How can I solve an equation in a While loop?

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mr mo
mr mo 2018-4-12
评论： Walter Roberson 2018-4-16
Hi. I have a equation like below.
[m1 m2 ; m3 m4] * [d1 ; d2] = [c1 ; c2]
The values of m1, m2, m3, m4 and c1, c2 change in a while loop.
I want to find the d1 and d2 values in every Iteration of the mentioned while loop.
I wrote this, but some times it return Nan values.
[d1 ; d2] = [m1 m2 ; m3 m4] \ [c1 ; c2];
How can I do that?
Thanks a lot.

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回答（2 个）

Walter Roberson
Walter Roberson 2018-4-12
That equation does not always have solutions. Your m might be singular or the calculation might overflow or might contain inf in ways lead to nan.
If you have the symbolic toolbox sometimes you can get further by putting sym() around the two sides of the \ operator, but if that helps you probably need to go back and do the calculation of the m and c values as sym.
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mr mo
mr mo 2018-4-16
编辑：mr mo 2018-4-16
Hi. Thanks. m1, m2, m3, m4, c1 and c2 are some values that changes in every iteration of the while loop.
For example they might be equal to these values 2.31, 0.94, 0.94, 0.44, -0.15, -0.03 respectively.
These values changes in the next iteration and so on. but at the end by using the above mentioned code, I receive Nan values of d1 and d2, and I am confused what to do.
Walter Roberson
Walter Roberson 2018-4-16
Please follow the debugging instructions I indicated with dbstop and so on.

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Torsten
Torsten 2018-4-12
Maybe m1, m2, m3, m4, c1, c2 are NaN ?
And use
D = [m1 m2 ; m3 m4] \ [c1 ; c2];
d1 = D(1);
d2 = D(2);
And always make sure that det(M) does not equal 0 before computing D.
Best wishes
Torsten.

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