How to make a graph of an equation if part of it is an array?

Question

DEHAO Lin 2020-12-18

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https://ww2.mathworks.cn/matlabcentral/answers/697245-how-to-make-a-graph-of-an-equation-if-part-of-it-is-an-array

评论： DEHAO Lin 2020-12-19

I am trying to make a graph of y, but I am getting "Unable to perform assignment because the left and right sides have a different number of elements." most of the time. I think the array r is the reason I am getting this problem, but I don't know how to make that graph happen. Can someone show my a path to solve this or provide some advice?

    h = 1;
    t = 1 : h : 100;
    r = 88 - 3.5*t;
    f = @(t,N) (r)*N*(1-N/51000000000);
    y = zeros(1,length(t));
    y(1) = 10;
    for ii = 2:length(t)
        y(ii) = y(ii-1)+ h*feval(f,t(ii-1),y(ii-1));
    end
    plot(t,y);

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Answer 1

VBBV 2020-12-18

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https://ww2.mathworks.cn/matlabcentral/answers/697245-how-to-make-a-graph-of-an-equation-if-part-of-it-is-an-array#answer_579135

编辑：VBBV 2020-12-18

在 MATLAB Online 中打开

%i
h = 1;
  t = 1 : h : 100;
  r = 88 - 3.5*t;
  f = @(t,N) (r)*N*(1-N/51000000000);
  y = zeros(length(t),length(t));
  y(:,1) = 10;
  for ii = 1:length(t)
      y(ii,:) = y(ii,:)+  h*feval(f,t(ii),y(ii));
  end
  plot(t,y);

Try this

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VBBV 2020-12-18

在 MATLAB Online 中打开

You can also try with this initial condition

%f true
y(1,:) = 10;

DEHAO Lin 2020-12-19

Thanks for the help! It works although it was not I was expected, maybe since I introduced the r(t) I should have make it a f(x,y,z) and a 3D graph. I haven't learn that in Matlab yet but I maybe soon in the future. Thanks again for finding a working code!

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Answer 2

Daniel Pollard 2020-12-18

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https://ww2.mathworks.cn/matlabcentral/answers/697245-how-to-make-a-graph-of-an-equation-if-part-of-it-is-an-array#answer_579105

You're right, the right side of line 10 has the same size as r (which itself has the same size as t), and you're trying to assign it to an element of a vector. You can't set a 1x1 object to equal a 100x1 object in matlab.

You can give f another argument, r, and then in feval call give it an argument of r(ii), or something like that. When I tried that y became mostly NaN or -Inf. I don't know if that's what you expected or not though.

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DEHAO Lin 2020-12-19

I don't know what NaN or -Inf, but I decided to replace the r(t) with a constant so that simplified the issue. I was just trying to have a graph that can show equilibrium. Thanks for the help!

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