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

2 次查看（过去 30 天）
Deividas David2021-11-21

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'.

### 回答（1 个）

Sulaymon Eshkabilov 2021-11-21
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
##### 1 个评论显示隐藏 无
Walter Roberson 2021-11-21
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.

### Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by