left riemann sum integration

so I have to make a numerical integrator using a few methods, just made a left Riemann sum calculator but I seem to be missing something

my code seems to be more accurate when I multiply my y array by dx in the for loop, but should I be multiplying dx at each iteration or at the very end?

also is it just my code or does Riemann sum require a lot of iteration to be accurate?

    % code
clear;
clc;
x1 = 1; x2 = 3;
f=@(x)  2*x.^5 - 3*x.^2 - 5;
c=1;
n=500;
dx=(x2-x1)/n;
for i=0:dx:n-1
     y(c)=dx*f(1+i*dx);

    c=c+1;
s=sum(y)*dx
z=integral(f,x1,x2)
  end

Answer 1

[Torsten]

Try

x1 = 1;
x2 = 3;
f=@(x)  2*x.^5 - 3*x.^2 - 5;
n=500;
dx=(x2-x1)/n;
summe=0.0;
for i=1:500
  summe=summe+f(x1+dx*(i-1));
end
summe=summe*dx;
z=integral(f,x1,x2);

Best wishes

Torsten.

[David Araujo]

How can i plot this?

[Sam Zavala]

I hope this is somewhat of a helpful answer but what we have been doing is build an array using a for loop and our time as telling us how many times to repeat it. Then you simply plot the array :)