Assign an input to an anonymous function

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Dave Regan
Dave Regan 2014-12-1
编辑： Walter Roberson 2025-3-27
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.

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回答（4 个）

Steven Lord
Steven Lord 2023-6-15
Ran in:
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
Dave Regan
Dave Regan 2014-12-2
编辑：Dave Regan 2014-12-2
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.
Joy Tian
Joy Tian 2014-12-16
But it doesn't explain to me what is @(x) mean. And how does it relate with anonymous function.
M_A_C
M_A_C 2023-6-15
编辑：Walter Roberson 2025-3-27
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 an excerpt 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 @()???
  4 个评论
显示 2更早的评论
M_A_C
M_A_C 2025-3-27
Steven thak you for clarifying that, my bad.
Steven Lord
Steven Lord 2025-3-27
That's okay, I just wanted to make sure you weren't being misled by something phrased incorrectly or ambiguously in the documentation.

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