MATLAB: Help!!! I keep getting error message “Array indices must be positive integers or logical values.”

Question

Hi this is my code for calcualting the forward difference of the following array.

t= [0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440 460 480 500 520];
v= [0 139 298 433 685 1026 1279 1373 1490 1634 1800 1986 2191 2417 2651 2915 3303 3516 3860 4216 4630 5092 5612 6184 6760 7327 7581];
h=20; 
for i = 1:length(t); 
   Dforward(i)= v(t(i)+h)-v(t(i))/h;
end

I believe my problem is the code keeps calcualting beyond the boundries of my array, and I'm not sure what todo to fix it. Please any advice would be great

Answer 1

Perhaps you mean this:

t = [0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440 460 480 500 520];
v = [0 139 298 433 685 1026 1279 1373 1490 1634 1800 1986 2191 2417 2651 2915 3303 3516 3860 4216 4630 5092 5612 6184 6760 7327 7581];
for k = 1 : length(t) - 1
	h = t(k + 1) - t(k);
	Dforward(k)= (v(k+1) - v(k)) / h;
end
% Plot original signal
subplot(2, 1, 1);
plot(t, v, 'b.-', 'MarkerSize', 30, 'LineWidth', 2);
grid on
xlabel('t', 'FontSize', 20);
ylabel('v', 'FontSize', 20);
title('v vs. t', 'FontSize', 20);
% Plot Dforward.
subplot(2, 1, 2);
plot(t(1:end-1), Dforward, 'b.-', 'MarkerSize', 30, 'LineWidth', 2);
grid on
xlabel('t', 'FontSize', 20);
ylabel('Dforward', 'FontSize', 20);
title('Dforward vs. t', 'FontSize', 20);