Plotting bisect method in matlab - matlab

Question

So I had a problem in which I needed to find roots using the bisect method:
Function:
function [ c,k ] = bisect(f,a,b,tol)
k=0;
while b-a > tol
c=(a+b)/2;
if sign(f(c)) == sign(f(b))
b=c;
else
a=c;
end
k=k+1;
end
Script:
f = #(x) (((1800).*log((160000)./(160000 - (x.*2600))) - (9.812).*x)./750) - 1;
a = 10;
b = 50;
tol = 1e-4;
[root, iter] = bisect(f,a,b,tol);
fprintf(' iterations = %i root = %15.10e ' ,iter, root);
This works perfectly, now I need to use an embedded function in Matlab to find the value of c for different values of q (in the above example q is a fixed number 2600) from 2000 to 3000 in increments of 10 and plot x vs q. In order to do this I have the following script:
function myFunction
h = 10;
a = 10;
b = 50;
tol = 1e-4;
function y = f(x)
y = (((1800).*log((160000)./(160000 - (x.*q))) - (9.812).*x)./750) - 1;
end
for q = (2000:h:3000)
k=0;
while b-a > tol
c=(a+b)/2;
if sign(f(c)) == sign(f(b))
b=c;
cArray(q) = c;
else
a=c;
cArray(q) = c;
end
k=k+1;
end
end
plot(q,cArray)
end
This code has no errors but when I run it there is no graph. Can someone please help me with this issue? I don't even know if my code to find c vs q is correct.

Answer 1

Major issues with your myFunction code:
The endpoints a,b should be reset within the for loop, so that the root search begins anew.
Using q as index in cArray(q) results in a too-large array filled with zeros for indices below 2000. You need an index variable to keep track of that array. (I used j below)
plot(q,cArray) should be plot(2000:h:3000,cArray), because q is not an array, it's a scalar.
Minor issues:
You never used k
You assign to cArray(q) within the while loop; this only needs to be done once the loop has ran.
Corrected function:
function myFunction
h = 10;
tol = 1e-4;
function y = f(x)
y = (((1800).*log((160000)./(160000 - (x.*q))) - (9.812).*x)./750) - 1;
end
j = 0;
for q = 2000:h:3000
a = 10;
b = 50;
while b-a > tol
c=(a+b)/2;
if sign(f(c)) == sign(f(b))
b=c;
else
a=c;
end
end
j=j+1;
cArray(j)=c;
end
plot(2000:h:3000,cArray)
end
and its output: