MATLAB: Custom selection functions for genetic algorithm

Question

I am brand new to genetic algorithms. I am trying to learn by reproducing the results I found in a paper. I am getting hung up on the selection method. The paper uses normalized geometric selection. It looks like this in not one of the canned methods available and that I need to write a custom function. I have an equation and most of the parameters that I need. The onlything I think I need is the rank of the individual. My big issue is that I am not clear on how to pass this to my function. I took a stab at it with the below function but this failed. I dont know the syntax for passing parameters to and from my function for this instance. I appreciate any help. I suspect the solution is very simple but I am really confused.

function [S] = Selection(r)
p = 60; % population size
q = 0.05; % probablity of selection
R = r; % rank of individual
q = p/(1-(1-q)^p);
S = q*(1-q)^(R-1); % normalized geometric equation

Answer 1

The selection function for ga must have the following signature

function parents = myfun(expectation, nParents, options)

The explanation of the input and output variables are given here: https://www.mathworks.com/help/gads/genetic-algorithm-options.html#f6593. Note that the output variables 'parents' must have the dimension of 1xnParents. To see an example of selection function, check the code of one of the MATLAB's built-in selection function

edit selectionremainder