How can I gather all the values from a loop into an array?

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Raquel Terrer van Gool 2020-5-5

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评论： Star Strider 2020-5-5

Hi,

I was wondering how can I gather all the values into an array because I need to plot the answer later. This is part of the code I am writing:

for i=x0:h1:xx-h1 % Where h1 is the step size
    y1=y1+dy(i,y1)*h1;
    i=i+h1;
end

If someone knows, please let me know! Thanks!

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Raquel Terrer van Gool 2020-5-5

在 MATLAB Online 中打开

This is the full code. When I try to do that, I get an error saying that the array indices must be positive and logical.

yi=@(x)exp(1/3*x.^3-1.2*x);
dy=@(x,y)y*x.^2-1.2*y;
x0=0; xx=2; y1=1; y2=1;
xp=[0:0.01:2];
 
h1=0.25;
h2=0.1;
 
for i=x0:h1:xx-h1 % Where h1 is the step size
    y1=y1+dy(i,y1)*h1;
    i=i+h1;
end
 
for i=x0:h2:xx-h2 % Where h2 is the step size
    y2=y2+dy(i,y2)*h2;
    i=i+h2;
end

Raquel Terrer van Gool 2020-5-5

Thank you very much for your help!

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Answer 1

Star Strider 2020-5-5

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https://ww2.mathworks.cn/matlabcentral/answers/523158-how-can-i-gather-all-the-values-from-a-loop-into-an-array#answer_430428

编辑：Star Strider 2020-5-5

在 MATLAB Online 中打开

Knowing only what you posted, I would do something like this:

i=x0:h1:xx-h1;                          % Where h1 is the step size
y1v = zeros(size(i));                   % Preallocate
for k = 1:numel(i)
    y1=y1+dy(i,y1(k))*h1;
    y1v(k) = y1;
end

That stores the existing values of ‘y1’ as vector ‘y1v’ so it stores the values while not otherwise disrupting the code. The ‘i’ vector is now separate, so to plot them later, something like this would likely work:

figure
plot(i, y1v)
grid

I did not test this, however it should work.

EDIT —

With the full code (not available when I first posted this), it changes to:

yi=@(x)exp(1/3*x.^3-1.2*x);
dy=@(x,y)y*x.^2-1.2*y;
x0=0; xx=2; y1=1; y2=1;
xp=[0:0.01:2];
 
h1=0.25;
h2=0.1;
i1=x0:h1:xx-h1;                          % Where h1 is the step size
y1v = zeros(size(i1));                   % Preallocate
for k = 1:numel(i1)
    y1=y1+dy(i1(k),y1)*h1;
    y1v(k) = y1;
end
i2=x0:h2:xx-h2;                          % Where h2 is the step size
y2v = zeros(size(i2));                   % Preallocate
for k = 1:numel(i1)
    y2=y2+dy(i2(k),y2)*h2;
    y2v(k) = y2;
end
 
figure
plot(i1, y1v,    i2, y2v)
grid