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Re: Self-teaching snag

• To: mathgroup at smc.vnet.net
• Subject: [mg74598] Re: Self-teaching snag
• From: dh <dh at metrohm.ch>
• Date: Tue, 27 Mar 2007 04:08:59 -0500 (EST)
• References: <eu7re0$b6p$1@smc.vnet.net>

Hi Todd,

try to figure how many time charge[_] is called when you calculate 

charge[35].

To circumvent this problem, you must save all function values of 

charge[]. This can be done like this:

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

Now, everything should work as you like.

Daniel



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[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

> 

> 

>  

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