Question

if 1st column detect 1, then add  1 -1 -1 to 2nd to 4th column 
if 1st column detect 2, then add -1  1 -1 to 2nd to 4th column 
if 1st column detect 3, then add -1 -1  1 to 2nd to 4th column 

example: A is 5×1 matrix

A=
1
2
3
2
1

i would like to get the result as below: A become 5×4 matrix

A =
1  1 -1 -1
2 -1  1 -1
3 -1 -1  1
2 -1  1 -1
1  1 -1 -1

the code i wrote below can not get the above result, please help…

if A(1:end,1) == 1
   A(1:end,2:4) = [1 -1 -1]
else if A(1:end,1) == 2
   A(1:end,2:4) = [-1 1 -1]
else 
   A(1:end,2:4) = [-1 -1 1]
end

Answer 1

Firstly, you’re comparing integers to vectors in your if statements. That won’t work. You need to loop over the entire vector, checking each element by itself. It is also preferable to preallocate the result matrix before modifying it, as allocation is an expensive operation:

A = [A zeros(size(A,1), 3)];
for i=1:size(A,1)
    if(A(i,1) == 1)
        A(i,2:4) = [1 -1 -1];
    elseif(A(i,1) == 2)
        A(i,2:4) = [-1 1 -1];
    elseif(A(i,1) == 3)
        A(i,2:4) = [-1 -1 1];
    end
end