What values are you going to store? The chosen_index is for sure a scalar because as soon as the condition is satisfied once the loop stops executing, your code is achieved using one line:
The_chosen_one = find(accumulation > p,1,'first')
If your question is how to store values in a loop then:
% an example
x = randi(10,1,10);
y = zeros(size(x)); % preallocate
for k = 1:numel(x)
y(k) = x(k) * 2;
end
