How to use Hash Tables (dictionaries) in MATLAB?

Question

I need to acces data by string index, like table('one') %returns 1. Is there such a data structure in MATLAB? How is it implemented?

(also cross-duplicate of http://www.mathworks.com/matlabcentral/answers/21130-dictionaries-of-hashtables-in-matlab) — Franck Dernoncourt, Mar 24 '12 at 10:45

score 55 · Accepted Answer · edited Jan 08 '15 at 12:26

55

In recent versions of MATLAB, there's the containers.Map data structure. See MATLAB Map containers for more. This removes some of the restrictions when using STRUCTs. For example

c = containers.Map
c('foo') = 1
c(' not a var name ') = 2
keys(c)
values(c)

edited Jan 08 '15 at 12:26

Nigel Davies

1,640
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answered Mar 26 '12 at 07:49

Edric

23,676
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score 17 · Answer 2 · answered Mar 24 '12 at 10:42

A structure can be used as a sort of hash table:

>> foo.('one')=1

foo = 

    one: 1

>> foo.('two')=2;
>> x = 'two';
>> foo.(x)

ans =

     2

To query whether a structure contains a particular field (key), use isfield:

>> isfield(foo,'two')

ans =

     1

The disadvantage of this scheme is that only strings that are also valid Matlab variable names can be used as keys. For instance:

>> foo.('_bar')=99;
??? Invalid field name: '_bar'.

To get around this restriction, use one of the solutions in the question linked by Oli.

Another disadvantage of this approach is that the names of the keys have to be less than `namelengthmax` (63) characters — rhombidodecahedron, Apr 23 '15 at 19:20

score 0 · Answer 3 · answered Sep 24 '19 at 13:03

Just to add to the previous answers (could not comment on the other good answers) : a struct lookup like this:

s={};
s.abc = 1;   %insert a value in the struct, abc is your lookup hash
out = s.abc; % read it back

The actual read back of the value is about 10 times faster with a struct than using a container. Full test code as follows if interesting

function s=test_struct_lookup_hash_speed
%% test how a struct lookup speed works vs container, interesting

    s = {};  % hash table
    v = {};  % reverselookup table for testing

    nHashes = 1E4;  % vary this to see if read speed varies by size (NOT REALLY)
    nReads  = 1E6;

    fprintf('Generating hash struct of %i entries\n', nHashes);
    tic
    for i = 1:nHashes
        hashStr = md5fieldname(randi(1E8));
        s.(hashStr) = i;
        v{end+1} = hashStr;  %reverselookup
    end
    toc

    fprintf('Actual HashTable length (due to non unique hashes?): %i, and length of reverse table: %i\n',length(fieldnames(s)), length(v) );

    fprintf('Reading %i times from a random selection from the %i hashes\n', nReads, nHashes);
    vSubset = [ v(randi(nHashes,1,3))  ];

    for i = 1:length(vSubset)
       hashStr = vSubset{i};

       % measure read speed only
       tic
       for j = 1:nReads
         val = s.(hashStr);
       end
       toc

    end


    %% test CONTAINERS
    fprintf('Testing Containers\n');
    c = containers.Map;

    fprintf('Generating hash struct of %i entries\n', nHashes);
    tic
    for i = 1:nHashes
        hashStr = md5fieldname(randi(1E8));
        c(hashStr) = i;
        v{end+1} = hashStr;  %reverselookup
    end
    toc

   fprintf('Reading %i times from a random selection from the %i hashes\n', nReads, nHashes);
   vSubset = [ v(randi(nHashes,1,3))  ];

    for i = 1:length(vSubset)
       hashStr = vSubset{i};

       % measure read speed only
       tic
       for j = 1:nReads
         val = c(hashStr);
       end
       toc

    end    



    %% Get a valid fieldname (has to start with letter)   
    function h=md5fieldname(inp)
    h = ['m' hash(inp,'md5')];
    end


end

How to use Hash Tables (dictionaries) in MATLAB?

3 Answers3

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