(TI-89) Expression as a function argument?

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Expression as a function argument?

(TI-89) Expression as a function argument?

Ben Woodworth 05 Mar 2014 01:24

(Sorry for posting a TI-89 question here, there wasn't a forum for it)

If I want, for example, to create a function to find the derivative of an expression, how do I set up the arguments to make a variable hold an expression?

I tried doing this (e=expression, v=variable), but it automatically evaluates the expression before deriving it:

derivative(e,v)
Prgm
Disp d(e,v)
EntPrgm

(d() is the derivative function on the TI-89)

Any idea how I'd go about this? Thanks!

Last edited on 05 Mar 2014 21:23 by Ben Woodworth Show more

Re: (TI-89) Expression as a function argument?

Xeda Elnara 05 Mar 2014 14:00

If it is automatically evaluating the expression, it is because your variable is already defined. For many languages, there are what are known as "Global variables" (variables that any program can access) and "Local variables" that only that specific program can access. Changing a Local variable does not change it as a Global variable.

Basically, if x is your variable and when you type "x" on the homescreen it returns a number, string, matrix, or whatever, x has been defined as a global variable. The fix would be:

derivative(e,v)
Prgm
Local e,v
Disp d(e,v)
EndPrgm

However, in a case like this, you might prefer to use a function variable. Programs are meant to do much fancier things, whereas functions are used like commands and return something. For example:

derivative(e,v)
Func
Local e,v
Return d(e,v)
EndFunc

Finally, the "68K Programming" section is actually the place for TI-89 stuff. The TI-89/92/Voyage 200 calculators (and a few others, I believe) are considered 68K calculators and have the same or similar BASIC language as well as a Motorola 68000 processor. The 82/83/84 and others are Z80 calculators, but typically "Z80 Programming" refers to assembly. Still, this does bring up a good point that maybe we should note that.

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Z80 Assembly>English>TI-BASIC>Python>French>C>0

Re: (TI-89) Expression as a function argument?

Ben Woodworth 05 Mar 2014 21:53

Okay, this is the function I have right now

test(e)
Func
 Local e
 Return String(e)
EndFunc

This program is always returning "true" or "false" when I run it. If I type this into the home screen…

2→x
               2
test(x^2=4)
          "true"

… it will return "true", and it will return "false" if I set x to anything other than 2 or -2. Do you know why it's doing this?

Last edited on 05 Mar 2014 21:53 by Ben Woodworth Show more

Re: (TI-89) Expression as a function argument?

Toothless the Dragon 06 Mar 2014 02:42

Well, I don't know the TI-89, but…

The only possible numbers squared that equal 4 is 2 or -2, right? o.O

-3^2=9
-2^2=4
-1^2=1
0^2=0
1^2=1
2^2=4
3^2=9

Kinda self-explanatory if ya ask me. :P

Last edited on 06 Mar 2014 03:53 by Toothless the Dragon Show more

Re: (TI-89) Expression as a function argument?

Ben Woodworth 06 Mar 2014 02:58

lol I want the program to return "x^2=4", not "true" or "false" :p

Last edited on 06 Mar 2014 02:59 by Ben Woodworth Show more

Re: (TI-89) Expression as a function argument?

Toothless the Dragon 06 Mar 2014 03:47

Uhm… how about string("x^2=4")?

http://tibasicdev.wikidot.com/68k:string

Or Text "x^2=4"? I wish I had a TI-89, partly to help ya out. :/ The other part just because all of those new commands seem so awesome. I mean, a dialogue box? 20% cooler than what the TI-84 has. :P

Last edited on 06 Mar 2014 03:48 by Toothless the Dragon Show more

Re: (TI-89) Expression as a function argument?

Xeda Elnara 06 Mar 2014 12:35

Yes, as I said above about global variables and local variables. You are making 'e' a local variable that you are passing "x^2=4" to. Since x is already a global variable, it gets evaluated, so 2^2=4 which is a true statement, so the expression evaluates to True. Converted to a string, this is just "true" . I don't have batteries for my 89 at the moment, but maybe try to make x a local variable, too? That will only work if the inputs are parsed after local variable definition, though, so it might not work.

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Z80 Assembly>English>TI-BASIC>Python>French>C>0

Re: (TI-89) Expression as a function argument?

Ben Woodworth 06 Mar 2014 23:59

Hmm I tried adding x as a local variable too, and it seems like the expression is getting evaluated before it's even being sent to the function.

Re: (TI-89) Expression as a function argument?

