Interpreting Matlab function simp

Ein Krieger

Hello, guys

Hey guys,

Got stuck with function integration using Simpson's rule and need your help.

Please first refer to picture attached for full idea of my question:

The Matlab command related to it is:

for i=1:nr

u1d(i)=4.0*pi*r(i)^2*u(it,i)

end

I1=simp(0.0,r0,nr,u1d)/(4.0/3.0*pi*r0^3)

I1 is

nr=21

r0=1.0;

Does it mean that I1 is integrated 21 times between boundaries 0 and r0?

kreil

Can you post more information? The code you posted references variables that you never define. Make it so that your code block can be copy/pasted into matlab.

Ein Krieger

Yes. Sure

I have attached all commands with order from Pic.1 to Pic.3

kreil

simp() is not a matlab function, so the information about the input arguments is not available in the documentation. I suggest looking at the function file for simp() to find this info.

kreil

EDIT: I found information about this function in the MATLAB file exchange.

Code:

function s = simp(f, a, b, h)
x1 = a + 2 * h : 2 * h : b - 2 * h;
sum1 = sum(feval(f, x1));
x2 = a + h : 2 * h : b - h;
sum2 = sum(feval(f, x2));
s = h / 3 * (feval(f, a) + feval(f, b) + ...
           2 * sum1 + 4 * sum2);

It appears that the inputs are:

f=function, a=initial value, b=end value, h=interval size

Ein Krieger

I have found it as separate m.file. Here are the commands:

function uint=simp(xl,xu,n,u)
h=(xu-xl)/(n-1);
uint(1)=u(1)-u(n);
for i=3:2:n
uint(1)=uint(1)+4.0*u(i-1)+2.0*u(i);
end
uint=h/3.0*uint;
But why here different variables are used such as xl and xu?

It seems to me that we use r in integration?

kreil

At a glance it looks like

xl = beginning of interval
xu = end of interval
n = number of slices
u = function

So in your case of

Code:

I1=simp(0.0,r0,nr,u1d)

You are integrating u1d from 0 to r0 with nr intervals

Ein Krieger

Quote by kreil

At a glance it looks like

xl = beginning of interval
xu = end of interval
n = number of slices
u = function

So in your case of

Code:

I1=simp(0.0,r0,nr,u1d)

You are integrating u1d from 0 to r0 with nr intervals

So you mean simp can be uniformly used for every type of variable assuming their correct order?

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kreil Recognitions: Gold Member	Can you post more information? The code you posted references variables that you never define. Make it so that your code block can be copy/pasted into matlab.

Ein Krieger	Yes. Sure I have attached all commands with order from Pic.1 to Pic.3 Attached Thumbnails

kreil Recognitions: Gold Member	Interpreting Matlab function simp simp() is not a matlab function, so the information about the input arguments is not available in the documentation. I suggest looking at the function file for simp() to find this info.