error: eval: TRY must be a string

Hey guys, I'm trying to find zero in selected interval from a function graphic but all it gives me is an aerror that TRY must be a string.

The script is :

f = @(x) 0.1*x - sin(2 * x) + 0.25;
for i = 1:3
  [a, b] = fgraf(f, -3, 3);
  f = fzero(@(x) 0.1*x - sin(2*x) + 0.25, [a, b]);
  disp(strcat("Zero found in the interval: ", num2str([a, b])));
  disp(ans);
  disp("");
end

fgraf is:

function [k, d] = fgraf (fun, a, b)

dx = (b - a) / 100;                   % x step
x1 = a : dx : b;
y = x1;
[m, n] = size(x1);
for k = 1 : n                         % cycle begins
  x = x1(k);
  y(k) = eval(fun);                   % function calculation
end                                   % cycle stops
plot(x1, y);                          % forming graph
grid on; xlabel('x');
ylabel(fun);
title('Tiriamos funkcijos grafikas');
[k, ky] = ginput(1);                
[d, dy] = ginput(1);               

end

[Geoff Hayes]

@Deividas David - I don't understand this line of code (which may or may not be the source of the error)

y(k) = eval(fun);  

What are you trying to evaluate? Can't you just do

for k = 1 : n                         % cycle begins
  x = x1(k);
  y(k) = fun(x);                   % function calculation
end  

instead? Since fun is your function handle that requires an input.

Answer 1

[DGM]

Why won't something like this work?

f = @(x) 0.1*x - sin(2 * x) + 0.25;

for i = 1:3
    [a, b] = fgraf(f, -3, 3);
    z = fzero(@(x) 0.1*x - sin(2*x) + 0.25, [a, b]); % don't overwrite f
    
    fprintf('Zero found in the interval: %s\n',mat2str([a, b],5))
    fprintf('zero is at is %.4f \n\n',z) % don't use ans
end


function [k, d] = fgraf(fun, a, b)
    x = linspace(a,b,100); 
    y = fun(x); % no loops, no eval
    
    plot(x, y);
    grid on; 
    
    xlabel('x');
    ylabel(char(fun)); % fixed
    title('Tiriamos funkcijos grafikas');
    
    [k,~] = ginput(1);
    [d,~] = ginput(1);
end