Xeda Elnara 07 Mar 2014 13:43

Ah, well then there isn't really a workaround for that. You will just have to either delete x (ClearAZ will clear all single-letter variables) and instead of storing a number to it, either do nothing, or store equations to it. For example, you can do sin(x)+3→t, then as long as x hasn't been stored to, it doesn't evaluate and stays "sin(x)+3" .

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Z80 Assembly>English>TI-BASIC>Python>French>C>0

Re: (TI-89) Expression as a function argument?

Ben Woodworth 08 Mar 2014 18:25

That seems like it's the only way, maybe this is possible with assembly programs. I'm wondering if there's a way to look at the source code for things like the derivative/solve/etc functions to see how they work and maybe learn from that.

Re: (TI-89) Expression as a function argument?

MasterChief2552 08 Mar 2014 19:14

As far as I know, the calculator uses only iterative methods for commands like solve( (solves an expression for a variable), fnInt (calculates the integral, the area below (or above) a function) or nDeriv (numerical derivation, calculates the gradient at X of an function).
So, for all these commands, the calculator only uses iterative methods and does not really calculate it.
For example the nDeriv command:
A example of finding the derivative at a location X in an equation y with Ti 84 Basic:

:Input “Equation“, Y1
:Input “X: “, C
:Disp “The gradient at the location
:Disp C
:Disp “of
:Disp Y1
:Disp “is equal to
:C-10^-4→X
:expr(Y1→A
:C+10^-4→X
:expr(Y1→B
:(B-A)/2*10^-4
:Disp Ans

As we all know, the iterative derivative (using lim) is defined as:

For precision you will have to minimize Deltax, so that lim DeltaX → 0
This is, why I chose 10^-4 ;)
Of course this was not iterations, but the fnInt command uses iterations (it uses the method, where you part the function area into many rectangles)
And here an example for the iterative solving:

:Disp “Input the equation in such a way that it is equal to 0
:Input “Equation: 0=“, Strq
:Input “Guess“, G
:while inString(Str1, “X
:inString(Str1, “X
:sub(Str1, 1, Ans-1)+“L1“+sub(Str1,Ans+1,length(Str1)-Ans-1→Str1
:End
:0→D
:{G,G→L1
:while (D<100) and (10^-4<min(abs(expr(Str1
:D+0.1→D
:L1(1)-D→L1(1
:L1(2)+D→L1(2
:End
:If D=100
:Then
:Disp “No solution found
:Else
:Disp “Zero at
:10^-4<abs(expr(Str1
:If Ans(1):Then
:Disp G-D
:Else
:Disp G+D
:End:End

(Did not test it)
And here an example for the integral of a function being calculated iteratively using the rectangle method:

:Input “Equation: f(x)=“,Str1
:Input “Start: “,A
:Input “End: “,B
:Input “Iterations: “, M //this is the number of iterations. The greater it is, the greater also the precision is. Something like 100 should do a good job
:0→F
:(B-A)/M→D
:For(X,A,B,D
:expr(Str1
:F+Ans*D→F
:End
:Disp “The integral is equal to
:Disp F

Or here an optimized version:

:Input “Equation: f(x)=“,Str1
:Input “Start: “,A
:Input “End: “,B
:Input “Iterations: “, M //this is the number of iterations. The greater it is, the greater also the precision is. Something like 100 should do a good job
:(B-A)/M→D
:Disp “The integral is equal to
:Disp sum(seq(expr(Str1)*D,X,A,B,D

(I also did not test this one)
However I only wanted to show you the methods, how the calculator only iteritavely calculates things like this

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Wake me, when you need me.

Last edited on 08 Mar 2014 20:06 by MasterChief2552 Show more

Re: (TI-89) Expression as a function argument?

Xeda Elnara 11 Mar 2014 15:31

You are thinking of the wrong calculator :P The TI-89 has a computer algebra system and supports evaluating integrals and derivatives. For example, d(sin(x),x) would return cos(x), assuming x has not been stored to.

@Ben: Maybe, but that is a lot more effort than simply using a Delvar. Even the built in commands work that way. For example, see above. If you have 3 in the 'x' variable, then d(sin(x),x) returns cos(3), not cos(x).

This is a really easy problem to deal with and any user should expect the outcome if they have a value stored in a variable. I believe it is even discussed in the manual that TI provides.

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Z80 Assembly>English>TI-BASIC>Python>French>C>0

Re: (TI-89) Expression as a function argument?

MasterChief2552 11 Mar 2014 17:20

Oh, sorry, I did not know this.
But it is very amazing to know that the calculator can evaluate derivatives and integrals now :O

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Wake me, when you need me.