# Need some clarification

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Hey all, I've got some code that's trying to solve a system of 6 equations using fsolve. It runs a loop, because the six equations represent variables in a data set, so multiple iterations are desired. Here is the code itself:

%data.csv should be defined as the file with measured values.

data = csvread('data_c.csv');

assert (mod(size(data, 1), 6) == 0, ...

'Input data must have an integer multiple of 6 rows');

assert (size(data, 2) == 2, ...

'Input data must have exactly two columns.');

nsys = size(data, 1) / 6;

soln = zeros(nsys, 6);

options=optimset('MaxFunEvals',1e10,'MaxIter',25000);

for k = 1 : nsys,

F = c_generate(data(6*(k-1) + (1:6), 1:end));

guess = [1 1 1 1 1 1];

soln(k, :) = fsolve(F, guess,options);

end

fid=fopen('results.csv','w');

fprintf(fid,'%5.5f, %5.5f, %5.5f, %5.5f, %5.5f, %5.5f\n',soln);

fclose(fid);

And here is the function file:

function F = c_generate(data)

assert (ndims(data) ==2, ...

'System parameters ''p'' must be 2D matrix.');

assert (all(size(data) ==[6,2]), ...

'System parameters must be 6-by-2 matrix.');

y = data(:,1);

n = data(:,2);

F = @(x) (x(1)+(x(2).*(y.^2))+(x(3)/(y.^2))+(x(4)/(y.^4))+(x(5)/(y.^6))+(x(6)/(y.^8))-n.^2);

end

When I run it, I get an error:

??? Error using ==> plus

Matrix dimensions must agree.

Error in ==>

c_generate>@(x)(x(1)+(x(2).*(y.^2))+(x(3)/(y.^2))+(x(4)/(y.^4))+(x(5)/(y.^6))+(x(6)/(y.^8))-n.^2)

at 9

F = @(x)

(x(1)+(x(2).*(y.^2))+(x(3)/(y.^2))+(x(4)/(y.^4))+(x(5)/(y.^6))+(x(6)/(y.^8))-n.^2);

Which leads to the usual red cascade. Since I'm using the skeleton of an older function file to create this new one, I don't know what it means by matrix sizes must agree. Since the x's are being solved for, don't they represent 1x1 matrices? How should I fix this, what does this mean?

##### 0 Comments

### Accepted Answer

Matt Tearle
on 17 Feb 2012

Just from looking at the code, it looks as though you're dividing a scalar by a column vector:

y = data(:,1);

...

...x(3)/(y.^2)...

This will result in a matrix right-divide, which will return a row vector. But

x(2).*(y.^2)

will give a column vector. And adding these together, of course, gives our good friend "Matrix dimensions must agree".

I suspect you want to do ./ instead of /.

##### 2 Comments

Matt Tearle
on 17 Feb 2012

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