programming a piecewise function on the 48 G
#1

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?

#2

Maybe you need to press "-> NUM"?

#3

> 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 >>
>>

--

#4

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.

#5

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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