Saving output of for loop after each iteration (for a function)

Question

JD 2021-4-1

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/789589-saving-output-of-for-loop-after-each-iteration-for-a-function

评论： Image Analyst 2021-4-2

y = 0:0.05:1;
u= 2*y-(2*y.^3)+y.^4;
a = 0:0.01:1;
b = 4000; 
Carray=zeros(1,101)
for i=1:size(a)
        c=A_function(y,u,b,a(i));
        Carray(i)=c;
end

I want to run the function 'c' for all values of 'a'

and I want to store value of 'c' for each 'a'

The error i get is "Unable to perform assignment because the left and right sides have a different number of elements"

Can someone please let me know what I am doing wrong?

Thanks

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Answer 1

Sulaymon Eshkabilov 2021-4-1

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https://ww2.mathworks.cn/matlabcentral/answers/789589-saving-output-of-for-loop-after-each-iteration-for-a-function#answer_664029

编辑：Sulaymon Eshkabilov 2021-4-1

在 MATLAB Online 中打开

Here is the corrected code of yours:

y = 0:0.05:1;
u= 2*y-(2*y.^3)+y.^4;
a = 0:0.01:1;
b = 4000; 
C=zeros(numel(a),numel(u));
for ii=1:size(a)
        C(ii,:)=A_function(y,u,b,a(ii));
end

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JD 2021-4-1

I fixed the error by changing this line “For ii=1:size(a)-1”

But now my C matrix is still all 0’s

Steven Lord 2021-4-1

在 MATLAB Online 中打开

%{
C=zeros(numel(a),numel(u));
for ii=1:size(a)
%}

I agree with the first line but not the second. Before looking at the output of the code below, how many lines of text does it display?

a = 0:0.1:1;
for k = 1:size(a)
fprintf("k is %d\n", k)
end
k is 1

Now compare this with using numel instead of size.

a = 0:0.1:1;
for k = 1:numel(a)
fprintf("k is %d\n", k)
end
k is 1
k is 2
k is 3
k is 4
k is 5
k is 6
k is 7
k is 8
k is 9
k is 10
k is 11

Why the difference in behavior? When any of the inputs to the colon function or : operator is non-scalar and non-empty, MATLAB will just use the first element of that input.

sz = size(a)
sz = 1×2
     1    11
v = 1:sz % equivalent to 1:sz(1)
v = 1

While your code would have worked for a column vector, the numel approach works in general to allow you to loop through all the elements in an array.

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Answer 2

Image Analyst 2021-4-1

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https://ww2.mathworks.cn/matlabcentral/answers/789589-saving-output-of-for-loop-after-each-iteration-for-a-function#answer_664074

在 MATLAB Online 中打开

Try this:

y = 0:0.05:1;
u = 2*y-(2*y.^3)+y.^4;
a = 0:0.01:1;
b = 4000;
rows = length(a)
columns = 101;
Carray = zeros(rows, columns);
for row = 1 : rows
	resultVector = A_function(y, u, b, a(row));
	Carray(row, :) = resultVector;
end
function c = A_function(y, u, b, aValue)
c = randi(9, 1, 101); % Whatever it may be...
end

Obviously replace the A_function with your actual function that returns a vector. Carray will not be all zeros.

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JD 2021-4-1

编辑：JD 2021-4-1

在 MATLAB Online 中打开

function c=OS_Eigen2D(y,u,b,a,N)
% Set up as a function, the following input parameters are required.
% INPUT:
% y  
% u 
% b 
% a
% N     =   (Optional) 
Discretization order for Chebyshev polynomials
%           N is defaulted to be N= 101 if otherwise specified; No need to
%           change in general
% OUTPUT
% c  
% N is the number of data points for the Gauss-Lobato interpolation (optional)
 if ~exist('N','var')
     % N parameter does not exist, so default it to something
      N = 101;
 end
%%SET-UP DIFFERENTIATION MATRICES
[D,yGL]=cheb(N);
D2=D^2;
D2=D2(2:N,2:N);
S=diag([0; 1./(1-yGL(2:N).^2); 0]);
D4=(diag(1-yGL.^2)*D^4-8*diag(yGL)*D^3-12*D^2)*S;
D4=D4(2:N,2:N);
D0=eye(N-1);
%%INTERPOLATE TO GAUSS-LOBATTO POINTS AND TRANSFORM COORDINATES
ymax=max(y);
y_trnsfrm=2.*y./ymax-1;
dzdy=2/ymax;
uGL=interp1(y_trnsfrm,u,yGL,'spline');
dduGL=(dzdy^2)*(D^2)*uGL;
uGL=uGL(2:end-1);
dduGL=dduGL(2:end-1);
D2=(dzdy^2)*D2;
D4=(dzdy^4)*D4;
%%DISCRETIZE O-S EQUATION
ak2=a^2;
R0=D2-ak2*D0;
R1=(1i/b/a)*(D4-2*ak2*D2+(ak2^2)*D0);
R1=R1+(uGL*ones(1,length(uGL))).*R0-(dduGL*ones(1,length(uGL))).*D0;
%%SOLVE
c=eig(R1,R0);
end
%%Chebyshev  compute D = differentiation matrix, x = Chebyshev grid
 function [D,x] = cheb(N)
  if N==0, D=0; x=1; return, end
  x = cos(pi*(0:N)/N)'; 
  c = [2; ones(N-1,1); 2].*(-1).^(0:N)';
  X = repmat(x,1,N+1);
  dX = X-X';                  
  D  = (c*(1./c)')./(dX+(eye(N+1)));      % off-diagonal entries
  D  = D - diag(sum(D'));                 % diagonal entries
 end

JD 2021-4-1

Hi Image Analyst,

Did that work for you? I have to turn in my work tonight so just wondering if you were able to troubleshoot why it’s not working with the actual function.

Thanks