Mapper used for transform coding

After doing the transform of an image i have to do the mapper with this condition

Z_i= 2 y_i 0<=y_i<=b

   2|y_i|    -1-b <=y_i<=0

    b+|y_i| other wise.

where b=y_min, y_i is the wavelet coefficients

I am having following doubts.

1.When i try to give min of the coefficients with command u=min(y_i) i am getting many values.Whats the reason?how to get a single value for b?

2. if true

    % for y_i=0:b 
        Z_i=2 y_i 
             end
    if Y_i= -1-b :0
       Z_i=2|y_i
else
   Z_i= 0; 

end

Is this the way to code the above mapper function in matlab?

Thank you so much for your patience to read my doubt.Expecting a kind reply.