Define a Function

[paulh]

Hi,

Am a newbie and have a question. I want to define a function such as x^3+x^2 +1 and use that function in my program. Is
there a command such as DEF function that will define that function so I can evaluate it for certain values?

Thanks,
Paul

[carlgundel]

Aug 2, 2020 23:39:00 GMT paulh said:

Hi,

Am a newbie and have a question. I want to define a function such as x^3+x^2 +1 and use that function in my program. Is
there a command such as DEF function that will define that function so I can evaluate it for certain values?

Thanks,
Paul

 Sure.


print myFn(5)
end

function myFn(x)
 myFn = x^3+x^2+1
end function



[Rod]

Generally speaking we don’t need to define functions or define variables and variable types in Just BASIC because running the program is a two step process. Firstly your code is compiled, that takes care of all the definitions. Variable types are simplified down to integer, float or string. You will recognise a float when you see a . within a numeric variable.

After compiling the code is then interpreted line by line and run.

All seamless and behind the scene thus simplifying coding with Just BASIC.

[paulh]

Guys,


Thanks for the tips. Just (no pun intended) what I needed.

Paul

[carlgundel]

Aug 3, 2020 12:19:59 GMT Rod said:

Generally speaking we don’t need to define functions or define variables and variable types in Just BASIC because running the program is a two step process. Firstly your code is compiled, that takes care of all the definitions. Variable types are simplified down to integer, float or string. You will recognise a float when you see a . within a numeric variable.

After compiling the code is then interpreted line by line and run.

All seamless and behind the scene thus simplifying coding with Just BASIC.

That's an important point.  The function declaration does not have to be above the code that uses the function.  In Liberty BASIC, and indeed in most languages that support user functions and subroutines, the code that defines functions is allowed to be anywhere in the program code.

[paulh]

Again, thanks for your help. I am not clear on one thing regarding this.

You gave me as an example:

print myFn(5)
end

function myFn(x)
myFn = x^3+x^2+1
end function

I am wondering if it is possible to enter a function in some form on an INPUT statement at the beginning of the program instead of having it written in the
program itself as in the example x^3+x^2+1? I

[paulh]

I guess I would want something like:

Input <some function>
print myFn(5)
end

function myFn(x)
myFn = <some function>
end function

Thanks,
Paul

[B+]

Aug 7, 2020 21:53:13 GMT paulh said:

I guess I would want something like:

Input <some function>
print myFn(5)
end

function myFn(x)
myFn = <some function>
end function

Thanks,
Paul

Been there, done that! Create a txt file with formulas and variables and their current value. The app allows you to add a formula, AKA Function, variable or change variable values see here:

justbasiccom.proboards.com/thread/482/handy-formula-saver

Frankly it's easier to edit the text file with formulas and values and just run the app to evaluate the formula at the current variable settings.
Just follow the example txt file given in the thread.

[Rod]

I am getting confused by function and formula. Functions are built into the program and can be reused, they are defined at the start by writing them into the code which is compiled and run. We can't introduce a new function after that.

Are you trying to get a maths formula to compute or are you trying to add new functions to the program flow?

[tsh73]

I believe he really looks for EVAL
www.libertybasicuniversity.com/lb4help/EVAL.htm
JB does not have EVAL function, but its bigger (paid, could be tried as unregistered shareware) cousin LibertyBASIC has. (link on forum homepage, top right)

Of course folks get creative and invent theirs own EVAL-like things
But sometimes (code from that LB helpfile topic)
'ask for an expression
   input "Type a numeric expression>"; code$
   answer = eval(code$)
   print answer
*this* clearly looks like reason to get LB.

[tenochtitlanuk]

Yes- move up to LB and have eval() available!

   global expr$

   print "My function of x ( eg x^3 +x^2 +1)  "
   input expr$

   print " Evaluating "; expr$; " with x =5 yields "; myFn( 5)
   end

   function myFn( x)
       myFn =eval( expr$)
   end function

Outputs..
My function of x ( eg x^3 +x^2 +1)
?x^3 +x^2 +1
Evaluating x^3 +x^2 +1 with x =5 yields 151


Of course it gets more fun if you evaluate over a range of x..


   global expr$

   print "My function of x ( eg x^3 +x^2 +1)  "
   input expr$

   for x =0 to 10
       print " Evaluating "; expr$; " with x ="; x; " yields "; myFn( x)
   next x

   end

   function myFn( x)
       myFn =eval( expr$)
   end function


My function of x ( eg x^3 +x^2 +1)
?x^3 +x^2 +1
Evaluating x^3 +x^2 +1 with x =0 yields 1
Evaluating x^3 +x^2 +1 with x =1 yields 3
Evaluating x^3 +x^2 +1 with x =2 yields 13
Evaluating x^3 +x^2 +1 with x =3 yields 37
Evaluating x^3 +x^2 +1 with x =4 yields 81
Evaluating x^3 +x^2 +1 with x =5 yields 151
Evaluating x^3 +x^2 +1 with x =6 yields 253
Evaluating x^3 +x^2 +1 with x =7 yields 393
Evaluating x^3 +x^2 +1 with x =8 yields 577
Evaluating x^3 +x^2 +1 with x =9 yields 811
Evaluating x^3 +x^2 +1 with x =10 yields 1101


And even more fun if you go for a graphical output. And then you'll want to auto-scale so it 'fills the paper'. Before you know it, a full graphical calculater!

You might look at www.diga.me.uk/indexDemo.html for some of my graphing examples. LB is available FREE with a nag screen. As well as eval() it'll give you access to the fabulous 'lsn' lesson addition in LB. Download my lsn files and load/run in LB ( remember to make sure the file selecter is set to show files with the '.lsn' extension.

PS One LB 'gotcha'. Don't copy/paste into an input prompt- it doesn't work!

[tenochtitlanuk]

.. so, fun continued, ...




   print " My function of x ( eg x^3 +x^2 +1)  "
   input expr$

   WindowWidth  =550
   WindowHeight =550

   open "Graphed function" for graphics_nsb as #wg

   #wg "trapclose quit"
   #wg "down"
   #wg "line  10 250 510 250"
   #wg "line 260  10 260 510"


   for x =-10 to 10 step 0.02
       #wg "set "; int( 250 +25 *x); " "; int( 250 -eval( expr$))
   next x

   #wg "getbmp scr 1 1 511 511"
   bmpsave "scr", "demoEvalGraph.bmp"

   wait

   sub quit h$
       close #wg
       end
   end sub


[B+]

Get this, get that... Here is paulh example run with the code from link I gave:

Instructions:

Use menu item #3
paulh = x^3 + x^2 + 1

Use menu item #1 to set x
x = 5

Use menu item #5 to evaluate the function at x = 5

Here is Output:

        Formula Evaluation Menu

  Current Formula: paulh = x^3 + x^2 + 1

1 for setting / resetting a variable and value
2 for getting a list of variables and values
3 for settimg a new formula
4 for selecting a formula from list saved
5 for Evaluating Current Formula with current variable values
6 for saving all current variables = values and formulas
7 for quitting

   Please enter your choice number 5
x^3 + x^2 + 1
paulh = 151

Press enter to continue...




JUST BASIC did this!!!


This code needs to show variable values too, I guess, above x was set with "x = 5" using menu item 1.

And sure, defining a function has a technical meaning as demo'd by Carl but I don't think Paul was being technical.