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Re: precedence for ReplaceAll?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg110554] Re: precedence for ReplaceAll?
  • From: Leonid Shifrin <lshifr at gmail.com>
  • Date: Fri, 25 Jun 2010 07:27:32 -0400 (EDT)

Hi Matthias,

the problem is that in all rules you used, symbols <a> and <b> evaluated to
their respective values A*c and B*c before any rule substitution took place,
both in the rules and in the expressions to which you apply the rules. This
can be easily tracked by using Trace:

In[1]:=
a:=A c
b:=B c

In[3]:= Trace[a/.{a->x,b->y},ReplaceAll]

Out[3]= {A c/.{A c->x,B c->y},x}

In[4]:= Trace[b/.{a->x,b->y},ReplaceAll]

Out[4]= {B c/.{A c->x,B c->y},y}

In[5]:= Trace[a+b/.{a->x,b->y},ReplaceAll]

Out[5]= {A c+B c/.{A c->x,B c->y},x+y}

In[6]:= Trace[a*b/.{a->x,b->y},ReplaceAll]

Out[6]= {A B c^2/.{A c->x,B c->y},A B c^2}

So you see, the actual expressions used by ReplaceAll  are not at all those
you started with, hence the results. Presumably, you wanted to prevent
evaluation of both expressions and rules, at least until the rule applies.
You can use Unevaluated and HoldPattern for this purpose:

In[7]:=
Unevaluated[a] /. {HoldPattern[a] -> x, HoldPattern[b] -> y}

Out[7]= x

In[8]:= Unevaluated[b] /. {HoldPattern[a] -> x, HoldPattern[b] -> y}

Out[8]= y

In[9]:= Unevaluated[a + b] /. {HoldPattern[a] -> x,
  HoldPattern[b] -> y}

Out[9]= x + y

In[10]:= Unevaluated[a*b] /. {HoldPattern[a] -> x, HoldPattern[b] -> y}

Out[10]= x y

You can use Trace as above to see that now the actual expressions used in
ReplaceAll are what you probably had in mind. However, if you have a choice,
I wouldn't make the assignments to <a> and <b>  in the first place, or make
them local rules and apply after all other rules have been applied.

Hope this helps.

Regards,
Leonid





On Thu, Jun 24, 2010 at 1:27 AM, Matthias Fripp <mfripp at gmail.com> wrote:

> I am having trouble using ReplaceAll to replace symbols that already
> have a delayed assignment.
>
> e.g., this input:
>
> In[287]:=
> a := A c
> b := B c
> a /. {a -> x, b -> y}
> b /. {a -> x, b -> y}
> a + b /. {a -> x, b -> y}
> a * b /. {a -> x, b -> y}
>
> gives this output:
>
> Out[289]= x
>
> Out[290]= y
>
> Out[291]= x + y
>
> Out[292]= A B c^2
>
> All of this works as expected except for the final term. I would have
> expected to get the result "x y". Is there any way to force
> Mathematica to produce that result?
>
> If on the other hand the original assignment is a := A + c and b := B
> + c, I get an unexpected output for the sum, but the expected output
> (x y) for the product. If I insert d instead of one of the c's, I get
> various other (unpredictable) kinds of result.
>
> My first guess is that Mathematica is doing a sort of "double
> ReplaceAll", where it first tries the pattern given in the delayed
> assignment, and any symbols matched by that are not tested against the
> explicit ReplaceAll. But that doesn't explain why the sum works and
> not the product. Am I thinking about this the wrong way?
>
> Thanks for any help you can give!
>
> Matthias Fripp
>
>


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