       Re: Re: Re: odd mathematica blindspot

```Hi,
In Solve[a^x==b,x] with a and b symbols you get an exact symbolic solution
This will also be with your function definition for sol
sol[a_,b_]:=Solve[a^x==b,x]
if you call it with symbols
sol[a,b]
you will get the same solution.
As for the numerical example of mixing exact representation of
(9999999999/10000000000)
on previos posts.
As in any other programming environments you need to get used to
Mathematica's convention and use it correctly in order to get along.
yy[] means the HEAD of yy which is List.
so yy[]{x} means List[Times[List,x]] which makes no sense
you may see it by
yy[]{x} // FullForm

In order to get the values you can always type something like
{x,y}/.yy
or
{x,y}/.yy[] for the first result
you can do assignment as well with
{a,b}={x,y}/.y[]
This is actually a double assignment. a will be assigned the value 1
and b will be assigned the value 1
yehuda
On 4/28/05, Edward Peschko <esp5 at pge.com> wrote:
> On Tue, Apr 26, 2005 at 09:52:28PM -0400, yehuda ben-shimol wrote:
> > Hi,
> > First Mathematica solves it with no problem. I wonder which version
> > you are using. I tested it on ver. 5.1 on Win XP.
> > Second, the (rather simple) problem is
> > Solve[a^x==b,x] will return
> > {{x->Log[b]/Log[a]}}
> > so if you assign
> > sol = Solve[a^x==b,x];
> > you can check it for many values of a and b which in your case will lead to
> > sol={{x->-Log/Log[(999999999999/10000000000000)]}}
> > to extract this value you need to use replacement rules
> > x/. sol[] will give you the number
> > x/.sol will give you the number as a list of a single element.
>
> .... because the problem is so simple, that's why its a blindspot
> in mathematica IMO.
>
> Anyways, I tried it again -
>
> Mathematica 5.1 for Linux
> Copyright 1988-2004 Wolfram Research, Inc.
> -- Terminal graphics initialized --
>
> In:= Off[Solve::ifun];
>
> In:= sol[a_, b_] := Solve[a^x == b, x]
>
> In:= sol[(9999999999/10000000000), .5]
>
> Out= {]
>
> In:= sol[5,15];
>
> {{ x -> Log/Log }}
>
> The really odd thing is that it doesn't come up with a *symbolic* solution,
> it just says that none exists. Perhaps it is evaluating (9.../10....) as one,
> and hence triggering Solve to make the problem insoluble.
>
> Perhaps there's an issue of precision? However, FindRoot works perfectly fine
> on the same expression, so its a bit strange.
>
> btw - the notation for extraction of elements is rather.. odd. Is it basically an
> array of array of arrays? I would expect:
>
> yy = {{ x -> 1, y -> 1}, { x -> 2, y -> 2 }}
>
> to be accessed via:
>
>         yy[]{x}
>
> and the keys of the solution to be accessed by:
>
>         keys(yy[])
>
> or somesuch.
>
> Is there a good tutorial somewhere on the use of datastructures in mathematica?
> sort of a pocketguide on getting around?
>
> Ed
>
>

```

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