I am getting this preallocation warning

[Jan]

Please post code as formatted text, not as screen shot. Then it can by re-used by copy&paste to create an answer.

Your get this warning. Correct. So what is your question?

[Vânia Gonçalves]

clear; clc; close all
% Function Definition (Enter your Function here):
syms X Y;
f = 100*(Y-(X^2))^2 + (1-X)^2;
% Initial Guess (Choose Initial Guesses):
x(1) = -1.1;
y(1) = 0.5;
e = 10^(-3); % Convergence Criteria
i = 1; % Iteration Counter
% Gradient and Hessian Computation:
df_dx = diff(f, X);
df_dy = diff(f, Y);
J = [subs(df_dx,[X,Y], [x(1),y(1)]) subs(df_dy, [X,Y], [x(1),y(1)])]; % Gradient
ddf_ddx = diff(df_dx,X);
ddf_ddy = diff(df_dy,Y);
ddf_dxdy = diff(df_dx,Y);
ddf_ddx_1 = subs(ddf_ddx, [X,Y], [x(1),y(1)]);
ddf_ddy_1 = subs(ddf_ddy, [X,Y], [x(1),y(1)]);
ddf_dxdy_1 = subs(ddf_dxdy, [X,Y], [x(1),y(1)]);
H = [ddf_ddx_1, ddf_dxdy_1; ddf_dxdy_1, ddf_ddy_1]; % Hessian
S = inv(H); % Search Direction

% Optimization Condition:
while norm(J) > e
    I = [x(i),y(i)]';
    lam = J*J'./(J*H*J');
    x(i+1) = I(1)-lam*S(1,:)*J';
    y(i+1) = I(2)-lam*S(2,:)*J';
    i = i+1;
    J = [subs(df_dx,[X,Y], [x(i),y(i)]) subs(df_dy, [X,Y], [x(i),y(i)])]; % Updated Jacobian
    ddf_ddx_1 = subs(ddf_ddx, [X,Y], [x(i),y(i)]);
    ddf_ddy_1 = subs(ddf_ddy, [X,Y], [x(i),y(i)]);
    ddf_dxdy_1 = subs(ddf_dxdy, [X,Y], [x(i),y(i)]);
    H = [ddf_ddx_1, ddf_dxdy_1; ddf_dxdy_1, ddf_ddy_1]; % Updated Hessian
    S = inv(H); % New Search Direction
end

[Vânia Gonçalves]

x(i+1) = I(1)-lam*S(1,:)*J';

y(i+1) = I(2)-lam*S(2,:)*J';

In this lines of code i recieved a warming from matlab about preallocating for speed, and i don't know how to preallocate

[Jan]

@Vânia Gonçalves: A general hint is to search in the net at first. Asking an internet search engine for "Matlab preallocate" shows useful explanations already.

[Vânia Gonçalves]

hi

i already searched in the internet, but i didn´t find any usefull explanation, so i hoped find help here.

but thanks anyawy

Answer 1

[Steven Lord]

This is a Code Analyzer warning not a run-time warning.

If you're not sure what preallocation is, why it can be important, or how to do it see the first example on this documentation page.

[Vânia Gonçalves]

Thank you very much !!

Answer 2

[Jan]

Seriously? If I ask Google, I get e.g.:

• https://www.mathworks.com/help/matlab/matlab_prog/preallocating-arrays.html
• https://www.mathworks.com/matlabcentral/answers/99124-how-do-i-pre-allocate-memory-when-using-matlab

For your code:

...
maxIter = 1000;
x = zeros(maxIter, 1);
y = zeros(maxIter, 1);
while norm(J) > e && i < maxIter
    ...
end
x = x(1:i);  % Crop unused elements
y = y(1:i);

[Vânia Gonçalves]

Thanks for your help

i tried you advice

is it normal to take a long time to run?

[Jan]

What does "long" mean? Seconds or hours?

Symbolic calculations need some time. You can use the profiler to find the bottleneck:

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