How can I fix the error "Index in position 2 exceeds array bounds. Index must not exceed 1.

Hello! the code below was from a lab that was given in class. I inpputted it in MATLAB but it keeps giving me an error. I tried to change things like (),[], ',' but it still gives me the same error. I'm out of options how to fix it. Can anyone please help? thank you! the code is:

for k = 1:length(x_points)

fprintf('\nA %s exists at x = %4.2f, y = %4.2f.\n\n',outcomes{1,k},...

double(x_points(k)),double(y_points(k)))

end

fplot(y,[double(x_points(1))-1 double(x_points(length(x_points)))+1])

grid minor

[Voss]

Looks like outcomes has 1 column, but x_points has more than 1 element, so when you get to the second iteration of the for loop (k = 2), outcomes{1,k} (i.e., outcomes{1,2}) doesn't exist.

Check that outcomes and x_points and y_points are all defined correctly. You can post the code that creates those variables, if you have it, and someone can probably tell you how to fix it.

[Trixiajane Hess]

Hi so these are the codes prior to the first post:

clear,clc,close all
syms x
y = x^4-(16/3)*x^3+8*x^2-4; % initialize function
dydx = diff(y); % find derivative
d2ydx2 = diff(dydx); % find second derivative

x_points = solve(dydx); % find roots of derivative dy/dx = 0
y_points = subs(y,x,x_points);

for k = 1:length(x_points)
    eval(k) = subs(d2ydx2,x,x_points(k)); % solving 2nd derivative for x value
    if double(eval(k)) > 0 % checking to see if loocal minima
        outcomes(k) = "local minima";
    elseif double(eval(k)) == 0 % checking to see if inflection point
        outomes(k) = "inflection point";
    else
        outcomes(k) = "local maxima"; % otherwise, point is local maxima
    end
end

Thank you!

Answer 1

[Voss]

outcomes was defined as a string array, so it should be indexed with () not {}. And there was a typo after the elseif line ("outomes" instead of "outcomes"). Fixing those two things seems to have it working now:

clear,clc,close all
syms x
y = x^4-(16/3)*x^3+8*x^2-4; % initialize function
dydx = diff(y); % find derivative
d2ydx2 = diff(dydx); % find second derivative

x_points = solve(dydx); % find roots of derivative dy/dx = 0
y_points = subs(y,x,x_points);
for k = 1:length(x_points)
    eval(k) = subs(d2ydx2,x,x_points(k)); % solving 2nd derivative for x value
    if double(eval(k)) > 0 % checking to see if loocal minima
        outcomes(k) = "local minima";
    elseif double(eval(k)) == 0 % checking to see if inflection point
        outcomes(k) = "inflection point";
    else
        outcomes(k) = "local maxima"; % otherwise, point is local maxima
    end
end

for k = 1:length(x_points)
    fprintf('\nA %s exists at x = %4.2f, y = %4.2f.\n\n',outcomes(k),...
        double(x_points(k)),double(y_points(k)))
end

A local minima exists at x = 0.00, y = -4.00.


A inflection point exists at x = 2.00, y = 1.33.


A inflection point exists at x = 2.00, y = 1.33.

fplot(y,[double(x_points(1))-1 double(x_points(length(x_points)))+1])
grid minor

[Trixiajane Hess]

Oh! it worked! I brushed through the codes, rechecked it so many times but I guess I still missed the typo. thank you!

[Voss]

You're welcome! Let me know if you have any questions. If not, please mark the answer as 'Accepted'. Thanks!