I'm trying to use IxSet for database-like purposes, so I've built an index of items and I need to do exact match on some field.
Query operator (#=) http://hackage.haskell.org/packages/archive/ixset/1.0.2/doc/html/Data-IxSet.html#v:-64--61- returns an IxSet a type, but I need only to either get 1 or 0 results. To check if it's 0 results, I do null items on that, but how do I get first item?
Due to description of IxSet type http://hackage.haskell.org/packages/archive/ixset/1.0.5/doc/html/Data-IxSet.html#t:IxSet I don't see any typeclass that has operation like head or fst.
Ok, found an answer. You should use some available function like getOne http://hackage.haskell.org/packages/archive/ixset/1.0.5/doc/html/Data-IxSet.html#v:getOne , for example.
p.s.: I find that a bit strange to first create your type from list of items and then add operations that duplicate list-operations, but maybe I'm just newbie :)
Related
When working with lists in Haskell, i can simply load my file into ghci and type head list or last list to get the information that I need. But if I have a list of lists, lets say: list = [[1,2,3],[4,5,6]], how can I get the first element (head) of the first list (in this case, 1), or the last element of the second list (in this case, 6), and so one?
If all you need is the first or last element, concat will flatten the list for you.
There is an indexing function (!!), so for your examples, head . (!!0) and last . (!!1) . If your question is more general, then please elaborate. Indexing is not great because it can throw errors if you attempt to index past the end of the list, so usually we try to work around that, eg. by saying "well I want to do the same thing to every element of the list so I don't really need the index" (map function) or "if I really do need the index then don't use it directly") (zip [0..], or use of eg. a record data type).
Also, Hoogle is your friend if you've not met it before. If you can break down your functions into simple ones you think might be standard, then search their type signatures, that's usually a good place to start. Hoogle [a] -> Int -> a even if you don't find exactly what you want, often if you find something similar and browse it's module or source code you can find something helpful.
Given a Julia object of composite type, how can one determine its fields?
I know one solution if you're working in the REPL: First you figure out the type of the object via a call to typeof, then enter help mode (?), and then look up the type. Is there a more programmatic way to achieve the same thing?
For v0.7+
Use fieldnames(x), where x is a DataType. For example, use fieldnames(Date), instead of fieldnames(today()), or else use fieldnames(typeof(today())).
This returns Vector{Symbol} listing the field names in order.
If a field name is myfield, then to retrieve the values in that field use either getfield(x, :myfield), or the shortcut syntax x.myfield.
Another useful and related function to play around with is dump(x).
Before v0.7
Use fieldnames(x), where x is either an instance of the composite type you are interested in, or else a DataType. That is, fieldnames(today()) and fieldnames(Date) are equally valid and have the same output.
suppose the object is obj,
you can get all the information of its fields with following code snippet:
T = typeof(obj)
for (name, typ) in zip(fieldnames(T), T.types)
println("type of the fieldname $name is $typ")
end
Here, fieldnames(T) returns the vector of field names and T.types returns the corresponding vector of type of the fields.
In boto3 there's a function:
ec2.instances.filter()
The documentation:
http://boto3.readthedocs.org/en/latest/reference/services/ec2.html#instance
Say it returns a list(ec2.Instance) I wish...
when I try printing the return I get this:
ec2.instancesCollection(ec2.ServiceResource(), ec2.Instance)
I've tried searching for any mention of an ec2.instanceCollection, but the only thing I found was something similar for ruby.
I'd like to iterate through this instanceCollection so I can see how big it is, what machines are present and things like that.
Problem is I have no idea how it works, and when it's empty iteration doesn't work at all(It throws an error)
The filter method does not return a list, it returns an iterable. This is basically a Python generator that will produce the desired results on demand in an efficient way.
You can use this iterator in a loop like this:
for instance in ec2.instances.filter():
# do something with instance
or if you really want a list you can turn the iterator into a list with:
instances = list(ec2.instances.filter())
I'm adding this answer because 5 years later I had the same question and went round in circles trying to find the answer.
