Thomas is correct that this is a bit of an odd request unless explained better.
There are a number of implicit assumptions that need to be revisited here. Python Lists are what they are. They are not in any way tagged. They are not linked lists or binary trees or dictionaries or whatever you are looking for. They are a mutable object with an order at any given time and no memory or history of an earlier status. They support not just insertion but also deletion and replacement and other things. But generally, if your time span between deciding on additions and implementing them will contain no deletions, then one simple solution is to re-order your insertion to always do the last one first. The indices will only change at and above an insertion point. Your remaining insertions will always be at an untouched region where the indices remain the same, for now. A second choice as Thomas points out is to adjust your indices. An example might be if you have a collection of proposed insertions and each contains an index number and payload. Each time you insert the next payload at the insertion point, you invoke a function that goes through your remaining Collection and finds any with an index that is higher and increments it. Obviously there are issues if dealing with adding multiple times to the same index or adding multiple items at once. The above could be encapsulated in some kind of VIEW in some languages including of course some that use pointers. I will add by pointing out a way to do a multi-insertion at once if you know all the insertions at the same time. Take your list that you want to change by adding at say positions 9, 3 and 6. Now DON"T insert anything. Forget the concept. Instead, and this is drastic, make a NEW list. The new list is loosely old[0:2] + new_at_3 + old[3:5] + new_at_6 + old[6:8] +new_at_9 + old[9:] Something carefully written like that using concatenation means you do not lose track of indices and end up with a new extended list you can feel free to save under the old name and let the prior one be garbage collected. Maybe one of the above hints at what could work for you, or others may supply a better answer, or maybe you reevaluate what you are doing or explain it some more. -----Original Message----- From: Python-list <python-list-bounces+avi.e.gross=gmail....@python.org> On Behalf Of Thomas Passin Sent: Friday, March 3, 2023 1:04 PM To: python-list@python.org Subject: Re: Python list insert iterators On 3/3/2023 3:22 AM, Guenther Sohler wrote: > Hi Python community, > > I have a got an example list like > > 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 > T T > > and i eventually want to insert items in the given locations > (A shall go between 2 and 3, B shall go between 6 and 7) > > Right now i just use index numbers to define the place: > > A shall insert in position 2 > B shall insert in position 6 > > However when i insert A in position 2, the index for successful insertion > of B gets wrong > (should now be 7 instead of 6) > > No, it's not an option to sort the indexes and start inserting from the > back. > The most elegant option is not to store indexes, but list iterators, which > attach to the list element > and would automatically move, especially if an element is inserted before. > > I could not find such functionality in python lists of [ 1,2,3 ] > > Does python have such functionality ? > if yes, where can i find it ? You should be more clear about how to establish the desired insertion point. In your example, you first say that the insertion of B should be between 6 and 7. But after A gets inserted, you say that B's insertion point should change. How is anyone able to know what the correct insertion point should be at any time? If the rule is that B should get inserted after a particular known element, you can find out the index of that element with list.index() and insert just after that. If the rule is "There is an imaginary location that starts out after index 6 but moves depending on previous insertions", then you will probably need to capture a record of those insertions and use it to adjust the invisible insertion point. But this synchronization could be tricky to keep correct depending on what you want to do to this list. So you need to specify clearly what the rules are going to be. -- https://mail.python.org/mailman/listinfo/python-list -- https://mail.python.org/mailman/listinfo/python-list
