How can I solve system of linear equations in SymPy?
Solution 1
SymPy recently got a new Linear system solver: linsolve
in sympy.solvers.solveset
, you can use that as follows:
In [38]: from sympy import *
In [39]: from sympy.solvers.solveset import linsolve
In [40]: x, y, z = symbols('x, y, z')
List of Equations Form:
In [41]: linsolve([x + y + z - 1, x + y + 2*z - 3 ], (x, y, z))
Out[41]: {(-y - 1, y, 2)}
Augmented Matrix Form:
In [59]: linsolve(Matrix(([1, 1, 1, 1], [1, 1, 2, 3])), (x, y, z))
Out[59]: {(-y - 1, y, 2)}
A*x = b Form
In [59]: M = Matrix(((1, 1, 1, 1), (1, 1, 2, 3)))
In [60]: system = A, b = M[:, :-1], M[:, -1]
In [61]: linsolve(system, x, y, z)
Out[61]: {(-y - 1, y, 2)}
Note: Order of solution corresponds the order of given symbols.
Solution 2
In addition to the great answers given by @AMiT Kumar and @Scott, SymPy 1.0 has added even further functionalities. For the underdetermined linear system of equations, I tried below and get it to work without going deeper into sympy.solvers.solveset
. That being said, do go there if curiosity leads you.
from sympy import *
x, y, z = symbols('x, y, z')
eq1 = x + y + z
eq2 = x + y + 2*z
solve([eq1-1, eq2-3], (x, y,z))
That gives me {z: 2, x: -y - 1}
.
Again, great package, SymPy developers!
Solution 3
import sympy as sp
x, y, z = sp.symbols('x, y, z')
eq1 = sp.Eq(x + y + z, 1) # x + y + z = 1
eq2 = sp.Eq(x + y + 2 * z, 3) # x + y + 2z = 3
ans = sp.solve((eq1, eq2), (x, y, z))
this is similar to @PaulDong answer with some minor changes
- its a good practice getting used to not using
import *
(numpy has many similar functions) - defining equations with
sp.Eq()
results in cleaner code later on
Solution 4
Another example on matrix linear system equations, lets assume we are solving for this system:
In SymPy
we could do something like:
>>> import sympy as sy
... sy.init_printing()
>>> a, b, c, d = sy.symbols('a b c d')
... A = sy.Matrix([[a-b, b+c],[3*d + c, 2*a - 4*d]])
... A
⎡ a - b b + c ⎤
⎢ ⎥
⎣c + 3⋅d 2⋅a - 4⋅d⎦
>>> B = sy.Matrix([[8, 1],[7, 6]])
... B
⎡8 1⎤
⎢ ⎥
⎣7 6⎦
>>> A - B
⎡ a - b - 8 b + c - 1 ⎤
⎢ ⎥
⎣c + 3⋅d - 7 2⋅a - 4⋅d - 6⎦
>>> sy.solve(A - B, (a, b, c, d))
{a: 5, b: -3, c: 4, d: 1}
Solution 5
You can solve in matrix form Ax=b
(in this case an underdetermined system but we can use solve_linear_system
):
from sympy import Matrix, solve_linear_system
x, y, z = symbols('x, y, z')
A = Matrix(( (1, 1, 1, 1), (1, 1, 2, 3) ))
solve_linear_system(A, x, y, z)
{x: -y - 1, z: 2}
Or rewrite as (my editing, not sympy):
[x]= [-1] [-1]
[y]= y[1] + [0]
[z]= [0] [2]
In the case of a square A
we could define b
and use A.LUsolve(b)
.
Aniket Vij
Updated on July 09, 2022Comments
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Aniket Vij almost 2 years
Sorry, I am pretty new to sympy and python in general.
I want to solve the following underdetermined linear system of equations:
x + y + z = 1 x + y + 2z = 3
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Sartaj Singh almost 9 yearsIt should be noted, that linsolve is not yet available in any release. Currently accessible only through the development version.
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BhishanPoudel over 7 yearsI am using sympy 0.7.6, First I could not get linsolve so used solve, Second The Augmented matrix and Ax = b form gives EMPTY LIST [ ] answer, only first method gives solution as like above, how can we fix this?
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Brian Tompsett - 汤莱恩 about 5 yearsAdding further explanation would help readers understand why your answer is better than the others and enable them to vote for you. Expand your answer using the edit function.