You need to index L and Q:
L = linspace(0, 200, 2000);
Q = ones(1, length(L)); % Initialize to 1
for k = 2 : length(L)
if L(k) <= 10
Q(k) = 1;
elseif (L(k) > 10) && (L(k)<= 100)
Q(k)=0.32 * L(k) - 2.2;
elseif L(k) > 100
Q(k) =300/sqrt(L(k));
end
% Q=(Q1+Q2+Q3)
end
loglog(L, Q, 'LineWidth', 3);
grid on;
xlabel('L', 'FontSize', 20)
ylabel('Q', 'FontSize', 20)
Of course you don't need a loop - you can do it vectorized if you want but it's your homework so I didn't try to modify it too much.
