Index exceeds matrix dimensions

Hello, I am having trouble with my code. It keeps on giving me the "Index exceeds matrix dimensions" error and tells me

 Error in A9TRAPOchoa (line 20) n=n(in)

Is there any way to fix this?

function I=A9TRAPOchoa(fun,interval,n)

%Inputs: 
%integ-the function to be evaluated

%interval-[a b]=the integration limits

%n-number of segments

%Outputs:
%I-integral estimate

fun=@(x) (4.*x.^3)-(42.*x)+700;

interval=[0 10];

n=[2 3 4 5 10 15 20 30 40 50];

for in=1:length(n)

      n=n(in);
  end

a=interval(1);

b=interval(2);

x=a;
h=(b-a)/n;
s=fun(a);

for j=1:n-1

      x=x+h;
      s=s+(2*fun(x));
  end
  s=s+fun(b);
  I=(h/2)./s;
  end

Thank you for your time.

Answer 1

[ANKUR KUMAR]

There are few corrections which you have to do.

1. You have defined fun in between of the function. So, there is no need to write in the function call. So, your first line reduces to

function I=A9TRAPOchoa(interval,n)

2. I could not able to understand the reason behind using this code in between of the program.

for in=1:length(n)
 n=n(in);
end

You can delete these 3 lines. And by deleting these lines, there is no need to put n in the function call. So, now your first line reduces to

function I=A9TRAPOchoa(interval)

3. Instead of writing

x=a; h=(b-a)/n; s=fun(a);
You should to write 

x=a; h=(b-a)./n; s=fun(a);

Now, your function is correct. For, reference, I am pasting the correct function by omitting your errors.

function I=A9TRAPOchoa(interval)
fun=@(x) (4.*x.^3)-(42.*x)+700;
interval=[0 10];
n=[2 3 4 5 10 15 20 30 40 50];
a=interval(1);
b=interval(2);
x=a; h=(b-a)./n; s=fun(a);
for j=1:n-1
    x=x+h;
    s=s+(2*fun(x));
end
s=s+fun(b);
I=(h/2)./s;
end

Now, if you type A9TRAPOchoa([5 8]) in command window, then you will get your answer.

NOTE: You have already defined n in between of the function. So, if you need to change every time, then delete the line, where you have defined n in the function and put n in the function call. If you want to change n every time, then use

function I=A9TRAPOchoa(interval,n)
fun=@(x) (4.*x.^3)-(42.*x)+700;
interval=[0 10];

a=interval(1);
b=interval(2);
x=a; h=(b-a)./n; s=fun(a);
for j=1:n-1
    x=x+h;
    s=s+(2*fun(x));
end
s=s+fun(b);
I=(h/2)./s;
end

Type A9TRAPOchoa([5 8],[1:45]) in command window to get the desired result.