Increasing value of filter till required number of solutions each reached

I need a while loop to increase the length, n, of a filter until I have 8 values that satisfy a result. The code that I have does not seem to work.

index = [];
while length(index)>8
    n = 1;
    filtered_ipam_relative_hand_position = moving_average_filter(ipam_relative_hand_position, n);
    velocity=zeros(size(filtered_ipam_relative_hand_position));
    for i=1:length(data)
    for j= 1:3
        x = i-1;
        y = i+1;
        if x == 0
            x = 1;
        else
            if i == length(data)
                y = length(data);
            end
        end
        velocity(i,j) = (filtered_ipam_relative_hand_position(y,j)- filtered_ipam_relative_hand_position(x,j))/0.08;
    end
    end
     magnitude_velocity = zeros(length(velocity),1);
     for i=1:length(data)
         magnitude_velocity(i)=((velocity(i,1))^2 + (velocity(i,2))^2 + (velocity(i,3))^2)^0.5;
     end
     for i=1:length(data)-1
         if (magnitude_velocity(i+1,1)<0.01) && (magnitude_velocity(i,1)>0.01)
           index = [index i];
         end
     end
     n=n+1;
end

Answer 1

[Walter Roberson]

length([]) is 0, which is <= 8, so your "while" loop never executes.

[Afzal]

How should I define 'index' then?

[Walter Roberson]

The problem is not your definition of "index", the problem is your "while" condition.