# Help with a vector

2 views (last 30 days)
Lavorizia Vaughn on 30 Sep 2021
Commented: Lavorizia Vaughn on 30 Sep 2021
Hello. I have come up with the following code:
function p = findmanyzeros(f, a, b, n, tol)
x = a + (b-a)*(0:n)/n;
fx = f(x);
p = [];
for i = 1:n
if sign(fx(i)) ~= sign(fx(i+1))
p(end+1) = findzero(f, x(i), x(i+1), tol);
end
end
My instructions were to Implement a MATLAB function findmanyzeros of the form function p=findmanyzeros(f, a, b, n, tol)
which finds zeros in the interval [a, b] using the following strategy:
1. Compute n+1 equidistant points xk, k=0,...,n, between a and b
2. For k = 1,...,n, if f(xk) and f(xk1) have different signs, compute a zero using findzero
3. The output vector p should contain all the computed zeros
Lavorizia Vaughn on 30 Sep 2021
Edited: Cris LaPierre on 30 Sep 2021
f= @(x) cos(x)-x
f = function_handle with value:
@(x)cos(x)-x
findzeros(f, 0, 1, 10, 10^-4)
Unrecognized function or variable 'fx'.

Error in solution>findzeros (line 7)
if sign(fx(i)) ~= sign(fx(i+1))
function p = findzeros(f,a,b,n,tol)
x=a+(b-a)*(0:n)/n;
p=[];
for i = 1:n
if sign(fx(i)) ~= sign(fx(i+1))
p(end+1) = findzero(f, x(i), x(i+1), tol);
end
end
end

Cris LaPierre on 30 Sep 2021
Edited: Cris LaPierre on 30 Sep 2021
You have not defined a variable fx inside your findzeros function. Do you mean to use f?
You have created a recursive function but you have not defined an exit criteria. Since sign(f(i) always is the same as sign(f(i+1)), p is never assigned a value, and your result is an empty vector.
f= @(x) cos(x)-x;
f(1:10)
ans = 1×10
-0.4597 -2.4161 -3.9900 -4.6536 -4.7163 -5.0398 -6.2461 -8.1455 -9.9111 -10.8391
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Lavorizia Vaughn on 30 Sep 2021
thank you ive figured it out.

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