? on For Next Loops

[Anonymous]

222: !BRICK TIES

FOR dist=0 TO 30' STEP brick_tie
ADDx dist
GOSUB 111

NEXT dist

RETURN

Will this work in an object? It's just putting 1 in and that's it. The brick_tie is a parameter for a length. GOSUB 111 is the actual lines for the brick tie.

Thanks for any help,
Michele

[Anonymous]

Thanks Tom.
I know someone who has a strong computer science background
and I will ask him some of these questions and maybe he will
have some answers.
Thanks,
Peter Devlin

[Oleg]

Peter wrote:
Hello Tom,
Does the C++ for loop have the "step" part like GDL ?
Peter Devlin

C++ can use steps of course and more.
A problem may arise for both FOR and WHILE, as result of roundings and comparing floating numbers. ( GDL, C++,...).
Both variants above are IMHO almost equivalent and not quite correct.

IMHO better
FOR dist=0 TO 30 ' + brick_tie / 2 STEP brick_tie

[Anonymous]

Hello Oleg,
Great to hear from you.
You say," Both variants above are IMHO almost equivalent
and not quite correct."
Could you elaborate a little please.
How are they both not quite correct ?
Thank you,
Peter Devlin

[Oleg]

Peter wrote:
Hello Oleg,
Could you elaborate a little please.
How are they both not quite correct ?
Thank you,
Peter Devlin

Well. Some tests ( AC9 )
What do you think will printed

counter = 0
bstep = 3
FOR dist=0 TO 3 STEP bstep
  counter = counter + 1
NEXT dist
PRINT counter

Imagine there was some calculations.
bstep is same, but what will be printed now

counter = 0
bstep = 0.3/0.1
FOR dist=0 TO 3 STEP bstep
  counter = counter + 1
NEXT dist
PRINT counter

Same for WHILE

counter = 0
bstep = 0.3
dist = 0
WHILE dist < 0.3 DO 
  dist = dist + bstep
  counter = counter + 1
ENDWHILE
PRINT counter

counter = 0
bstep = 1 - 0.7
dist = 0
WHILE dist < 0.3 DO 
  dist = dist + bstep
  counter = counter + 1
ENDWHILE
PRINT counter

The result will depend on actual values and calculations that's mean unstable.

Oleg

[Anonymous]

Hello Oleg,
Thank you for your considerable elaboration.
I will have to copy your code into a lib part and
get my head around what is happening.
Thank you,
Peter Devlin

[Anonymous]

Hello Oleg,
I did an experiment with your second piece of code.

counter = 0
bstep = 0.3/0.1 !!!=3
FOR dist=0 TO 3 STEP bstep
counter = counter + 1
NEXT dist
PRINT counter !!!! prints 1

PRINT 0.3/0.1 !!!! prints 3

When I change the code to this.

counter = 0
bstep = int(0.3/0.1) !!!=3
FOR dist=0 TO 3 STEP bstep
counter = counter + 1
NEXT dist
PRINT counter !!!! prints 2

It is as though there is a secret fractional part
of the calculation of 0.3/0.1 that is not displayed
in what is printed in PRINT 0.3/0.1 and that there
is a rounding-up in the internal calculations.

How can this be ???
Is PRINT able to lie ?
I wonder how many places PRINT can display.

Peter Devlin

[Oleg]

PRINT does not lie. It rounds number to some signs before display ( it seems to 6 signs ).

In any case it needs to be ready, that issues may arise at any comparisons of floating numbers. ( like 0.3 is not equal 1-0.7 )
And if it important in script, use some tricks, like ABS, EPS etc
( It seems there is some guide from GS about that )

FOR loop uses internally comparison too and it may results to unexpected results. Therefore, when a loop uses floating numbers, and not integer, better to use it like
FOR dist=0 TO 3 + bstep / 2 STEP bstep

Oleg

[Anonymous]

Hello Oleg,
Yes, this is something that I have only recently come to understand.
Now, all I need to do is remember to put in that little extra to
help AC make it to the finish line.
Thanks Oleg,
Peter Devlin