Convert symbolic inequality to matrix form

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Cedric Kotitschke
回答： Nathan Hardenberg 2023-7-4
Let's say I have a symbolic inequality:
2*x + 3*y <= 5
Is there a way to elegantly extract the matrices A, b for the equivalent inequality A*[x;y] <= b?
I know the function equationsToMatrix but that only works for equalities.
Thanks!
  3 个评论
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Nathan Hardenberg
Nathan Hardenberg 2023-7-4
Ran in:
My thought was the same as the one from @Torsten (I think). But I would not consider it an elegant way
syms x y
eq = 2*x + 3*y <= 5;
eq1 = lhs(eq) == rhs(eq) % convert to equality
eq1 = 
[A, b] = equationsToMatrix(eq1, [x y])
A = 
b = 

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回答（2 个）

Gandham Heamanth
Gandham Heamanth 2023-7-4
Yes, in MATLAB, you can use the symbolic math toolbox to extract the matrices A and b from a symbolic inequality. Here's how you can do it:
syms x y;
% Define the symbolic inequality
inequality = 2*x + 3*y <= 5;
% Extract the coefficients of x and y
coefficients = coeffs(inequality, [x, y]);
% Extract the matrix A and vector b
A = [coefficients(1), coefficients(2)];
b = coefficients(3);
% Display the matrices A and b
disp('Matrix A:');
disp(A);
disp('Vector b:');
disp(b);
Note that this code assumes you have the Symbolic Math Toolbox installed in MATLAB.
  2 个评论
Nathan Hardenberg
Nathan Hardenberg 2023-7-4
Ran in:
The coeffs functions does not seem to work.
syms x y;
% Define the symbolic inequality
inequality = 2*x + 3*y <= 5;
% Extract the coefficients of x and y
coefficients = coeffs(inequality, [x, y])
coefficients = 
% Extract the matrix A and vector b
A = [coefficients(1), coefficients(2)];
Index exceeds the number of array elements. Index must not exceed 1.

Error in indexing (line 936)
R_tilde = builtin('subsref',L_tilde,Idx);
b = coefficients(3);
% Display the matrices A and b
disp('Matrix A:');
disp(A);
disp('Vector b:');
disp(b);
As an advice: you can past your code and mark it as code. Then you can run it in the browser and it is easier to read
Cedric Kotitschke
Cedric Kotitschke 2023-7-4
But this assumes a very specific structure of the inequality. I also want to be able to deal with those structurs:
syms x y
ineq1 = 2*x <= 2-y;
ineq2 = 0 >= 4*x + 6*y;
ineq3 = 3 >= 5*y;
etc.

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Nathan Hardenberg
Ran in:
Moved my comment to an answer (to be accepted only if satisfied):
syms x y
eq = 2*x + 3*y <= 5; % inequality
eq1 = lhs(eq) == rhs(eq) % convert to equality
eq1 = 
[A, b] = equationsToMatrix(eq1, [x y]) % use equationsToMatrix-function
A = 
b = 

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