Trying to separate Quaternion coefficients into array.

function g = actfun(q)                  % Activity function is LTF
    gw = (abs(q.w+1)-abs(q.w-1))/2;
    gx = (abs(q.x+1)-abs(q.x-1))/2;
    gy = (abs(q.y+1)-abs(q.y-1))/2;
    gz = (abs(q.z+1)-abs(q.z-1))/2;
    g = quaternion(gw,gx,gy,gz);
end    

Here q is a quaternion

Unrecognized method, property, or field 'w' for class 'quaternion'.

Error in plotQVNNs>actfun (line 71)

gw = (abs(q.w+1)-abs(q.w-1))/2;

Answer 1

[James Tursa]

Use the double( ) function to convert to numeric array, then use regular indexing. E.g., try this

function g = actfun(q)                  % Activity function is LTF
    Q = double(q);
    gw = (abs(Q(1)+1)-abs(Q(1)-1))/2;
    gx = (abs(Q(2)+1)-abs(Q(2)-1))/2;
    gy = (abs(Q(3)+1)-abs(Q(3)-1))/2;
    gz = (abs(Q(4)+1)-abs(Q(4)-1))/2;
    g = quaternion(gw,gx,gy,gz);
end    

This could also be vectorized

function g = actfun(q)                  % Activity function is LTF
    Q = squeeze(double(q))';
    g = quaternion((abs(Q+[1,1,1,1])-abs(Q-[1,1,1,1]))/2);
end    

However, if these are unit quaternions then this function just returns the inputs. For a unit quaternion

Q(1) is between -1 and 1, so Q(1)+1 is between 0 and 2, so abs(Q(1)+1) = Q(1)+1

Q(1) is between -1 and 1, so Q(1)-1 is between -2 and 0, so abs(Q(1)-1) = -(Q(1)-1) = -Q(1)+1

So you get

(abs(Q(1)+1)-abs(Q(1)-1))/2 =

(Q(1)+1 - (-Q(1)+1) ) / 2 =

( 2*Q(1)) / 2 =

Q(1)

Same for the other elements. So I presume this function is intended to work on non-unit quaternions?