First off, the return type in the documentation is wrong (still). As you say, it states that the return type is: list(ec2.Instance)
where it should be:ec2.instancesCollection.
At the time of writing there's an open issue in github covering this - https://github.com/boto/boto3/issues/2000.
When you call the filter method a ResourceCollection is created for the particular type of resource against which you called the method. In this case the resource type is instance which gives an instancesCollection. You can see the code for the ResourceCollection superclass of instancesCollection here:
https://github.com/boto/boto3/blob/develop/boto3/resources/collection.py
The documentation here gives an overview of the collections: https://boto3.amazonaws.com/v1/documentation/api/latest/guide/collections.html
To get to how to use it and actually answer your question, what I did was to turn the iterator into a list and iterate over the list if the size is > 0.
testList = list(ec2.instances.filter(Filters=filters))
if len(testList) > 0;
for item in testList;
.
.
.
This may well not be the best way of doing it but it worked for me.
I have a house with rooms that are defined with connections for when you can go from one room to another eg.
connection(garage,sidehall).
connection(sidehall,kitchen).
connection(kitchen,diningroom).
canget(X,Y):-connection(X,Y).
canget(X,Y):-connection(X,_),
write('player goes from '),write(X),write(' to '),write(Y),nl,
canget(_,Y).
Im trying to figure out how make it so the player can only get from one room to another when they have a specific item, such as you can only be in the kitchen when items = gloves.
canget(X,Y,Item):-connection(X,Y,Item),canbein(Y,Item).
canget(X,Y,Item):-connection(X,Somewhere,Item),canbein(Somewhere,Item),canget(Somewhere,Y,Item).
tried defining canbein with:
canbein(kitchen):- item(sword).
canbein(sidehall):- item(hat).
but that doesnt work!
Have defined my items as such, not sure if this is right either:
item(gloves,sword,helm,cheese).
Basically, have i declared my item values correctly?
How can i use the specific item value to make canget x to y false?
Thank you!
Well, I see a few problems with your code. Firstly, you call canbein with two arguments (from canget predicate). However, canbein is defined as single-argument predicate. Therefore, the call always fails as no canbein/2 predicate exists.
I suggest the following modification:
canbein(kitchen, sword).
canbein(sidehall, hat).
Than, the item definition is not required. Let's think about what happens during the unification of
canget(X,Y,Item) :- connection(X,Y,Item), canbein(Y,Item).
Let's assume the following setting X=sidehall, Y=kitchen, Item==sword. This predicate should be OK. Assuming the conection predicate is OK, prolog tries to find canbein(Y, Item) i.e. canbein(kitchen, sword) and it succeeds.
On the contrary, if the Item is different the unification fails, hence it works as expected.
The second problem is the item predicate. By your definition, it expects 4 arguments. That's nonsense, of course. You should declare it like
item(gloves).
item(sword).
item(helm).
item(cheese).
However, I don't think this predicate is necessary at all. Just to be clear, try to call item(X) and obtain all results (the four declared). Try it with the prior definition - what should you even ask for?
I hope it helps :)
test:1 AND NOT bool:true
returns 5 documents
but
test:1 AND (NOT bool:true)
returns 0 documents
Why?
Please Explain me the value of parentheses in lucene query formation
When you place (NOT bool:true) in parentheses it becomes a subquery, which is executed independent of the query test:1. NOT clauses in Lucene ONLY remove elements from the result set, they don't find anything. In SQL, for instance, you implicitly start with every value available, and filter elements which don't match clauses out. In Lucene, you start with nothing, and find results based on the clauses. The query NOT bool:true tells it what not to match, but doesn't give Lucene anything to find and return. Any query of the form:
(any query finding results) AND (NOT something)
Will find zero results, because, on it's own, NOT something finds nothing, and (something) AND (nothing) returns nothing. You can perform a search like that, by getting all values first, before the lonely NOT clause, like:
test:1 AND (*:* AND NOT bool:true)
However, that will perform very poorly, and your first example:
test:1 AND NOT bool:true
Is definitely the correct one.