So I have a query like this one
let query =
query {
for person in people do
select person
}
And I'd like to have it sequenced.
let sequence : seq<Person> = query
But I can't find any information on how to do it, maybe I've become bad at using search engines.
I'm getting unexpected type compiling expections using things like |> seq.ofList and ToList().
The expression was expected to have the type seq<Person> but here has the type Generic.List<Person>.
The result of a query expression has type IQueryable<_>, which is a subtype of IEnumerable<_> (for which seq<_> is a synonym), so you can simply change the type:
let mySeq : seq<_> = myQuery
Or, if you want to avoid a type annotation, use the built-in seq function, which does the same thing:
let mySeq = seq myQuery
Related
Having 2 lists, I want check which values of List1 are in List2. I'm trying as below but I get error
List1 = {3,2,8,7,5}
List2 = {1,3,4,2,6,7,9}
= List.Transform(List1, each Text.Contains(List2, _))
Expression.Error: We cannot convert a value of type List to type Text.
Details:
Value=[List]
Type=[Type]
My expected output would be 3,2,7.
How can I do this?
See List.Intersect Documentation
Intersect = List.Intersect({List1,List2})
#horseyride has probably the best answer but using your original logic, you could also write the intersection like this:
List.Select(List1, each List.Contains(List2, _))
This uses Select instead of Transform since you are trying to select/filter instead of changing the elements and uses the appropriate List type instead of Text for the Contains part.
Am having a request in the following design :- example.com/?param1=blah¶m1=blah1....param1=blahn. Notice that I want to have param1 with several parameters. Now to capture the values of the parameter param1, I do the following
searchQery = request.GET.getlist('param1')
where I get a list with values for param1. I want to use a complex query
for item searchQuery:
val = MyModel.objects.filter(Q(item__startswith=searchQuery[0])| Q(item__startswith=searchQuery[1])| Q(item__startswith=searchQuery[2])) all the way to searchQuery[n]
In short I want to iterate through the values of the list, dynamically and am currently lost there.
from operator import or_
val = MyModel.objects.filter(reduce(or_,(Q(item__startswith=x) for x in searchQuery)))
Use reduce to combine a generated sequence of Q expressions.
Scala noob i'm afraid:
I have the following declared class variable which will the objects I read from the database:
val options = mutable.LinkedList[DivisionSelectOption]()
I then use JPA to get a List of all rows from a table:
val divisionOptions = em.createNamedQuery("SelectOption.all", classOf[SelectOption]) getResultList
/* Wrap java List in Scala List */
val wrappedOptions = JListWrapper.apply(divisionOptions)
/* Store the wrappedOptions in the class variable */
options += wrappedOptions
However, the last line has an error:
Type Expected: String, actual JListWrapper[SelectOption]
Can anyone help with what I am doing wrong? I'm just trying to populate the options object with the result of the DB call.
Thanks
What (probably) is happening is that a JlistWrapper[SelectOption] isn't a DivisionSelectOption, so the method += isn't applicable to it. That being the case, it is trying other stuff, and giving a final error on this:
options = options + wrappedOptions
That is a rewriting Scala can do to make things like x += 1 work for var x. The + method is present on all objects, but it takes a String as parameter -- that's so one can write stuff like options + ":" and have that work as in Java. But since wrappedOptions isn't a String, it complains.
Roundabout and confusing, I know, and even Odersky regrets his decision with regards to +. Let that be a lesson: if you thing of adding a method to Any, think really hard before doing it.
This is the error I'm getting and I have no idea why: "Error: Unbound record field label length "
Does anyonw know?
let rastavi str =
let sublist = ref [] in
let list = ref [] in
for i = ((str.length str)1) [down]to 0 do
if str.[i] =' ' then (str.[i] :: !sublist)
else (list := (!sublist:: !list)) sublist = []
done ;;
You're using OO notation to get the length of a string. OCaml uses functional notation. So it looks like this:
String.length str
Not like this:
str.length (* OO notation, not in OCaml *)
Edit:
Side comment: this solution is very much an imperative take on the problem. If you're trying to learn the FP mindset, you should try to think recursively and immutably. Since this looks like homework, it's very likely a functional solution is what you want.
But here are a few other problems in your original code:
You have two expressions next to each other with nothing in between. If you want to "do" two things, you need to separate them with a semicolon ; (however, this is imperative style)
You're using = which compares two values for equality. If you want to assign a value to a reference you need to use :=. (Imperative style, again.)
I would like to create a lookup table in OCaml. The table will have 7000+ entries that, upon lookup (by int), return a string. What is an appropriate data structure to use for this task? Should the table be externalized from the base code and if so, how does one go about "including" the lookup table to be accessible from his/her program?
Thanks.
If the strings are addressed using consecutive integers you could use an array.
Otherwise you can use a hash table (non-functional) or a Map (functional). To get started with the Map try:
module Int =
struct
type t = int
let compare = compare
end ;;
module IntMap = Map.Make(Int) ;;
If the table is too large to store in memory, you could store it in an external database and use bindings to dbm, bdb, sqlite,...
let table : (int,string) Hashtbl.t = Hashtbl.create 8192
To store the table in a separate file (e.g. as an array), simply create a file strings.ml with the content:
let tbl = [|
"String 0";
"String 1";
"String 2";
...7000 more...
|]
Compile this with:
ocamlc -c strings.ml
As explained in the manual, this defines a module Strings that other Ocaml modules can reference. For example, you can start a toplevel:
ocaml strings.cmo
And lookup a string by accessing a particular position in the array:
Strings.tbl.(1234) ;;