python: less ugly solution to the intersection of 3 lines in 3 dimensional space

I've tagged this as homework because it was originally a school project, it was accepted but I'm not satisfied with the code.

I've used cramers rule to find the intersection of 3 lines in three dimensional space given the x, y and z coefficients but the code is hideous.

```
def trinomial():

print "Trinomials require 3 equations with three variables each."
firstX = input ("Please enter the coeficcient of X in the first equation: ")
firstY = input ("Please enter the coefficient of Y in the first equation: ")
firstZ = input ("Please enter the coefficient of Z in the first equation: ")
firstA = input ("and what's after the = sign?")
secondX = input ("Please enter the coefficient of X in the second equation: ")
secondY = input ("Please enter the coefficient of Y in the second equation: ")
secondZ = input ("Please enter the coefficient of Z in the second equation: ")
secondA = input ("And what's after the = sign?")
thirdX = input ("Please enter the coefficient of X in the third equation: ")
thirdY = input ("Please enter the coefficient of Y in the third equation: ")
thirdZ = input ("Please enter the coefficient of Z in the third equation: ")
thirdA = input ("And what's after the = sign?")
D= firstXsecondYthirdZ+firstYsecondZthirdX+firstZsecondXthirdY-thirdXsecondYfirstZ- thirdYsecondZfirstX-thirdZsecondXfirstY
DX = firstAsecondYthirdZ+firstYsecondZthirdA+firstZsecondAthirdY-thirdAsecondYfirstZ-thirdYsecondZfirstA-thirdZsecondAfirstY
DY = firstXsecondAthirdZ+firstAsecondZthirdX+firstZsecondXthirdA-thirdXsecondAfirstZ-thirdAsecondZfirstX-thirdZsecondXfirstA
DZ = firstXsecondYthirdA+firstYsecondAthirdX+firstAsecondXthirdY-thirdXsecondYfirstA-thirdYsecondAfirstX-thirdAsecondXfirstY
#the next two if statements evaluate the D's, or determinants, to see if it's an inconsistant or identical systerm

• Instead of having variables firstX, secondX, thirdX, replace them with a list X

• Don't put input and calculations in the same function, separate them

-
You do:

if D == 0:
some stuff
if D != 0:
other stuff

use else instead

• Your three lines calculating D, DX, DY, DZ are very similar. Make it a function.

• The terms being subtractd parallel the ones being added. Extract the pair into a function.

Here is my result:

DEPENDENT = 0
INCONSISTENT = 1
GOOD = 2

def determinant(X, Y, Z):
    def term(x, y, z):
        return X[x]*Y[y]*Z[z] - X[z]*Y[y]*Z[z]
    return term(0, 1, 2) + term(2, 0, 1) + term(1,2,0)

def trinomial(X, Y, Z, A):

    D  = determinant(X, Y, Z)
    DX = determinant(A, Y, Z)
    DY = determinant(X, A, Z)
    DZ = determinant(X, Y, A)
#the next two if statements evaluate the D's, or determinants, to see if it's an inconsistant or identical systerm
#if it's not, then it does the math to solve the equation
    if D == 0:
       if DX == 0 and DY == 0 and DZ == 0:
           return DEPENDENT, None
       else:
           return INCONSISTENT, None
    else:
       return GOOD, [DX/D, DY/D, DZ/D]

def main():
    print "Trinomials require 3 equations with three variables each."
    X.append( input ("Please enter the coeficcient of X in the first equation: ") )
    Y.append( input ("Please enter the coefficient of Y in the first equation: ") )
    Z.append( input ("Please enter the coefficient of Z in the first equation: ") )
    A.append( = input ("and what's after the = sign?") )

    X.append( input ("Please enter the coeficcient of X in the second equation: ") )
    Y.append( input ("Please enter the coefficient of Y in the second equation: ") )
    Z.append( input ("Please enter the coefficient of Z in the second equation: ") )
    A.append( = input ("and what's after the = sign?") )

    X.append( input ("Please enter the coeficcient of X in the third equation: ") )
    Y.append( input ("Please enter the coefficient of Y in the third equation: ") )
    Z.append( input ("Please enter the coefficient of Z in the third equation: ") )
    A.append( = input ("and what's after the = sign?") )

    result, answer = trinomial(X, Y, Z, A)
    if result == DEPENDENT:
        print "System is dependant, there are infinite solutions"
        print
    elif result == INCONSISTENT:
        print "System is inconsistant, there are no solutions"
        print
    else:
        print "the solution to the set of equations is ("+ ",".join(map(str, answer)) + ")."
        print

But if you do any amount of python programming with this stuff you'll want to learn how to use numpy, which make your code:

def trinomial(X, Y, Z, A):
    system = numpy.array([X, Y, Z]).T
    unknowns = numpy.array([A])

    return numpy.linalg.solve(system, unknowns)

StackExchange Code Review Q#2895, answer score: 4