Initializing an Empty List

Hello,

With the actual version of NQE not providing direct way to initialise an Empty list

ex:

let empty_list = ]

empty_list = ]

A workaround for this is to construct the creation of that list and providing a value with the same data type you need

// an empty List<String>
FALLBACK =
foreach x in n""]
where x != ""
select x;

// an empty list of Numbers
FALLBACK_NUMBERS=
foreach x in i0]
where x != 0
select x;

Taking this logic further we can create a function that provides us an empty list of whatever generic type. by taking a default value of that type as an input, then comparing against that value

The below block contains the full PoC code

// an empty List<String>
FALLBACK =
foreach x in  ""]
where x != ""
select x;

// an empty list of Numbers
FALLBACK_NUMBERS=
foreach x in  0]
where x != 0
select x;

// An empty list of a generic type T
emptyListT(default_val)=
foreach x in  default_val]
where x != default_val
select x;

foreach a in  ""]
let empty_list_of_strings = FALLBACK
let empty_list_of_numbers = FALLBACK_NUMBERS

// this provides a List<IpAddress>
let empty_list_of_ips = emptyListT(ipAddress("127.0.0.1"))

foreach ip in empty_list_of_ips
select {ip}

Finally, Thanks to @cariddir for encouraging me to start publishing :)

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We can complement this with some extra functions, to add/remove items from a list


addToList(value, list) =
  min(foreach x in [0]
      select if value in list then list else list + [value]);

removeFromList(value, list) =
  min(foreach x in [0]
      select list - [value]);

sample use of this as shown below

foreach x in [0]
// this provides a List<IpAddress>
let iplist = emptyListT(ipAddress("127.0.0.1"))
let iplist = addToList(ipAddress("1.1.1.1"), iplist)
// this next line won't be added (duplicate)
let iplist = addToList(ipAddress("1.1.1.1"), iplist)
// 1.1.1.2 will be added
let iplist = addToList(ipAddress("1.1.1.2"), iplist)
// 1.1.1.1 will be removed
let iplist = removeFromList(ipAddress("1.1.1.1"), iplist)
// 1.1.1.4 won't be removed, although it's not included in the list in any case.
let iplist = removeFromList(ipAddress("1.1.1.4"), iplist)
foreach ip in iplist
select { ip }

the expected output is 1.1.1.2

Ahmed and Andy,

Thank you for providing this information. However, NQE is not like Python. You don’t need to initialize a list before you added entries to it. You can create a list an any time.

I have had to use an Empty list for times when I adding things like two records together, that don’t all have the same fields. For example, the query from the Forward Library FQ_d885c4d4d82ea9d4fee7469bed2b66e1df77509e. In this case it has an “emptyNumberList = fromTo(1, 0);” because you have interfaces that are not trunks that would be added to lists of interface that are trunks. Then the interface that is not a trunk would have an empty vlan list.

You can also add things into a list with a plus sign. You can add lists together.

Initializing an empty list may be something you are used to, so you can probably still work that way. However, I just wanted to point out that it is not required in the NQE language. Notice, I don’t need to initialize “twoLists” or “threeLists”.

I apologize if I’m interpreting your post wrong.

Regards,

-Tyson

Oh, and I forgot to show subtracting a entry from a list.