System of Linear algebraic eqn. to matrix form

Question

Nunthadech Rodcheuy 2012-6-3

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回答： Kevin O 2013-12-6

I would like to convert system of algebraic equation to Matrix form

I knew that in Mathematica "CoefficientsArray[expression,{x,y,z}]" will help me pull out those coefficients of unknowns and it can also arranges in order.

For ex. unknowns are x, y, z

eqn : 2x+3y=0

>>> then in Mathematica i can do [2 3 0] or even [3 0 2], my point is, it can manage the order . >>>However, so far in MATLAB that i knew, the cmd is "coeffs(expr)", i got [2 3] and that's it no 0 for z

Does anyone know how to assign an order as well as include 0 coefficient in the out put array?

Thk

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Answer 1

Nunthadech Rodcheuy 2012-6-5

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I've got a sol. for those who are in the same situation

try using >>>subs()

for ex.

[subs(f,{x,y,z},{0,0,1}) subs(f,{x,y,z},{0,1,0}) subs(f,{x,y,z},{1,0,0})]

result is

ans = [0 3 2]

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Answer 2

Walter Roberson 2012-6-3

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You can specify the indeterminents for ceoffs

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Nunthadech Rodcheuy 2012-6-3

Could you please give an sample code?

for ex. input 2*x+3*y+0*z output [3 0 2]

i've tried specify in the following way but it's not work for me , out put is still an equation.

syms x y z

>> coeffs(2*x+3*y,[x y z])

ans =

2*x+3*y

Anyway, thk a lot, i really appreciate your suggestion

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Answer 3

Kevin O 2013-12-6

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在 MATLAB Online 中打开

For anyone using versions earlier than 2013, which has the equationstoMatrix function, I used this block of code to create a 6X6 coefficient matrix from a vector of equations and variables. It isn't pretty, but that's probably why they added the new function:

 % for i=1:6
    K(i,:)=[subs(EQ(i),[a1 a2 a3 a4 a5 a6],[1 0 0 0 0 0]) ...
        subs(EQ(i),[a1 a2 a3 a4 a5 a6],[0 1 0 0 0 0]) ...
        subs(EQ(i),[a1 a2 a3 a4 a5 a6],[0 0 1 0 0 0]) ...
        subs(EQ(i),[a1 a2 a3 a4 a5 a6],[0 0 0 1 0 0]) ...
        subs(EQ(i),[a1 a2 a3 a4 a5 a6],[0 0 0 0 1 0]) ...
        subs(EQ(i),[a1 a2 a3 a4 a5 a6],[0 0 0 0 0 1])];
end