObj
smObj = 
SimpleMap containing 0 stored values, max index of 0.

Using a SimpleMap for persistent data storage within a value class

classdef ValueClass
	properties (Hidden)
		smPersistent;
	end
	
	methods
		function ValueClass
			% - Initialise with an empty map
			smPersistent = SimpleMap();
		end
		
		function fResult = LongCalculation(vcObj, nIndex)
			% - Check whether the result has already been calculated
			if (~isempty(vcObj.smPersistent(nIndex)))
				fResult = vcObj.smPersistent(nIndex);
			else
				% - No, so compute the result from scratch
				fResult = PerformLongCalculation(nIndex);
				
				% - Cache the result
				vcObj.smPersistent(nIndex) = fResult;
			end
		end
	end
end

The property smPersistent will maintain data assigned to it from within methods of ValueClass, even though it is not a handle class. This is useful for internal hidden caching of results, not as useful for user-facing properties.

In the example above, a SimpleMap is used for "memoization" of the result of a long calculation. The result is cached, and this cache is maintained even though the object vcObj is not returned by the LongCalculation method.

This is most useful if the semantics of a class do not lend themselves to being a handle class.