Calling separate functions in Matlab

Question

I'm trying to call a a function that allows the user to input an equation of their choosing, and then use a separate function to approximate it using the trapezoidal rule in numerical analysis.

Here's my code for the user input equation:

    function f = myf(x)
    y=@(x) input('Input Equation:  ');
    f=y(x);
    end

And here's the code for applying the trapezoidal rule:

    function T=trapez(f,a,b,n)
    h=(b-a)/n;
    x=[a+h:h:b-h];
    T=h/2*(feval(f,a)+feval(f,b)+2*sum(feval,f,x));
    fprintf('The approximation by trapezoida rule is: %f with step h: %f\n',T,h);
    end

My issue is trying to use the equation determined by the first function as an input in the second.

    >> f=myfun
    Input Equation: exp(x^2)

    f = 

    @(x)exp(x^2)


    f = 

    @(x)exp(x^2)

    >> trapez(f,0,1,15)
    Error using feval
    Not enough input arguments.

    Error in trapez (line 4)
    T=h/2*(feval(f,a)+feval(f,b)+2*sum(feval,f,x));

Answer 1

Here is the function to input the function,

function f=myf
y=input('Input equation: ','s');
eval([ 'f=@(x)' y ';'])

and use f=myf from another function.

Also your trapez unction needs some modification:

function T=trapez(f,a,b,n)
    h=(b-a)/n;
    x=[a+h:h:b-h];
    T=h/2*(f(a)+f(b)+2*sum(arrayfun(f,x)));
    fprintf('The approximation by trapezoida rule is: %f with step h: %f\n',T,h);
    end

---

Depending on how the function is to be input, myf could be different. This should work if you give the input as, e.g.: x^2 (so just the function, not extra syntax)

y=input('Input equation: ','s')
eval([ 'f=@(x)' y])

And a sample input/output:

Input equation: x^2
y =
x^2
f = 
    @(x)x^2

and then you can do f(2) to find 2^2.

Alternatively, if you want to input the function with its argument, e.g.: @(y) y^2

y=input('Input equation: ','s')
eval(['f=' y])

and sample output:

>> y=input('Input equation: ','s')
    eval(['f=' y])
Input equation: @(t) t^2
y =
@(t) t^2
f = 
    @(t)t^2
>> f(2)
ans =
     4