Warning: Variable appears to change size on every loop iteration (within a script). Consider preallocating for speed.

Hello. I am working on a project where I've created a while-loop to calculate 3 power values and 4 cost values. I need to run this loop 60 times. I receive the warning shown in the summary and 3x4 double for my cst variable and 4x3 double in my power variable. I'm trying to store the values in a table so I can see all 60 values of P1, P2, P3, F1, F2, F3, and Ft. I'm very new to MATLAB and I cannot figure out how to get past the fact that I can apparently only get to 12 values each for cost and power. Here is my code:

% Group 2 - Part I.b. Optimal Power Flow with Gradient Search
% Start fresh - clear command window, variables, and plots
clc, clearvars, close all
% Initialize the values
P1 = 300;
P2 = 200; 
P3 = 850 - P1 - P2;
n = 0;
k = 60;
alpha = 10;

% Initialize the cost functions
F1 = 561 + 7.92 * P1 + 0.00156 * P1^2;
F2 = 310 + 7.85 * P2 + 0.00194 * P2^2;
F3 = 78 + 7.97 * P3 + 0.00482 * P3^2;
Ft = F1 + F2 + F3;

% Initialize the gradient of the cost functions
gradientF1 = (7.92 + 0.003124 * P1 - 7.97 - 0.00964 * P3); 
gradientF2 = (7.85 + 0.00388 * P2 - 7.97 -0.00964 * P3); 
gradientF3 = [gradientF1;gradientF2]; 

while n < k
    x = [P1;P2];
    n = n + 1;
    % Update power values
    P1 = P1 - alpha * gradientF1;
    P2 = P2 - alpha * gradientF2;
    P3 = 850 - P1 - P2;
    power(n,1) = P1; %#ok<*SAGROW>
    power(n,2) = P2;
    power(n,3) = P3;
    cst(n,1) = F1;
    cst(n,2) = F2;
    cst(n,3) = F3;
    cst(n,4) = Ft;
end 

% Initialize arrays
power_array = zeros(3,60);
cst_array = zeros(4,60);

% Store power values
power_array(k, :) = [P1, P2, P3];

% Store cost values
cst_array(k, :) = [cst1, cst2, cst3, cst4];

% Display results
disp('Final Power Values:');
disp(power_array(end, :));
disp('Final Cost Values');
disp(cst(end, :));

% Plot power values
figure;
plot(3:60, power);
title('Power Values');
xlabel('Iterations');
ylabel('Power (MW)');
legend('P1', 'P2', 'P3');

% Plot cost values
figure;
plot(4:60, cst);
title('Cost Values');
xlabel('Iterations');
ylabel('Cost ($/hr)');
legend('F1', 'F2', 'F3', 'F4');

Answer 1

[Walter Roberson]

power = zeros(k, 3);
cst = zeros(k, 4);

before the while loop.