can sage stop ordering terms?

savecancel

Your example:

sage: y == C.add(-A*x,hold=True)/B
y == (C - A*x)/B

Another example:

sage: a = (-2*x).add(x^2,-x^3, hold=True); a
-2*x + x^2 - x^3

Further operations reorder the expression

sage: -2*a
2*x^3 - 2*x^2 + 4*x

You have to start with a symbolic expression:

sage: -2.mul(a,hold=True)
AttributeError: 'sage.rings.integer.Integer' object has no attribute
'mul'

sage: Expression(SR,-2).mul(a,hold=True)
-2*(-2*x + x^2 - x^3)

sage: a.mul(-2,hold=True)
-2*(-2*x + x^2 - x^3)

Don't know why kcrisman gets a reordered expression

sage: B.add(A,hold=True)
B + A

Definitely we want that Sage reorders... We want A+B+A to become 2A + B...

I do not think it would be hard to manage a pretty print that put first the "positive" terms and then the negative ones (knowing that negative are considered as mul(x,-1)).

Here is a simple function that do it on the first level.

def pretty_print(E):
    if E.operator() == operator.add:
        pos = []
        neg = []
        for elt in E.operands():
            if (elt.operator() == operator.mul and elt.operands()[-1] < 0) or (elt.is_numeric() and elt < 0):
                neg.append(-elt)
            else:
                pos.append(elt)
        pos_str = ' + '.join(map(repr,pos))
        neg_str = ' + '.join(map(repr,neg))
        if len(neg) == 0:
            print pos_str
        elif len(neg) == 1:
            print pos_str + ' - ' + neg_str
        else:
            print ' + '.join(map(str,pos)) + ' - (' + ' + '.join(map(str,neg)) + ')'

    else:
        print E

Then inside Sage

sage: var('x,y,z,t')
sage: pretty_print(x-y+z)
x + z - y
sage: pretty_print(z-x-y)
z - (x + y)
sage: pretty_print(y-x-3*z+t)
t + y - (x + 3*z)
sage: pretty_print(3*x*y - 2*z*t - 3)
3*x*y - (2*t*z + 3)