 a= ceil(5*rand(5,5));
 a > 3
 a < 3
 a == 3

 %  b(i,j)  = {  a(i,j)      if a(i,j) >3
 %            {  10          if a(i,j) == 3
 %            {   -1         if a(i,j) < 3

 c1 = a > 3
 c2 = a == 3
 c3 = a < 3

 b = a .* c1 + 10 .* c2 - 1 .* c3

 ind1 = find(b == 10)
 b(ind1) = 20
