how to efficiently encode two values as a unique entitiy

Dear all,

With respect to the above subject, I wonder if we can make the following code more efficient and compact (reduce the ifelse)..

A=3;
B=3;
C=[];
if A==1 && B==1
    C=1;
elseif  A==1 && B==2
    C=2;
    elseif  A==1 && B==3
    C=3;
    elseif  A==2 && B==2
    C=4;
    elseif  A==2 && B==3
    C=5;
    elseif  A==3 && B==1
    C=6;
    elseif  A==3 && B==2
    C=7;
    elseif  A==3 && B==3
    C=8;
end

In addition, I also wonder if we can scale it up for A=[2;2;3;1] and B= [1;1;3;1]

Thanks in advance

[Star Strider]

Duplicate: How to efficiently encode two values as a unique entitiy (link)

Answer 1

[Jan]

A = 3;
B = 3;
Q = [1, 1, 1, 2, 2, 3, 3, 3; ...   % A
     1, 2, 3, 2, 3, 1, 2, 3; ...   % B
     1, 2, 3, 4, 5, 6, 7, 8].';    % C
idx = ismember(Q(:, 1:2), [A, B], 'rows');
C   = Q(idx, 3);

I do not know what " scale it up for A=[2;2;3;1] and B= [1;1;3;1]" means.

Answer 2

[Jan]

Start with a simplification by omitting repeated comparisons:

A = 3;
B = 3;
C = [];
if A == 1
   if B == 1
      C = 1;
   elseif B == 2
      C = 2;
   elseif B == 3
      C = 3;
   end
elseif A == 2
   if B == 2
      C = 4;
   elseif B == 3
      C = 5;
   end
elseif  A == 3
   if B == 1
      C = 6;
   elseif B == 2
      C = 7;
   elseif B == 3
      C = 8;
   end
end

This should run faster, but is not much nicer. What about switch ?

A = 3;
B = 3;
C = [];
switch A
    case 1
      switch B
          case 1
             C = 1;
          case 2
             C = 2;
          case 3
             C = 3;
      end
    case 2
       switch B
          case 2
             C = 4;
          case 3
             C = 5;
       end
    case 3
       switch B
           case 1
              C = 6;
           case 2
              C = 7;
           case 3
              C = 8;
       end
   end
end
   

Nope, not nicer also.