Array indices must be positive integers or logical values. Error in sym/subsref (line 870)

I have been having an issue with my code, I am writting a direct method for finding a root from a user input. However, I spent an hour to debug my code with no improvement. A help from the community would be very apperciated.

The error message output is as follow:

Array indices must be positive integers or logical values.

Error in sym/subsref (line 870)

R_tilde = builtin('subsref',L_tilde,Idx);

Error in direct_method (line 21)

fx_value(ii)=fx_given(x(ii))

thank you in advance, my code is as follow:

function Root_Found=direct_method

syms x

fx_given=input('Input the desired function:');
x1=input('Input the lower limit guess value of the root:');
x2=input('Input the upper limit guess value of the root:');
fprintf("The chosen roots values: [x1,x2]=[%.1f,%.1f] \n",x1,x2);
N=input('Input the number of iterations desired:');

x=linspace(x1,x2,N)';
fx_value=zeros(N);

for ii=1:N
fx_value(ii)=fx_given(x(ii))
end

figure
plot(x,fx_value)
title('Direct Method')
xlabel('x')
ylabel('y')
grid on

Root_Found=fx_value;
end

Answer 1

[Walter Roberson]

syms x
fx_given=input('Input the desired function:');

We can predict from the error message that whatever the user is entering in response to the prompt is not either an anonymous function or a symbolic function defined with symfun() . For example the user may be entering a formula such as

x^2 + 3*x - sin(x)

Then

fx_value(ii)=fx_given(x(ii))

with fx_given not being a function handle or symbolic function, fx_given(VALUE) would be an attempt to index fx_given at location indicated by VALUE.

If the user is entering a symbolic expression, then you should use matlabFunction to convert the user expression into a function handle, or else you should code as

fx_given(x) = input('Input the desired function:');

[Hadi Alawwad]

thank you Mr. Roberson, that fixed my code. much appeciated.

Answer 2

[Image Analyst]

A thorough discussion of the error is in the FAQ:

https://matlab.fandom.com/wiki/FAQ#.22Subscript_indices_must_either_be_real_positive_integers_or_logicals..22

[Hadi Alawwad]

thank you for the link, I will be adding it to my resources.