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HP Forum Archive 08

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programming a piecewise function on the 48 G
Message #1 Posted by Fernando Leyton on 30 June 2002, 8:19 a.m.

I'm taking a first calculus course at Paris VI and i want to programm a piecewise function on the HP 48 calculator. The function is :

F(x)= 3x if x<-5 = x/2 -5 if -5<x<0 = exp(x) if x>0

I programmed: << ---> X << CASE ' x<-5' THEN ' 3x' END ' -5< x < 0 ' THEN ' x/2 -5 ' END ' x > 0' THEN ' exp(x)' END END >> >>

But i don't get the numeric result, only the algebraic expression corresponding to the input domain?

      
Re: programming a piecewise function on the 48 G
Message #2 Posted by Ellis Easley on 30 June 2002, 11:45 a.m.,
in response to message #1 by Fernando Leyton

Maybe you need to press "-> NUM"?

      
Re: programming a piecewise function on the 48 G
Message #3 Posted by Rupert (Northern Italy, EU) on 30 June 2002, 12:27 p.m.,
in response to message #1 by Fernando Leyton

> But i don't get the numeric result, only the algebraic expression > corresponding to the input domain?

Just add EVAL after any algebraic expression you need to evaluate. It looks like there is a test who needs to be re-written, too:

<< ---> X << CASE 'X < -5' THEN '3*X' EVAL END 'X > -5 AND X < 0' THEN 'X/2-5' EVAL END 'X > 0' THEN 'EXP(X)' EVAL END END >> >>

--

      
Re: programming a piecewise function on the 48 G
Message #4 Posted by Vieira, Luiz C. (Brazil) on 30 June 2002, 11:44 p.m.,
in response to message #1 by Fernando Leyton

Hi;

Just to know: does the final expression, when evaluated, return the expected result? If so, a single ->NUM after the last END is enough, because all possible CASEs return an algebraic expression. Say, you could try: 

« -> X
  « CASE
    'X<-5' THEN '3*X' END
    'X>-5 AND X<0' THEN 'X/2-5' END
    'X>0' THEN 'EXP(X)' END
    END ->NUM
  »
»
(the ->NUM can also come between both » and »). If you wanna try an RPL solution (no need of a ->NUM adding): 
« -> X
  « CASE
    X -5 < THEN X 3 * END
    X -5 > X 0 < AND THEN X 2 / 5 - END
    X 0 > THEN X EXP END
    END
  »
»
(I know you did not ask for it, but I am a compulsive RPL programmer... my shrink said so.)

Hope you have success.

      
Re: programming a piecewise function on the 48 G
Message #5 Posted by Arnold Steekelenburg on 1 July 2002, 9:29 a.m.,
in response to message #1 by Fernando Leyton

Hi Fernando,

Another way is this:

Type in:

'F(X)=IFTE(X<-5,3*X,IFTE(X>-5 AND X<0,X/2-5,EXP(X)))'

Then press left-shift STO (= Define) Press VAR and you'll see the variable (function) F.

Type a value and press the softkey for F and this will be evaluated immediately.

Right-shift F will give you the program the HP48 has created as a result of the DEFinition.

IFTE = short IF THEN ELSE

Using this construction you can plot piecewise functions too. Arnold

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