Assign an input to an anonymous function

Basically what I'm trying to do is ask the user to input a function of x 'f(x)' then assign it as an anonymous function then have the user input what x value they would like the function to be evaluated at...

This code isn't complete, but heres what I have so far with the output it gives me:

    function [] = NewtsMeth
    syms x
    l=input('Please enter f(x) = ');
    func=@(x) (l);
    Xest=input('Pleae enter an initial guess = ');
    k=func(Xest);
    d=diff(func,x)
    s=k
    end

output:

    >> NewtsMeth
    Please enter f(x) = 6*x-8
    Pleae enter an initial guess = 3

    d =
    6

    s =
    6*x - 8

Obviously this isn't Newton's method but I'm just trying to figure this input/evaluate part out.

Answer 1

[Dave Regan]

I did it like this...

function []=test
syms x
digits(9);
func= input('Please enter f(x) = ');
Xest= input('Pleae enter an initial guess = ');
d=diff(func,x);
x=Xest;
for i = 0:15
    ds=eval(d);
    fs=eval(func);
    x = x-((fs)./(ds));
    vpa(x)
    if x(i-1)==x(i)
        break
    end
end
end

Didn't end up using anonymous function.

The if loop is a little messed up beacuse I'm trying to find a way to break it once the answer repeats but I basically just changed x from symbolic and assigned the value of the guess to it then evaluated the function using "eval()"

I'm sure there's a better way to do it.

Answer 2

[Joy Tian]

But it doesn't explain to me what is @(x) mean. And how does it relate with anonymous function.

Answer 3

[M_A_C]

Hi,

I'm in an annoying loop of errors and discontinued functions.

The original scipt can be found at https://docs.google.com/document/d/1vyy_w8r2hxR-CEMJrooRg4nuO6M_jS8FY9qQRRDCwF0/edit

The script asks for an input that is a function. That function goes to the argument of the inline function, but it is discontinued. Then I tried to use the Anonymous Function function, but I keep getting errors because for some reason @() does not allow me to evaluate the equation related to that function. Then I tried to apply the str2func to my input, but str2func is also being discontinued, and I get errors anyways.

This is the exert of the code that generates the error:

clc

e1=input("Enter F(Y)=","s");

% f_c = str2sym(e1);

F=@(Y) e1;

n=input('Enter No. of Iteration=');

a=input('Enter Initial Guess a=');

b=input('Enter Initial Guess b=');

while (F(a)*F(b)>0) I get the error here because F(a) = '2*x + x^2'

1-The original code works pretty well with inline.

2-Please, Can anyone answer how to use input and @()???

[M_A_C]

I just found a solution, but still using str2func!

Instead of entering 2*x + x^2 I input @(x) 2*x + x^2

and then F = str2func(e1)

Any suggestions to avoid that type of input and str2func are wellcome.

Thank you

Answer 4

[Steven Lord]

If you have a string or a char vector containing an equation:

s = '2*x + x^2 + y'

s = '2*x + x^2 + y'

let's vectorize it so it can be called with arrays of data rather than just scalars.

vs = vectorize(s)

vs = '2.*x + x.^2 + y'

We can use the symvar function (which despite the name is in MATLAB itself, not Symbolic Math Toolbox) to identify the variables used in the equation.

vars = symvar(vs)

vars = 2×1 cell array
    {'x'}
    {'y'}

String operations will let us build the input arguments section of our anonymous function.

inputArguments = "@(" + join(vars, ",") + ") "

inputArguments = "@(x,y) "

Combining the input arguments section and the vectorized equation text by appending strings lets us build the input to str2func.

f = str2func(inputArguments + vs)

f = function_handle with value:
    @(x,y)2.*x+x.^2+y

We can check that the function returns the correct result by evaluating the anonymous function and performing the same computations manually.

check1 = f(1:10, 2)

check1 = 1×10

5 10 17 26 37 50 65 82 101 122

check2 = 2*(1:10) + (1:10).^2 + 2

check2 = 1×10

5 10 17 26 37 50 65 82 101 122

isequal(check1, check2)

ans = logical
   1