Input parser: one parameter dependent from another parameter - matlab

Question

I am using the input parser functionality a lot but now I am facing the following issue:
I am coding an object that computes the rate-of-change of time series, i.e. one input argument is a matrix of doubles (timeSeries). A second input parameter is the time period used to compute the rate-of-change (lag). lag can be a vector wherby different rate-of-changes will be computed. A third input vector is called weightVector. weightVector will be used to compute an average of all rate-of-changes and applies the appropriate weights to the respective rate-of-change results.
Now, I always like to define some default values when using input parser. I would like to define weightVector to be an equal weighted vector. However, the length of the default weightVector is dependent on the length of lag. For example, if lag = [1,2,3], then weightVectorshould equal [1/3, 1/3, 1/3]. How am I supposed to code a situation like this? My current code for the constructor looks like this:
function obj = roc(timeSeries, varargin)
%% Input parser
% 1. Create input parser instance
p = inputParser;
% 2. Default values for input arguments
default_lag = 1;
default_weightVector = 1/length(lag); % This line is causing
% problems as LAG isn't
% defined, yet.
% 3. Validation of input arguments
valid_lag = {'vector', 'nonempty', 'integer', 'positive'};
check_lag = #(x) validateattributes(x, {'numeric'}, valid_lag);
valid_weightVector = {'vector', 'nonempty'};
check_weightVector = #(x) validateattributes(x, {'numeric'}, ...
valid_weightVector);
% 4. Add input arguments to input scheme
p.addRequired('timeSeries');
p.addParamater('lag', default_lag, check_lag);
p.addParameter('weightVector', default_weightVector, check_weightVector);
% 5. Parse input arguments
parse(p, timeSeries, varargin{:});
% 6. Assign results to variables
lag = p.Results.lag;
weightVector = p.Results.weightVector;
%% Main code
end % Constructor

Answer 1

function obj = roc(timeSeries, varargin)
%% Input parser
% 1. Create input parser instance
p = inputParser;
% 2. Default values for input arguments
default_lag = 1;
default_weightVector = 1;
% 3. Validation of input arguments
valid_lag = {'vector', 'nonempty', 'integer', 'positive'};
check_lag = #(x) validateattributes(x, {'numeric'}, valid_lag);
% 4. Add input arguments to input scheme
p.addRequired('timeSeries');
p.addParameter('lag', default_lag, check_lag);
p.addParameter('weightVector', default_weightVector);
% 5. Parse input arguments
parse(p, timeSeries, varargin{:});
% 6. Assign results to variables
lag = p.Results.lag;
if check_weightVector(p.Results.weightVector, lag) == true
weightVector = p.Results.weightVector;
end
function vout = check_weightVector(weightVector, lag) % validation function
if length(lag) ~= length(weightVector)
error('lengthWeightVector:WrongNumberOfElements', 'The number of elements in "weightVector" must correspond to the number of elements in "lag"');
elseif sum(weightVector) ~= 1
error('sumWeightVector:SumNotEqualToOne', 'The sum of elements in "weightVector" must equal 1');
end
vout = true;
end
%% Main code
end