can't reorganizing a list

Hi Philip,

This is due to you needing to understand that most operations in SKILL are what are called "non-destructive". This means that if you do (for example:

l1='(1 2 3 4)
l2='(5 6 7 8)
append(l1 l2) => (1 2 3 4 5 6 7 8)

but the lists l1 and l2 will not have changed. This is to avoid "side effects", and also to be more efficient. So for example, if I was to do:

l3=l2

Both l3 and l2 are pointing at the same data in memory - and since most list functions do not modify the lists in place, I can use the variables l3 and l2 quite safely.

The same is true with the reverse function. If you were to look at the return value of reverse(l_pos) you would see that the list was reversed - but it did not touch the original list. You'd have to do: l_pos=reverse(l_pos) to do that - then it should work fine.

Note however that sortcar  is a destructive function (this is highlighted in the documentation). For efficiency reasons, the sort is done in place. Because of this, your variable l_pos may change after the operation. What actually happens is that the first value that the variable points to will continue to be the first value in the resulting list. So if you did:

l4='(4 1 2 5 6 3)
sort(l4 'lessp) => (1 2 3 4 5 6)
l4 => (4 5 6)

What this means is:

1. You need to be aware of the fact that sort/sortcar etc are destructive and so can have side effects
2. You should always use the return value of sort/sortcar rather than relying on the fact that it will destructively modify the list passed to the sort function. 

So if you do:

l_pos=sortcar(l_pos 'lessp)

it will do what you want.

I'm not sure how copying the list into l_bla would alter any of this. I'm not sure how you "transcribed" it.

Regards,

Andrew.

hi andrew,

very thanks and ... i was slow on the uptake(is this right in english^^). sure...l_pos = reverse(l_pos) .... i don't know why i missed out the "l_pos="  =)

so thanks for that really good description and it answered my question.

transcribe i thought was the englisch meaning to write data from one poin to another?! ... like from one list to another list.
and i realize this with a simple for loop

regards,
philip

"slow on the uptake" is correct English ;->

I knew what you meant by "transcribe" - just not exactly how you'd done the transcription. I hope it wasn't a for loop, because that's not a good way of dealing with a list. It would have been OK if you'd done newList=foreach(mapcar item oldList item) or better still newList=copy(oldList) - but even then, I don't see how it would have fixed the problem!

Regards,

Andrew.