How do I solve "Conversion to logical from sym is not possible" error on line 26

clear; clc;

sym h;

f = @(x) 10*pi*h^2-((pi*h^3)/3)-1000;

F = @(x) 20*pi*h-pi*h^2; %calculated by hand

n = 6;

eps = 1*10^-(n+1); %epsilon value

h0 = 6;

for i = 1:20

f0 = vpa(subs('f','h','h0'));

f0_prime = vpa(subs('F','h','h0'));

Y = (h0-f0)/f0_prime; % The Formula

error = abs(Y-h0);

if error < eps %checking the error value after each iteration <===== LINE 26

break

end

h0 = Y;

end

Y = Y - rem(Y,10^-n)

Answer 1

[Jemima Pulipati]

Hello,

From my understanding, you are trying to use a symbolic variable in an 'if' condition. But the if condition checks for a definite value and here since there is a symbolic variable, it throws an error.

Here are few observations from the code:

1. The function handles have to be defined with the variable that is present in the expression
2. While passing a function handle to subs(), you need not place it in quotes as this is a variable
3. You may have to use double() to actually substitute the numeric values while referencing f0, f0_prime.

Here is the modified code

clear; clc;
sym h;
f = @(h) 10*pi*h^2-((pi*h^3)/3)-1000; 
F = @(h) 20*pi*h-pi*h^2; %calculated by hand
n = 6;
eps = 1*10^-(n+1); %epsilon value
h0 = 6;
for i = 1:20
f0 = vpa(subs(f,'h','h0'));
f0_prime = vpa(subs(F,'h','h0'));
Y = (h0-double(subs(f0)))/double(subs(f0_prime)); % The Formula
error = abs(Y-h0);
if error < eps     %checking the error value after each iteration <===== LINE 26
break
end
h0 = Y;
end
  
Y = Y - rem(Y,10^-n)

Here are some answers from the community which might be of relevance to you.

https://www.mathworks.com/matlabcentral/answers/69143-conversion-to-logical-from-sym-is-not-possible-error-shows-up-when-i-try-to-execute-the-following

https://www.mathworks.com/matlabcentral/answers/383061-conversion-to-logical-from-sym-is-not-possible

https://www.mathworks.com/matlabcentral/answers/264727-how-to-sole-the-solve-the-error-conversion-to-logical-from-sym-is-not-possible-in-my-program

https://www.mathworks.com/matlabcentral/answers/319055-problem-with-conversion-to-logical-from-sym-is-not-possible