Need help with error in for loop

Using Gold-Section search method to determine max value of f(x)

want to iterate 20 time and each time making the braket smaller to find value

clc
clear
r=((sqrt(5)-1)/2);
d=r*(6)
xL(1)=-2;
xu(1)=4;
x1(1)=xL+d;
x2(1)=xu-d;
f1= zeros(1,20);
f2= zeros(1,20);


for i=1:20
    f1(i)=4*x1(i)-1.8*(x1(i)^2)+1.2*(x1(i)^3)-0.3*(x1(i)^4)
    f2(i)=4*x2(i)-1.8*(x2(i)^2)+1.2*(x2(i)^3)-0.3*(x2(i)^4)
    d(i)=r*(xu(i)-xL(i));
    if f1>f2;
        xL(i+1)=x2(i);
        xu(i+1)=xu(i);
        x1(i+1)=x2(i)+d(i)
        x2(i+1)=x1(i)
    elseif f2>f1;
        xL(i+1)=xL(i);
        xu(i+1)=x1(i);
        x1(i+1)=x2(i)
        x2(i+1)=x1(i)-d(i)
    end
end

Get error:

Index exceeds the number of array elements (1).

Error in Doherty_Nicholas_BIME450_Hw12 (line 14)

f1(i)=4*x1(i)-1.8*(x1(i)^2)+1.2*(x1(i)^3)-0.3*(x1(i)^4)

[KSSV]

Your variables all are scalars and you are trying to access them as vectors.

Answer 1

[Jan]

Your code tests f1<f2 and f1>f2, but not f1 == f2. In this case x1(i+1) and x2(i+2) are not created.

r = (sqrt(5)-1) / 2;
d = r * 6;
xL(1)=-2;
xu(1)=4;
x1(1)=xL+d;
x2(1)=xu-d;
f1= zeros(1,20);
f2= zeros(1,20);
for i = 1:20
    f1(i) = 4 * x1(i) - 1.8 * (x1(i)^2) + 1.2 * (x1(i)^3) - 0.3 * (x1(i)^4);
    f2(i) = 4 * x2(i) - 1.8 * (x2(i)^2) + 1.2 * (x2(i)^3) - 0.3 * (x2(i)^4);
    d(i)  = r * (xu(i) - xL(i));
    if f1 > f2
        xL(i+1) = x2(i);
        xu(i+1) = xu(i);
        x1(i+1) = x2(i) + d(i);
        x2(i+1) = x1(i);
    elseif f2>f1
        xL(i+1) = xL(i);
        xu(i+1) = x1(i);
        x1(i+1) = x2(i);
        x2(i+1) = x1(i) - d(i);
    else  % f1 == f2: What should happen then?!
        xL(i+1) = xL(i);
        xu(i+1) = xu(i);
        x1(i+1) = x1(i);
        x2(i+1) = x2(i);
    end
end

[Nick Doherty]

Thank you that helps a little bit

I am still getting the error where my arrays are not changing value are i increases. For example, my f1 and f2 are calculated once and stay constant for the 1x20double

clc

clear

r=((sqrt(5)-1)/2);

f1= zeros(1,20);

f2= zeros(1,20);

for i=1:20

d(1)=r*(6)

xL(1)=-2;

xu(1)=4;

x1(1)=xL(1)+d(1);

x2(1)=xu(1)-d(1);

f1(i)=4*x1(i)-1.8*(x1(i)^2)+1.2*(x1(i)^3)-0.3*(x1(i)^4)

f2(i)=4*x2(i)-1.8*(x2(i)^2)+1.2*(x2(i)^3)-0.3*(x2(i)^4)

d(i)=r.*(xu(i)-xL(i));

if f1>f2;

xL(i+1)=x2(i);

xu(i+1)=xu(i);

x1(i+1)=x2(i)+d(i);

x2(i+1)=x1(i);

elseif f2>f1;

xL(i+1)=xL(i);

xu(i+1)=x1(i);

x1(i+1)=x2(i);

x2(i+1)=x1(i)-d(i);

else f1==f2

xL(i+1) = xL(i);

xu(i+1) = xu(i);

x1(i+1) = x1(i);

x2(i+1) = x2(i);

end

end

[Nick Doherty]

Nevermind, solved.

For every check in the if statement, make sure it is f1(i) and f2(i)

Thanks