Mathematica 9 is now available
Services & Resources / Wolfram Forums
-----
 /
MathGroup Archive
2007
*January
*February
*March
*April
*May
*June
*July
*August
*September
*October
*Archive Index
*Ask about this page
*Print this page
*Give us feedback
*Sign up for the Wolfram Insider

MathGroup Archive 2007

[Date Index] [Thread Index] [Author Index]

Search the Archive

Re: Self-teaching snag

  • To: mathgroup at smc.vnet.net
  • Subject: [mg74597] Re: [mg74556] Self-teaching snag
  • From: Bob Hanlon <hanlonr at cox.net>
  • Date: Tue, 27 Mar 2007 04:08:26 -0500 (EST)
  • Reply-to: hanlonr at cox.net

To speed recursions up, add memory to the function definition so that it doesn't repeat prior recursions.

Clear[charge1];

charge1[0]=1; 
charge1[day_Integer?Positive]:=
    charge1[day]=0.95charge1[day-1];

charge1[20]

0.358486

charge1[35]

0.166083

However, the recursion can be avoided with

Clear[charge2,rate];

charge2[day_,rate_:0.95]:=rate^day;

Off[Solve::ifun];

d=day/.Solve[charge2[day]==0.15,day][[1]]

36.9857

Plot[charge2[day],{day,0,40},
    Epilog->{Red, AbsolutePointSize[4],
        Point/@Table[{n,charge1[n]},{n,0,40,4}],
        Blue,AbsoluteDashing[{5, 10}],
        Line[{{0,0.15},{d,0.15},{d,0}}]},
    ImageSize->500];

If the result for charge2 is not obvious it can be obtained from RSolve 

Clear[charge,day,rate];

charge[day]/.RSolve[{charge[day]==rate*
      charge[day-1],charge[0]==1},charge[day],day][[1]]

rate^day


Bob Hanlon

---- Todd Allen <genesplicer28 at yahoo.com> 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[0]=1.0 (* 100% *);
> charge[day_]:=(charge[day-1]-(0.05*charge[day-1]));
> charge[20]
> 
> 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[35], 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?
> 
> 
> Additionally,
> 
> When I try the following:
> 
> In[145]:=
> 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 
> 



  • Prev by Date: Re: Self-teaching snag
  • Next by Date: Re: Self-teaching snag
  • Previous by thread: Re: Self-teaching snag
  • Next by thread: Re: Self-teaching snag