MATLAB: Left riemann sum integration

for loopleft riemann sumnumerical 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

Best Answer

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.
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