Are optional parameters possible?

simonl

Hi All,

I'd like to create a method that has optional parameters, like:

PUB myMethod( x, y, z )

Where 'z' is optional; taking a default value if it's omitted in the call. Is this possible?

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Cheers,
Simon

---

www.norfolkhelicopterclub.com

You'll always have as many take-offs as landings, the trick is to be sure you can take-off again
BTW: I type as I'm thinking, so please don't take any offence at my writing style

Zap-o

Could you just set it to -1, so that in the method it would be ignored only if this condition is true?

simonl

@Zap-o: That was a rapid response - thanks.

I guess I could, but the programmer would need to remember to add the -1; in which case they may as well just enter the default value!

I should explain; I'm trying to write a beginners tutorial to Propeller communications, and I've just come across something where optional parameters would be useful (in a helper object). With optional parameters, the helper methods can default them to the most commonly used values, but be flexible enough to allow non-common values for the more experienced/needy.

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Cheers,
Simon

---

www.norfolkhelicopterclub.com

You'll always have as many take-offs as landings, the trick is to be sure you can take-off again
BTW: I type as I'm thinking, so please don't take any offence at my writing style

Chuck Rice

Not that I know of, but you can make multiple methods like:

methodXY(x,y) <-- This method just calls methodXYZ(x,y,-1)

methodXYZ(x,y,z)

I am not sure how expensive that would be in the overhead of nested calls, but it would make it simpler to understand (from the outside).

simonl

@Chuck Rice: Hey, that's a good plan! I'll give that a try - thanks.

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Cheers,
Simon

---

www.norfolkhelicopterclub.com

You'll always have as many take-offs as landings, the trick is to be sure you can take-off again
BTW: I type as I'm thinking, so please don't take any offence at my writing style

simonl

Ah! But then they'd need to use different method names [noparse]/noparse][noparse]:boohoo:[/noparse

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Cheers,
Simon

---

www.norfolkhelicopterclub.com

You'll always have as many take-offs as landings, the trick is to be sure you can take-off again
BTW: I type as I'm thinking, so please don't take any offence at my writing style

Zap-o

simonl,

I think I understand what you are after and frankly I don't think it is possible. You might be able to get around this limit by using a type of global variable. here is an example

var byte default

main
default := -1 


some pub (x , y  )


Case default 
   -1 : do woek 
   other :
        do other work

Chuck Rice

I know. Limitation of the language I think. The alternative, but might be too advanced for beginners would be to:

LONG ParameterList[noparse][[/noparse]10]

PUB

ParameterList[noparse][[/noparse] 0 ] := x
ParameterList[noparse][[/noparse] 1 ] := y
ParameterList[noparse][[/noparse] 2 ] := z

MethodName(@ParameterList)

Post Edited (Chuck Rice) : 4/12/2009 5:30:15 PM GMT

jazzed

Hi Simon,

Here's the approach I use. Given the "parameter" code block below, one can build a method that takes one argument which is an expression of the form: p(v1)+p(v2)+p(vN) and call the "dequeue" in the method to get the input values v1, v2, vN. The number of values are limited to the value of QLEN.


For example here is a method that uses the "parameter" code block (untested).

OBJ disp : "tv_text"

PUB main
  printv( p("s")+p("i")+p("m")+p("o")+p("n")+p("!") )
  repeat

PUB printv(arg) | value
' print values passed as sum of p(vN)

  repeat
    disp.dec(dequeue())
    disp.out(" ")
  while count
  disp.out($d)

The "parameter" code block (tested):

PUB p(v)
{{
This is the "parameter" method. The user will string together a
list of method calls to push parameters onto the print queue.

For example: snprintf(@buf, len, string("values %d %d %d\n"),p(1)+p(2)+p(3))
This will print "values 1 2 3\n" to the buffer.

@param v - value that will replace the printf % format specifier.
@returns 1. Using a list of p(v)+p(v)+p(v) as the arg to snprintf gives arg count. 
}}
  enqueue(v)
  return 1

CON
  QLEN = $3f     ' up to 31 parameters
  
VAR
  long queue[noparse][[/noparse]QLEN+1]
  long tail, head
  
PRI enqueue(val)
  long[noparse][[/noparse]@queue+head] := val
  head+=4
  head&= QLEN

PRI dequeue : rc
  rc := long[noparse][[/noparse]@queue+tail]
  tail+=4
  tail&= QLEN
  return rc
  
PRI count
  if head => tail
    result := head-tail
  else
    result := head+(QLEN-tail)
      
PRI flush
  head := tail

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--Steve


Propalyzer: Propeller PC Logic Analyzer
http://forums.parallax.com/showthread.php?p=788230