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

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!

[KSSV]

cannot exacly help unless full code is known. You save values in loop as:

n = 10 ;
y = zeros(n,1) ; 
for i = 1:n
y(i) = rand ;
end

[Raquel Terrer van Gool]

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]

Thank you very much for your help!

Answer 1

[Star Strider]

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

This runs without error, and appears to produce the correct result.

[Raquel Terrer van Gool]

I get an error refering to the following line:

y1v(k) = y1

Unable to perform assignment because the left and right sides have a different number of elements.

[Star Strider]

My code ran without error when I tested it. Are you running the same code that I posted?

[Raquel Terrer van Gool]

The edit ran with no problem!

[Star Strider]

Great!