2nd 3rd derivatives

trying to plot first 3 derrivatives for the following function: y = exp(-3.*x)

x = [-2:0.01:1];
y =  exp(-3.*x);
dy=diff(y)./diff(x);
d2ydx2=diff(y,2)./diff(x,2);

plot (x(3:end),d2ydx2)

it plots the original function and first derrivative but gives a weird graph for 2nd derrivative

any help would be appriciated

Answer 1

[Matt J]

x = [-2:0.01:1];
y =  exp(-3.*x);
dy=gradient(y,0.01);
ddy=gradient(dy,0.01);

plot (x(3:end),ddy(3:end))

Answer 2

[Torsten]

x = [-2:0.01:1];
y = exp(-3.*x);
dydx = diff(y)./diff(x);
d2ydx2 = diff(dydx)./diff((x(1:end-1)+x(2:end))/2);
d3ydx3 = diff(d2ydx2)./diff(x(2:end-1));
figure(1)
hold on
plot((x(1:end-1)+x(2:end))/2,dydx)
plot((x(1:end-1)+x(2:end))/2,-3*exp(-3*(x(1:end-1)+x(2:end))/2))
hold off

figure(2)
hold on
plot(x(2:end-1),d2ydx2)
plot(x(2:end-1),9*exp(-3*x(2:end-1)))
hold off

figure(3)
hold on
plot((x(2:end-2)+x(3:end-1))/2,d3ydx3)
plot((x(2:end-2)+x(3:end-1))/2,-27*exp(-3*(x(2:end-2)+x(3:end-1))/2))
hold off