matlab | logical subscripting - learning notes

http://www.mathworks.com/help/matlab/learn_matlab/indexing.html#f2-15124 clearly defines it -- "Suppose X is an ordinary matrix and L is a matrix of the same size that is the result of some logical operation. Then X(L)specifies the elements of X where the elements of L are nonzero."

Note if L has 5 non-zero elements, then length(X(L)) == 5.

I think L must be an array of booleans, not doubles.

For a matrix, see http://www.mathworks.com/help/matlab/math/matrix-indexing.html#bq7egb6-1

But here's a real illustration in my code:

  step = 1/200;
  steps = 2/step;
  reruns=500;

  % generate increments
  %rng(0,'twister'); % if we want repeatable
  incr = randn(steps,reruns)*sqrt(step);

  std(incr) % should  all be around 0.07
  hist(incr(:,1))

  % random walker positions
  p = cumsum(incr);

  % select a subset of Columns, using filter on
  % "200th ROW and 400th ROW" so
  % row expression = wildcard; column expression = filter on Row.
  % If we carelessly swap the expressions, matlab won't warn us!
  qualified = p(:, (p(200,:)>0 & p(400,:)>0));