Question

Possible Duplicate:
Hash tables in MATLAB

General Question

Is there any way to get a hashset or hashmap structure in Matlab?

I often find myself in situations where I need to find unique entries or check membership in vectors and using commands like unique() or logical indexing seems to search through the vectors and is really slow for large sets of values. What is the best way to do this in Matlab?

Example

Say, for example, that I have a list of primes and want to check if 3 is prime:

primes = [2,3,5,7,11,13];

if primes(primes==3)
    disp('yes!')
else
    disp('no!')
end

if I do this with long vectors and many times things get really slow.

In other languages

So basically, is there any equivalents to python’s set() and dict(), or similarly Java’s java.util.HashSet and java.util.HashMap, in Matlab? And if not, is there any good way of doing lookups in large vectors?

Edit: reflection on the answers

This is the running time i got on the suggestions in the answers.

>> b = 1:1000000;
>> tic; for i=1:100000, any(b==i);; end; toc
Elapsed time is 125.925922 seconds.

s = java.util.HashSet();
>> for i=1:1000000, s.add(i); end    
>> tic; for i=1:100000, s.contains(i); end; toc
Elapsed time is 25.618276 seconds.

>> m = containers.Map(1:1000000,ones(1,1000000));
>> tic; for i=1:100000, m(i); end; toc
Elapsed time is 2.715635 seconds

The construction of the java set was quite slow as well though so depending on the problem this might be quite slow as well. Really glad about the containers.Map tip. It really destroys the other examples, and it was instant in set up too.

Answer 1

Like this?

>> m = java.util.HashMap;
>> m.put(1,'hello,world');
>> m.get(1)
ans =
hello, world

Alternatively, if you want a Matlab-native implementation, try

>> m = containers.Map;
>> m('one') = 1;
>> m('one')
ans =
     1

This is actually typed – the only keys it will accept are those of type char. You can specify the key and value type when you create the map:

>> m =  containers.Map('KeyType','int32','ValueType','double');
>> m(1) = 3.14;
>> m(1)
ans =
  3.14

You will now get errors if you try to put any key other than an int32 and any value other than a double.

You also have Sets available to you:

>> s = java.util.HashSet;
>> s.put(1);
>> s.contains(1)
ans = 
     1
>> s.contains(2)
ans = 
     0