maps:from_list seems to preserve the reverse of the order of the list. This example is from the docs:
List = [{"a",ignored},{1337,"value two"},{42,value_three},{"a",1}],
maps:from_list(List).
Is this behavior guaranteed?
"Preserve the reverse order of the list" isn't really accurate, since that would mean the map would somehow present that order, and it won't do that. I think what you're really asking about is whether values associated with keys that appear later in the list are stored in the map, effectively dropping those that appear earlier in the list; if so, the maps:from_list/1 documentation provides the following guarantee:
If the same key appears more than once, the latter (right-most) value
is used and the previous values are ignored.
Related
In InfluxDB (1.5), I have a table where the fields have become inconsistently typed. Most rows in the table are Integer, however, some rows have become strings.
How is this possible? I thought, once a field's types were set (upon first insert), any insert into the table with incorrect typing would fail.
What do I do now? If I go back and attempt to overwrite the data in the inconsistent rows, I get errors saying the field is a string.
After some more research, here's what I've discovered:
Answer to Part 1:
InfluxDB uses a system they refer to as 'sharding' - while I don't know the specifics, I do know that data from the same measurement/table can be stored across multiple, different 'shards'.
According to the InfluxDB documentation, field types can differ between these shards, within the same field, on the same table.
Answer to Part 2:
In order to fix this, the currently-suggested answer is to make a new table, download all the data, and re-insert while ensuring the data that gets inserted is the proper types.
If you had a tag which changed type and became a field, this can be especially difficult to fix, the link above does not address that. To do selects only on tag or field, you can use tag_name::tag or field_name::field within a select statement.
The GROUP BY * clause suggested in the link is required in order to preserve tags, but seemed to cause issues when I used it.
My current solution is a PHP script that uses curl, downloads the points, chunks them, then re-inserts the points into the new table, ensuring each point that gets inserted is casted to the new, uniform type, and properly inserted.
The best way to stopping future issues, is simply not to have them. I went looking for how to lock field types in all cases, across all shards, for a particular measurement table.
Unfortunately, it seems impossible to guarantee 100% type consistency across all current and future shards. "Don't make mistakes because it's really difficult to clean up" seems to be InfluxDB's modus operandi.
There is a table I found below
My question is whether or not it is true that a single and double linked list have the same operation run times like the table seems to show. I would think in the deletion case for example, a double linked list would be better since we have access to previous. So is the table wrong on that being O(n) for singly linked lists?
If they are all the same, does this similarity hold for a circular one as well?
Thanks.
Here is my answer to your question:
No matter whether the double linked list enable you have access to previous or not, it doesn't affect the time complexity we calculate in terms of Big O notation, I think it does give you some convenience though.
Yes, they are all the same, and the similarity holds for a circular one as well.
I would like to know how I can prevent a duplicate entry (based on my own client/project definition of what that means-below), in an AppSheet mobile app connected to Google Sheets.
AppSheet talks alot about UNIQUEID() which they also encourage using and designating as the KEY field. row_number is another possibility.
This is fine for the KEY in the sense of its purpose is to be unique, meaningless, and uniquely identify a record, and relate to other tables.
However, it doesn't prevent a duplicate ("duplicate" again, as defined by my own client's business rules&process) from occurring. I mean, I assume the UniqueId() theoretically would, but that's abstract theory, because it would only produce unique ones anyway.
MY TABLE HAS THESE COLUMN: [FACILITY NUMBER] and [TIMESTAMP] (date and time of event). We consider it a duplicate event, and want to DISALLOW the adding of such a record to this table, if the 2nd record has the same DATE (time irrelevant), with the same FACILITY. (we just do one facility per day, ever).
In AppSheet how can I create some logic that disallows the add based on that criteria? I even basically know some ways I would do it. it just seems like I can't find a place to "put" it. I created an expression that perfectly evalutes to TRUE or FALSE and nothing else, (by referencing whether or not the FACILIY NUMBER on the new record being added is in a SLICE which I've defined as today's entries). I wanted to place this expression in another (random) field's VALIDIF. To me it seemed like that would meet the platform documentation. the other random field would be considered valid, only if the expression evaluated to true. but instead appsheet thought i wanted to conver the entire [other random column] to a dependent dropdown.
Please help! I will cry tears of joy when appsheet introduces FORM events and RECORD events that can be hooked into at the time of keying, saving, etc.
surprised to see this question here in stackoverflow --- most AppSheet questions are at http://community.appsheet.com.
The brief answer is that you are doing the right thing providing a Valid_If constraint. Your constraint is of the form IN([_THIS], ) so AppSheet is doing the "smart" thing by automatically converting that list into a dropdown of allowed values. From your post, it appears that you may instead want to say NOT(IN([_THIS], )) -- thereby saying that the value [_THIS] is valid as long as it is not in the list specified (making sure it is not a duplicate).
Old question, but in case someone stumbles upon the same:
The (not so simple) answer is given in https://help.appsheet.com/en/articles/961274-list-expressions-and-aggregates.
From the reference:
NOT(IN([_THIS], SELECT(Customers[State], NOT(IN([CustomerId],
LIST([_THISROW].[CustomerId])))))): when used as the Valid_If
condition for the State column, it ensures that every customer has a
unique value for State. In this example, we assume that CustomerId is
the key for the Customers table.
This could be written more schematic like this:
NOT(IN([_THIS], SELECT(<TableName>[<UnqiueColumnName>], NOT(IN([<KeyColumnName>], LIST([_THISROW].[<KeyColumnName>]))))))
Technically it says:
Get me a list of the current values of the column of the table
Ignore the value of the current row (identified by [_THISROW] and looking into the column)
Check, if the given value exists in the resulting list
This statement has to be defined - with the correct values for , & - as Valid_If statement.
What is the suggested approach for updating an objects value in an array, bearing in mind the array may have been reordered?
I'm wondering how dangerous using index based paths is, when an array could have possibly changed via a deletion, or reorder.
Would it be better to use objects instead, I wonder.
If you are using a mutable list, it is inherently unsafe to update an object by its position in a list. The right thing to do is to use deref. Assuming you have a list of references (the most common case) you can dereference a Model at its position in the list. This will ensure it points to the object's identity path rather than the index in the list. Then you can update the object directly without worrying about whether it has moved around in the list.
In my code I want to take advantage of ETS's bag type that can store multiple values for single key. However, it would be very useful to know if insertion actually inserts a new value or not (i.e. if the inserted key with value was or was not present in the bag).
With type set of ETS I could use ets:insert_new, but semantics is different for bag (emphasis mine):
This function works exactly like insert/2, with the exception that instead of overwriting objects with the same key (in the case of set or ordered_set) or adding more objects with keys already existing in the table (in the case of bag and duplicate_bag), it simply returns false.
Is there a way to achieve such functionality with one call? I understand it can be achieved by a lookup followed by an optional insert, but I am afraid it might hurt performance of concurrent access.