Greetings,
I am a completely new to SAGE as of a few days ago. I have used Maple
and Mathematica for years, and it is easy to do what I am describing
below in those systems. I assume it is also easy to do in sage, but I
have not been able to find it in the documentation.
Here's the story:
I am attempting to write a sage function where I need to solve a
system of equations, and then plug these solutions into an expression,
and then process the result further.
Since the solutions to the system come wrapped in brackets, and
subs_expr expects the substitution equations without any brackets
around them I do not see how to do this.
Here is a toy example in interactive mode:
***********************************************************************
sage: y=var('y')
sage: z=var('z')
sage: solns = solve( [x+y+z==5, y+z==3, x+z==1], [x,y,z])
sage: solns
[[x == 2, y == 4, z == -1]]
sage: ( x + y + z ).subs_expr(solns)
---------------------------------------------------------------------------
TypeError Traceback (most recent call
last)
/Users/andrewsills/.sage/<ipython console> in <module>()
/Applications/sage/local/lib/python2.5/site-packages/sage/calculus/
calculus.pyc in subs_expr(self, *equations)
3922 for x in equations:
3923 if not isinstance(x, SymbolicEquation):
-> 3924 raise TypeError, "each expression must be an
equation"
3925 R = self.parent()
3926 v = ','.join(['%s=%s'%(x.lhs()._maxima_init_(), x.rhs
()._maxima_init_()) \
TypeError: each expression must be an equation
*****************************************************************************
However, if I manually cut and paste my solutions into the expression,
I get the desired result:
*******************************************************************
sage: ( x + y + z ).subs_expr(x == 2, y == 4, z == -1)
5
*******************************************************************
Of course, I cannot manually cut and paste in the middle of a
function.
So, it would seem that I either need to find a way to, in effect,
remove the brackets that naturally occur in [[ x == 2, y == 4, z ==
-1 ]], or alternatively, find another way to substitute into ( x + y +
z ) where it is acceptable to have the brackets there.
I am sure this is extremely simplistic, and I appreciate your
patience.
Thanks,
Drew
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