On Fri, Jun 21, 2013 at 5:48 PM, peibol <pabe...@gmail.com> wrote:
> Hi all
>
> I've been searching just to check if what I want to do is feasible with
> sympy. The thing is that I have an expression ('a/b+c/d') for which I want
> to give several sets of values and for each set render the latex without
> evaluating it. I don't know how to avoid automatic evaluation and
> simplification. Any help would be fantastic!
>
> a,b,c,d = symbols('a b c d')
> fvars = [a,b,c,d]
> values = [1,4,2,4]; #example: 1/4+2/4
> exp=sympify('a/b + c/d'); #here I define the algebraic expression
> res=nsimplify(exp.evalf(subs = dict(zip(fvars,values)))) #here I get
> the result for later use
Why don't you just use subs here? Are your typical values floats
rather than ints?
>
> straux=urllib.quote(printing.latex(exp)) #urllib is for making the
> expression URL friendly... the thing us that latex(exp)
> #I have tried
> exp.subs(a,1...) and eval (making a=1, b=4... and it gives 3/4, not 1/4 +
> 2/4
>
> strchart='http://chart.apis.google.com/chart?cht=tx&chs=140x50&chf=bg,s,FFFFFF00&chl='
> + straux
>
> Thanks!
The classical way to get these expressions to not evaluate is to use
Add(evaluate=False). If all you care about is printing, you can use
this trick
In [25]: (a/b + c/d).subs([(var, Symbol("%s" % i)) for var, i in
zip(fvars, values)])
Out[25]:
1 2
─ + ─
4 4
In other words, use a Symbol with the name of the function you want.
Unfortunately, this still won't preserve the input order, which is
currently impossible to do. I think for printing, though, otherwise
identical expressions are sorted alphabetically (more or less), so if
this is an issue, you could find a reliable pattern, and use the
symbol_names argument to latex(). You could also just use symbol_names
in the first place, instead of creating Symbol('1').
Aaron Meurer
>
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