       Re: Self-teaching snag

• To: mathgroup at smc.vnet.net
• Subject: [mg74572] Re: [mg74556] Self-teaching snag
• From: Murray Eisenberg <murray at math.umass.edu>
• Date: Tue, 27 Mar 2007 03:54:56 -0500 (EST)
• Organization: Mathematics & Statistics, Univ. of Mass./Amherst
• References: <200703260704.CAA11373@smc.vnet.net>
• Reply-to: murray at math.umass.edu

```The trouble is that each and every time Mathematica calculates
charge[day] for some particular value of day, it must recurse all the
way back the initial value.  That is, once a value of charge is
calculated, Mathematica forgets it.

Look at the following, for example:

charge=1.0 (* 100% *);
charge[day_]:=(charge[day-1]-(0.05*charge[day-1]));
charge
0.358486

?charge   (* what does Mathematica know about charge? *)
Global`charge
charge=1.
charge[day_]:=charge[day-1]-0.05 charge[day-1]

Indeed, Mathematica knows nothing more about charge than the original
definitions, even though it calculated charge -- and hence in the
process had to calculate charge, charge, ..., charge, charge.

Now force Mathematica to "remember" any value of charge already
calculated -- in technical terms, "cache" the calculated values -- by
changing the definition to the following:

charge=1.0 (*100%*);
charge[day_] := charge[day] = (charge[day-1]-(0.05*charge[day-1]));

Watch what happens now when, say, you calculate charge (to keep the
output here short):

charge
0.814506

?charge
Global`charge
charge=1.
charge=0.95
charge=0.9025
charge=0.857375
charge=0.814506

Todd Allen wrote:
> Hi All,
>
>    I am trying to refresh my skills in basic problem
> solving using Mathematica, but am running into some
> difficulties which are beginning to make me suspicious
> of Mathematica itself.  (I probably should be
> suspicious of my own brain...but you know how that is
> :-)
>
> Here is the scenario:  I have written a basic function
> to tell me what percentage of battery power will
> remain in a battery after x number of days, provided
> that we start with a full charge and lose 5% of that
> charge per day.
>
> If you execute the following code in Mathematica
> (V5.1):
>
> charge=1.0 (* 100% *);
> charge[day_]:=(charge[day-1]-(0.05*charge[day-1]));
> charge
>
> I receive an output of 0.358486 for my query at the 20
> day mark.....so, no problem so far.
>
> However, when I try to ask for the output at
> charge, mathematica seems to enter an endless
> calculation.  I've let the computer run for as long as
> 5 minutes without getting an answer.  Is there
> something wrong with my function, my version of
> Mathematica or something else I haven't considered?
>
>
>
> When I try the following:
>
> In:=
> Solve[charge[day]==0.15,day];
>
> Mathematica gives me the error:
> "\$RecursionLimit::reclim: Recursion depth of 256
> exceeded."
>
> I am trying to ask Mathematica to tell my how many
> days it takes to reduce the battery power to 15
> percent, but I must be messing something up??
>
> If anyone has any pointers, I'd certainly appreciate
> it, because I am a little stuck right now.
>
> Best regards,
> Todd Allen
>
>
>
> ____________________________________________________________________________________
> We won't tell. Get more on shows you hate to love
> (and love to hate): Yahoo! TV's Guilty Pleasures list.
> http://tv.yahoo.com/collections/265
>

--
Murray Eisenberg                     murray at math.umass.edu
Mathematics & Statistics Dept.
Lederle Graduate Research Tower      phone 413 549-1020 (H)
University of Massachusetts                413 545-2859 (W)
710 North Pleasant Street            fax   413 545-1801
Amherst, MA 01003-9305

```

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