How can I generate a map key for this vector in MATLAB?

Question

I have a function that is looking at a number of elements. Each element is of the form of an 8x1 column vector. Each entry in the vector is an integer less than 1000. Every time I see such a vector, I'd like to add it to a list of "already seen" vectors, after checking to see that the vector is not already on this list. The function will examine on the order of ~100,000 such vectors.

Originally I tried using ismember(v', M, 'rows'), but found this to be very slow. Next I tried:

found = containers.Map('KeyType', 'double', 'ValueType', 'any');

Then each time I examine a new vector v, compute:

key = dot(v, [1000000000000000000000 1000000000000000000 1000000000000000 ...
              1000000000000 1000000000 1000000 1000 1]);

Then check isKey(found, key). If the key is not in the container, then found(key) = 1.

This seems like a pretty lousy solution, even though it does run considerably faster than ismember. Any help/suggestions would be greatly appreciated.

EDIT: Perhaps it would be better to use mat2str to generate the key, rather than this silly dot product?

Answer 1

The easiest way to generate a key/hash in your case would be to just convert the vector of integer values to a character array using char. Since your integer values never go above 1000, and char can accept numeric values from 0 to 65535 (corresponding to Unicode characters), this will give you a unique 8-character key for every unique 8-by-1 vector. Here's an example:

found = containers.Map('KeyType', 'char', 'ValueType', 'any');

v = randi(1000, [8 1]);  % Sample vector
key = char(v);
if ~isKey(found, key)
  found(key) = 1;
end