MATH 170A HW0

q1.m

A = rand(9);
x = rand(9,1);

disp("My implementation:");
disp(mul_vec_by_mat(A,x));
disp("Matlab implementation:");
disp(A*x);

function Ax = mul_vec_by_mat(A, x)
    [m, n] = size(A);
    if length(x) ~= n
        error('dimenstion mismatch!');
    end
    Ax = zeros(m,1);
    for i = 1:m
        for j = 1:n
            Ax(i) = Ax(i) + A(i,j) * x(j);
        end
    end
end

q2.m

A = rand(9,4);
B = rand(4,6);

disp("My implementation:");
disp(mul_mat_by_mat(A,B));
disp("Matlab implementation:");
disp(A*B);

function C = mul_mat_by_mat(A, B)
    [m, n] = size(A);
    [n_, p] = size(B);
    if n ~= n_
        error('dimenstion mismatch!');
    end
    C = zeros(m,p);
    for k = 1:n % k first for L1 cache friendly
        for j = 1:p % j needs to go before i since matlab uses "column major memory layout"
            for i = 1:m
                C(i,j) = C(i,j) + A(i,k) * B(k,j);
            end
        end
    end
end