You can use MATLAB's inputParser class:

http://www.mathworks.com/help/matlab/ref/inputparser-class.html http://blogs.mathworks.com/community/2012/02/13/parsing-inputs/

EDIT: I guess I'll just put the code in as well...

function y = f(x, extraOpts)
    p = inputParser;
    p.addRequired('x', @(x) length(x)>1);
    p.addOptional('visualize', 1, @isscalar);
    p.addOptional('N', 100, @isscalar);
    p.parse(x, extraOpts{:});
    do_things(x, extraOpts);
end

You might need to store the results of the parse, perhaps something along the lines of:

inputs = p.parse(x, extraOpts{:});
do_things(inputs)

END EDIT

Alternatively, you could do something like redefine your function to take the number of values in your struct though it's not as powerful as the input parser solution:

function y = f(N, visualize)
if nargin < 2
  visualize = 0
end
if nargin < 1
  visualize = 0
  N = 100
end