topic Re: How to insert a list into another list in Discussions

How to insert a list into another list

Craige_Hales — Fri, 09 Jun 2023 19:47:12 GMT

In I posted an answer that required inserting a list into another list of lists. The InsertInto function unpacks the inserted list into elements, which is not what I want. I used an assignment statement to the element one beyond the end of the list, which does insert the list as an element. Another choice is to wrap the list in another list before inserting it (so it unpacks the wrapped list, and inserts the original.)

Is there a better way to insert a list into a list? result and p are both lists.

result[N Items( result ) + 1] = p is a bit wordy but works and seems fairly clear that p is being stored just after the last element in the list. Good for extending, but can't insert anywhere else.

result={"a"};
p={"b","c"};
result[N Items( result ) + 1] = p; // wordy, but intent is clear
show(result);
/*:

result = {"a", {"b", "c"}}; // good

Insertinto( result, p ) won't work because it inserts p's elements, not p.

result={"a"};
p={"b","c"};
insertinto( result, p ) ; // looks right, but...
show(result);
/*:
result = {"a", "b", "c"}; // not what I want

insertinto( result, evallist( { p } ) ) works, but seems less readable than the assignment. Can insert anywhere but makes an extra copy.

result={"a"};
p={"b","c"};
insertinto( result, evallist( { p } ) ) ; // makes an extra copy of the list and hard to read
show(result);
/*:
result = {"a", {"b", "c"}}; // good: what I want

edit: then I found which is the same question. It looks like similar answers, leaning towards the evallist approach.

edit 18may2022: has a note about builtin function names, like list() or {} being different from unknown names.

Re: How to insert a list into another list

brady_brady — Tue, 18 May 2021 13:27:53 GMT

FWIW I always do it the EvalList ( ) way in your 2nd example. I guess you could do it this way, and not evaluate until you want / need to... not sure if this is buying you much (?)

result = {"a"};
p = {"b", "c"};
Insert Into( result, {p} ); //don't evaluate p here... but of course p is non-static
show(result[2][1]);    //can still access elements of p if desired
Show( Eval List( result ) );  //or can eval to see the whole thing

Re: How to insert a list into another list

ih — Tue, 18 May 2021 13:59:09 GMT

Not a direct solution but more as a work around I tend to use associative arrays, similar to what @pmroz suggested in your link.

Re: How to insert a list into another list

Craige_Hales — Tue, 18 May 2021 14:33:14 GMT

I might use that next time. It looks simpler, even if it is using a more complicated (under the covers) data structure, and since I'm already generating an index with nitems(...)+1 ... yes, it might be the better choice.

Re: How to insert a list into another list

Craige_Hales — Tue, 18 May 2021 14:37:38 GMT

Yes. I've used the evallist() approach and always feel like it needs a comment to explain. In my original code (link at top) I keep changing p between insertions, so it can't just insert a reference to p.