ForNext "Autostep"

[MLK]

nothing happens:

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For i=10 To 0
    Debug i
Next

just with STEP -1

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For i=10 To 0 Step -1
    Debug i
Next

its ok so far, but it would be nice to have an option like:

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For i=10 To 0 Autostep 1
    Debug i
Next

or something else.
because often i use:

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for x=y to z

and would like to use the same code, even if z<y

[Psychophanta]

Mmmh! good quest!
It should be good

[Christian]

I agree!

regards,
Christian

[guido]

The ugly aspect of AUTOSTEP is, that it adds a keyword for a single scenario. (It wouldn't work at all in Pascal where TO a DOWNTO are distinct keywords.) Typically the problem is solved with the arithmetic sign function available as SGN in ancient BASICs.

Given that this function is missing in PB, it can easily be supplied

Code: Select all

Procedure.l Sgn(in.l)
result.l = 0
IF in <> 0:  result.l = in.l/ABS(in.l): EndIf
ProcedureReturn result
EndProcedure

So the generic for loop transforms to:

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For x=y To z Step(Sgn(z-y))
   Debug Str(i)
   Next

Unfortunately now the compiler complains, that only a numeric constant is allowed after Step. Perhaps that is the restriction, which should be removed? It wouldn't mean much of a change internally, if one defines, that the value of the Step expression is only evaluated once before entering the loop.

[blueznl]

i consider for / next to be something simple and quick to create loops, i'd go for while / wend for anything more complex

[guido]

i consider for / next to be something simple and quick to create loops, i'd go for while / wend for anything more complex

In the extreme you could substitute all loops by if and goto . Striving for a code reflecting the problem it's out of question, that the purpose of a for loop is building a loop, where you know from the beginning, how often it is cycled and Repeat/Until While/Wend for those loops, where you don't. I can't recommend using the latter for the first scenario, even if it's possible...

[blueznl]

guido, i see your point

it's just a habit of mine to often use while / wends, as i often tend to mess with the variables / steps used for the loop inside the loop

i started with a basic (exbasic) that would not allow modification of for / next parameters ie.

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' exbasic
for a = 1 to 10
  if whatever condition is met :-)
    a = 10
  endif
next a

would not exit the loop, so i very quickly developped a habit to go for

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a=0
while a<10
  a = a+1
  if whatever condition is met :-)
    a = 10
  endif
wend

i have no problem with the step parameter, but you are right, it would be better if it would take a variable, not just a constant (weird, never checked it or used it, moment... yep, that's something that could be improved, just like you i wouldn't care if it would only be evaluated during first execution of the for / next line)

so, although i don't care about autostep, i think i'd like the following two things added:

- Step <iexpr>
- Sgn( <iexp> )

i'll add them to my ever growing wishlist...

[Fred]

A 'DownTo' keyword seems a cool solution, what do you think ?

[blueb]

If you're going to compare the 2 variables and STEP up or STEP down depending on their values then maybe AUTOSTEP is the better choice.
DOWNTO seems to infer that you'll always be STEPing down!

Then (if a user desires) they could use:

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For i=10 To 0 Autostep 1 
    Debug i 
Next

But still keep the existing STEP for compatiblity.

--blueb

[GedB]

Fred,

Introducing DOWNTO just worsens the problem, because you need separate keywords for either situation.

Imagine you are writing code to draw a line between two points, and you are using a for loop to step through.

The problem is that the points could be anywere on screen. p1 could be 100, 100 and p2 50, 50. With required an explicit step or a separate keywork you have to write extra code to see which value is the higher.

What you really want to do is to not have to worry which direction the step is in. if x1 is 100 and x2 is 50 then the compiler will understand that it needs to count down from 100 to 50.

Perhaps a TOWARDS key word? So when the code is

For a = X1 TOWARDS X2 STEP 2

The compiler will work out the necessary direction.

I think a new keyword would be necessary because some people may rely on the current behaviour.

[GPI]

A 'DownTo' keyword seems a cool solution, what do you think ?

No it isn't. It is a better solution to add the possibility to add a variable in the step-counter...

[Dare2]

votes for: Step (expression)

And puts in a plug for SGN and NOT

[blueznl]

yes
yes

and yes

(and still people claim i'm a naysayer )

[Danilo]

nothing happens:

Code: Select all

For i=10 To 0
    Debug i
Next

its ok so far, but it would be nice to have an option like:

Code: Select all

For i=10 To 0 Autostep 1
    Debug i
Next

The compiler could automatically check whats the step count, 1 or -1.

Without Step, the compiler could compare FROM and TO:

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If FROM < TO
  STEP = 1
Elseif FROM > TO
  STEP = -1
; Else
;   STEP = 0 ; equal, no step needed
Endif

With Step given:

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If FROM < TO
  STEP = Step
Elseif FROM > TO
  STEP = -Step
; Else
;   STEP = 0 ; equal, no step needed
Endif

Would make things more easy for a BASIC language if its
checked automatically.

EDIT: Just forget it, sorry. Looks like i misunderstood the problem.

[guido]

@GPI, @GedB, @Dare2: Let me join the club!

votes for: Step (expression)

And puts in a plug for SGN and NOT

I couldn't have summarized it better. A new keyword just for blurring the sign of the step width seems overkill. TOWARDS is a very naturally sounding suggestion, but since AUTOSTEP is immediately preceeding the value it influences, AUTOSTEP wins the direct comparison. I consider DOWNTO the worst alternative for the reason given by GPI .

Just my 2 ct...