[Gambas-user] Swap expressions with side effects

[Tobias Boege]

Hi,

I just spent two good hours over my implementation of an algorithm to
arrange some PointFs in an array so that the distance between P[i] and
P[i+1] is the smallest possible among all points P[j], j > i. I don't
want to bother you with details (well, I think I already did, sorry) but
the comparison of two points is not only a function of these two points
but also of some other reference point. So I used a line like this:

  Swap $aPoints[iInd + 1], $aPoints[GetNearest(iInd)]

which means: get me the index of the nearest point to $aPoints[iInd] and
swap $aPoints[iInd + 1] with the value there. This effectively sorts my
array into the desired order (For iInd = 0 To $aPoints.Max - 1).

Well, not really. Because Swap is not immune to side effects induced by
the expressions to swap. The wiki states that implicitly by saying that

  Swap A, B

is equivalent to

  tmp = A
  A = B
  B = tmp

So my code really was:

  tmp = $aPoints[iInd + 1]
  $aPoints[iInd + 1] = $aPoints[GetNearest(iInd)]
  $aPoints[GetNearest(iInd)] = tmp

which leads to an erroneous array because GetNearest() is a function of
$aPoints[iInd + 1] which changed between the two invocations. [ The funny
thing is that, due to the nature of GetNearest(), the result of the above
three lines for all iInd is that the array remains unchanged. ]

To cut a long story short, is this by design? Do you think it's worth
changing that behaviour (it wouldn't break any sane use of Swap)? If not
it's equivalently fine. I'll add a clear warning to the wiki, then.

Regards,
Tobi

-- 
"There's an old saying: Don't change anything... ever!" -- Mr. Monk