Riemann Sum, need help explaining error in my code on my second day of matlab

So im trying to do a midpoint sum program for a class, and i am using the functions polevaluu and polint. these are similar to polyvalu and polyint.

so far:

function reimsum(v,a,b,n)

% v is a row vector, the coefficients of a decreasing order polynomial

%a and b are the lower and upper ends and n is # of subintervals

dx = (b-a)/n;

% set value for dx

C = zeros(1,n);

% skeleton for resulting values to be injected

for i=1:n ;

      C(i)=(polevaluu(v,(a+(2*i-1))*dx))*2*dx

%the first element in C is equal to polevaluu(v, a+dx) times 2dx

% the second is equal to polevaluu(v, a+3dx) times 2dx

%each element is the area of a rectangle of the sum.

end

   polevaluu(polint(v),b)-polevaluu(polint(v),a)
  %exact value of integral

Sum(C)

%summation of areas of rectangles.

end

I am getting this error:

Error in ==> reimsum at 5 C(i)=(polevaluu(v,(a+(2*i-1))*dx))*2*dx

not sure exactly what it means, it's my second day using matlab.

Help anyone?

[Walter Roberson]

What error message does it generate? For example does it say "Indices must be non-negative integers or logicals" ? Does it say "unknown function polevaluu"?

[Campos]

just this: Error in ==> reimsum at 5

C(i)=(polevaluu(v,(a+(2*i-1))*dx))*2*dx

would a screen shot help?

[Walter Roberson]

Yes, a screen shot would help.

http://www.mathworks.com/matlabcentral/answers/7924-where-can-i-upload-images-and-files-for-use-on-matlab-answers

[Campos]

http://imageshack.us/photo/my-images/23/rsum.jpg/

The first time I used the code today it said "too many output arguments"

[Walter Roberson]

polevaluu is not a built-in MATLAB function. We would need to see the code for it.

[Campos]

if true
function polevaluu(A,x)
%used 2 u's because my MATLAB folder already had a function names
%'polevalu'
n = length(A);
% find length of the polynomial
for i = 0:n-1 ;
  A(n-i) = A(n-i)*x^i ;
end
%uses the nth minus i term and multiplies it by the scalar to the ith power
value = sum(A)
%prints the value of the sum.
end

end

should be the same as polyval

Answer 1

[Walter Roberson]

You are attempting to use polevaluu as a function that returns a value, but your line

function polevaluu(A,x)

is defined to mean that polevaluu does not return any value.

Possibly what you want is

function value = polevaluu(A,x)

[Campos]

What you are saying is that i have to edit my polevaluu?

or are you saying to insert that code into my reimsum function instead of what is in my for loop?

[Campos]

It worked! Thanks!

polint was also having that problem.

now however my reimsum function is returning

the values of every step, so every polevaluu and every poleint...

[Walter Roberson]

Your code for polevaluu is designed to "prints the value of the sum.". You probably do not want that. Change your line

value = sum(A)

to

value = sum(A);

notice the semi-colon