Matlab: Init variable if undefined

How can I declare/assign a variable only if it was never assigned before?

Context:

I'm trying to find k which minimizes a function calculateSomeDistance(k) of k. The minimal distance and the corresponding k value should be available (ie. in scope) for later use. How should I declare minDistance so that I can check whether it was already initialized before comparing it to the currently calculated distance?

% How should I declare minDistance?
minDistance=undefined; % Doesn't exist.
for ki=1:K,
  distance=calculateSomeDistance(ki);
  if(isUndefined(minDistance) || distance < minDistance)
    minDistance = distance;
    minK = ki;
  end
end
% Here minK and minDistance must be in scope

Is there a way to assign a null/undefined value to a variable in matlab/octave and later test for it in order to make the first valid assignment?

PS: Initializing minDistance to a very large number is very ugly, and not what I'm looking for.

Initializing minDistance when ki is 1 (ie. on first pass) is OK, but still not nice.