My first fzero calculates just fine but the one after gives me an error eval: TRY must be a string

Hello guys, so I've been trying to find zero in interval for a function using fzero. I get my first answer right but when it tries to go for other ones it just gives me an error eval: TRY must be a string

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                  

x0 = a; dx = eps;
  atlikta = 0;           
while ~atlikta           
  x = x0;
  f0 = eval(f);          
  x = x + dx;
  fd = eval(f);          
  df = (fd - f0) / dx;   
  x1 = x0 - f0 / df;     
if abs(x1 - x0) < eps
  atlikta = 1;          
else
  x0 = x1;             
end                      
spr = x1;
end      

Unrecognized function or variable 'f'.

Answer 1

[Sulaymon Eshkabilov]

There are a few inconsistencies in your codes, i.e.,

f is a fcn not defined as a fcn handle or fcn file. What is fgraf() fcn?

The variable "ans" is not defined.

Just presuming that the problem exrcise and interval, the following can be used

  F=@(x) 0.1*x - sin(2*x) + 0.25;
  a=-3; b=3;
  f = fzero(F, [a, b]);
  disp(strcat("Zero found in the interval: ", num2str([a, b])));
  disp(f);
  disp("");
  
x0 = a; dx = eps;
atlikta = 0;           
while ~atlikta           
  x = x0;
  f0 = F(x);          
  x = x + dx;
  fd = F(x);          
  df = (fd - f0) / dx;   
  x1 = x0 - f0 / df;     
if abs(x1 - x0) < eps
  atlikta = 1;          
else
  x0 = x1;             
end                      
spr = x1;
end 

[Walter Roberson]

@Sulaymon Eshkabilov

ans is a special name in MATLAB. Each time there is a computation that would normally return a value, and the value is not being assigned to a variable, then MATLAB assigns the value to ans . For example,

x = 3

x = 3

Produces a value but the value is assigned to a variable, ans is not assigned to

whos

  Name      Size            Bytes  Class     Attributes

  x         1x1                 8  double              

5

ans = 5

whos, ans

  Name      Size            Bytes  Class     Attributes

  ans       1x1                 8  double              
  x         1x1                 8  double              

ans = 5

expression that produces a value, value was not assigned to a variable, ans is assigned to

x+7;
whos, ans

  Name      Size            Bytes  Class     Attributes

  ans       1x1                 8  double              
  x         1x1                 8  double              

ans = 10

Expression, output was supressed, but ans was assigned to.

Notice in all of these that even though I called whos, ans did not become the potential output of whos (whos can be called as a function.) Some functions check to see whether any outputs are requested, and if not then they do not assign to any output variable; when that happens, ans is not changed.