# Interpreting Matlab function simp

by Ein Krieger
Tags: integration, matlab, simpson's rule
 P: 34 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? Attached Thumbnails
 P: 544 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.
 P: 34 Yes. Sure I have attached all commands with order from Pic.1 to Pic.3 Attached Thumbnails
 P: 544 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.
 P: 544 EDIT: I found information about this function in the MATLAB file exchange. 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
 P: 34 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?
 P: 544 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 I1=simp(0.0,r0,nr,u1d) You are integrating u1d from 0 to r0 with nr intervals
P: 34
 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 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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