# Integration in Matlab

#### ruby_duby

I have a function

B(m,n) = Integration of this between 1 and 0 (x.^m-1).*((1-x).^n-1)

For my m file i have

function Beta = B(m,n)
x=quad('@B',0,1);
Beta=(x.^m-1).*((1-x).^n-1);

however when i run my m-file it doesnt work. Can anybody help me please

#### MATLABdude

Science Advisor
I have a function

B(m,n) = Integration of this between 1 and 0 (x.^m-1).*((1-x).^n-1)

For my m file i have

function Beta = B(m,n)
x=quad('@B',0,1);
Beta=(x.^m-1).*((1-x).^n-1);

however when i run my m-file it doesnt work. Can anybody help me please
Two problems:
1) Your function is defined AFTER you call the quad function
2) Your function is improperly defined and used. Your function should be in terms of x (that's what you're varying), and not m and n (also, these had better be defined elsewhere, say m=1, n=2--quad will not give you a symbolic answer).

Go to the Mathworks documentation page for the quad function and carefully look at their example:
http://www.mathworks.com/access/helpdesk/help/techdoc/ref/quad.html

#### ruby_duby

thanks so much for your help but I still cant get it to work! I really find Matlab difficult and have virtually got every single book the library has to help me!

Ive tried fiddling around with my m-file but to no luck!

When i want to find B(2,3) and B(3,4) using ny m-file, i just get an error message.

I think its because i have no idea what im doing so any guidance is really appreciated

#### MATLABdude

Science Advisor
You didn't pay attention to what I said in my last post that you're not only using function handles incorrectly, but also the 'quad' function. To reiterate, hardcode your function (e.g. use x^2 or something, not x^m)! Without getting overly sophisticated, your function should not be in terms of m or n, but in terms of x. That's the only way you can use it in conjunction with the quad function. Quad is substituting in values only for x, not for m and n! Also, you cannot just declare a function mid-way through an m-file.

I'll take at face value that this isn't a homework problem. The following is how you can get what I think you're trying to achieve by having MATLAB do the substitution for you. Try declaring a more complex function along the lines of the following:

Code:
function integral=quadEvaluator(m,n)
% Evaluates the integral of (x.^m-1).*((1-x).^n-1)
% over the interval 0<x<1 and with given input values of m and n

functionString = ['(x.^', num2str(m), '-1).*((1-x).^', num2str(n), '-1)'];
% Creates the equation substituting in values of m and n
% Using the square braces concatenates everything together

fprintf('Evaluating the integral of ')
fprintf(functionString)
fprintf(' over the interval 0<x<1)
% Nice printout to verify the proper function is being evaluated

integral = quad(functionString, 0, 1);
When you try to evaluate something, it should give you an answer

>> junkVariable = quadEvaluator(2,3)

Evaluating the integral of (x.^2-1).*((1-x).^3-1) over the interval 0<x<1
junkVariable = 0.4333

In the future, it would also help if you posted what the error message was, instead of just saying that you have an error message.

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