if Loop to print values into a variable

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chong kai sheng 2020-6-4

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关闭： MATLAB Answer Bot 2021-8-20

How do i run though the loop and print the valid numbers in the 'value' variable?

The condition is to determine the 𝑟-value cases that achieve 𝐵 ≥ 10 at 𝑡 = 5.

colourmap = [228 26 28; 55 126 184; 77 175 74; ...
    152 78 163; 255 127 0]/255;
%variables
r = [0.05 0.1 0.5 1 10];
t = (0:0.01:100);
k = 15
B0 = 1;
%
for i = 1: length(r)
    dBdt =@(t,B) (r(i)).*B.*(1-(B./k));
    [t,B] = ode45(dBdt,t,B0);
    figure(6);
    loglog(t,B,'color',colourmap(i,:))
    grid on;
    hold on;
    if B>= 10
       value = []
    end
    
end
xlabel('Time');
ylabel('Bacteria growth');
title('B against t');
legend(num2str(r','r = %5.2f'),'location','northwest');

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Answer 1

Star Strider 2020-6-4

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Change the loop slightly, using the two new commented lines:

for i = 1: length(r)
    dBdt =@(t,B) (r(i)).*B.*(1-(B./k));
    [t,B] = ode45(dBdt,t,B0);
    figure(6);
    loglog(t,B,'color',colourmap(i,:))
    grid on;
    hold on;
    idx = B>= 10;                                           % Logic Index Vector
    value{i,:} = [t(idx), B(idx)];                          % Save To Cell Array
end

That ran without error whan I tested it just now.

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chong kai sheng 2020-6-4

Tks for the help.

However the ans i needed to achieve was r = 1 , 10

I initally though i had to loop though to find the values that meet the condition but i ran into a wall and got stuck.

Star Strider 2020-6-4

在 MATLAB Online 中打开

My pleasure.

I am not certain that I understand what you want to do.

Try this:

for i = 1: length(r)
    dBdt =@(t,B) (r(i)).*B.*(1-(B./k));
    [t,B] = ode45(dBdt,t,B0);
    figure(6);
    loglog(t,B,'color',colourmap(i,:))
    grid on;
    hold on;
    Bmtx(:,i) = B;                                          % Save To Matrix
end
t5idx = find(t >= 5, 1, 'first');                           % Time Index
colidx = find(Bmtx(t5idx,:) >= 10, 1, 'first');             % Column Index
value = Bmtx(t5idx,colidx)                                  % ‘value’