What is the canonical way in Crystal to insert a key into a Hash, but throw an error if the mapping did exist before. In principle, code like this:
map = Hash(String, Int32).new
if map.has_key?("foo")
raise "Mapping already exists"
else
map["foo"] = 42
end
I wondered if is it possible without hashing twice? I came up with this idea, but it is hard to read IMHO:
map = Hash(String, Int32).new
if map.put("foo", 42) {}
raise "Mapping already exists"
end
(If the mapping exists, Map#put returns the old value. Otherwise, the result of the block is called, which is nil and evaluates to false.)
Is there a recommended pattern?
Performance wise you already arrived at the optimal solution. I would probably use the fact that block calls get inlined to rewrite it slightly:
def update(hash, key, value)
hash.put(key, value) { return }
raise "Duplicate entry"
end
However there's an important pitfall here: The hash still gets updated in the error condition:
hash = {"foo" => "bar"}
update(hash, "baz", "quux")
pp hash # => {"foo" => "bar", "baz" => "quux"}
update(hash, "foo", "hello") rescue puts "Failed updating foo" # => Failed updating foo
pp hash # => {"foo" => "hello", "baz" => "quux"}
https://carc.in/#/r/bn1o
Without breaking into internal APIs I don't think there's a way to avoid this. In practice a hash lookup is not very expensive, after all its a data structure optimized for looking up a value by a key. Especially since you seem to be okay with raising in your API, which is a much more expensive operation in relation. So personally I would go for something like your first example.
Related
my #g = (1,2,3,4);
say reduce {is-prime}, #g; # ==> gives error
say reduce {is-prime *}, #g; #==> gives error
say reduce {is-prime}, (1,2,3,4); # ==> gives error
say so is-prime #g.all; # ==> gives error
How to check if all elements of list are prime in Raku?
The answers above are all helpful, but they fail to explain why your solution does not work. Basically reduce is not going to apply a function (in your case, is-prime) to every member of a list. You want map for that. The error says
Calling is-prime() will never work with signature of the proto ($, *%)
Because reduce expects an infix, thus binary, function, or a function with two arguments; what it does is to apply them to the first pair of elements, then to the result and the third element, and so on. Last statement does not work for a similar reason: you are calling is-prime with a list argument, not a single argument.
You're basically asking: are there any elements in this list which are not prime? I would write that as:
say "not all prime" if #g.first: !*.is-prime;
Please note though, that apparently 1 is not considered prime according to the is-prime function:
say 1.is-prime; # False
so the first would trigger on the 1 in your example, not on the 4.
There are of course may ways to do this. A very explicit way is using a for loop:
for #g -> $g {
if $g.is-prime {
say $g;
}
}
Or with a grep (you could leave the $_ implicit):
#g.grep({ $_.is-prime }).say
Both above are assuming you really want to filter the primes out. Of course you can also really check each number and get a boolean:
#g.map({ .is-prime }).say
There is a big problem with this:
say reduce {is-prime}, #g;
You created a lambda:
{ }
The only thing it does is calls a function:
is-prime
You didn't give the function any arguments though.
Is it just supposed to guess what the arguments should be?
If you meant to pass in is-prime as a reference, you should have used &is-prime rather than {is-prime}.
Of course that still wouldn't have worked.
The other problem is that reduce operates by recursively combining values.
It can't do that if it operates on one argument at a time.
The bare block lambda {}, takes zero or one argument, not two or more.
reduce is often combined with map.
It happens so often that there is a Wikipedia page about MapReduce.
say ( map &is-prime, #g ==> reduce { $^a and $^b } );
# False
say ( map &is-prime, 2,3,5 ==> reduce { $^a and $^b } );
# True
I wrote it that way so that map would be in the line before reduce, but perhaps it would be more clear this way:
say reduce {$^a and $^b}, map &is-prime, 2,3,5;
# True
reduce with an infix operator is so common that there is a shorter way to write it.
say [and] map &is-prime, 2,3,5;
# True
Of course it would be better to just find the first value that isn't prime, and say the inverse.
Since if there is even a single value that isn't prime that would mean they can't all be primes.
You have to be careful though, as you may think something like this would always work:
not #g.first: !*.is-prime;
It does happen to work for the values you gave it, but may not always.
first returns Nil if it can't find the value.
not (2,3,5).first: !*.is-prime;
# not Nil === True
not (2,3,4).first: !*.is-prime;
# not 4 === False
not (2,3,0,4).first: !*.is-prime;
# not 0 === True
That last one returned 0 which when combined with not returns True.
You could fix this with defined.
not defined (2,3,0,4).first: !*.is-prime;
# False
This only works if first wouldn't return an undefined element that happens to be in the list.
(Int,Any).first: Real
# Int
defined (Int,Any).first: Real
# False
You could fix that by asking for the index instead of the value.
You of course still need defined.
(Int,Any).first: :k, Real
# 0
defined (Int,Any).first: :k, Real
# True
The other way to fix it is to just use grep.
not (2,3,0,4).grep: !*.is-prime;
# not (0,4) === False
Since grep always returns a List, you don't have to worry about checking for 0 or undefined elements.
(A List is True if it contains any elements, no matter what the values.)
grep is smart enough to know that if you coerce to Bool that it can stop upon finding the first value.
So it short-circuits the same as if you had used first.
This results in some fairly funky code, with those two negating operators. So it should be put into a function.
sub all-prime ( +#_ ) {
# return False if we find any non-prime
not #_.grep: !*.is-prime
# grep short-circuits in Bool context, so this will stop early
}
This could still fail if you give it something weird
all-prime 2,3,5, Date.today;
# ERROR: No such method 'is-prime' for invocant of type 'Date'
If you care, add some error handling.
sub all-prime ( +#_ ) {
# return Nil if there was an error
CATCH { default { return Nil }}
# return False if we find any non-prime
not #_.grep: !*.is-prime
}
all-prime 2,3,5, Date.today;
# Nil
use the all junction:
say so all #g».is-prime; # False
I have found Doctrine\Common\Collections\Criteria to be a very useful concept, if they worked for me.
In a symfony controller, I am calling this code:
$criteria = Criteria::create()
->where(Criteria::expr()->gt('position', 0))
->orderBy(['riskPosition', Criteria::ASC]);
$positions= $this->getDoctrine()->getRepository(DataCategory::class)->matching($criteria);
dump($positions->count()); // dumps 1, correct!
dump($positions);
foreach($positions as $r)
dump($r); // -> Unrecognized field: 0
dump($positions) gives
LazyCriteriaCollection {#881 ▼
#entityPersister: JoinedSubclassPersister {#849 ▶}
#criteria: Criteria {#848 ▼
-expression: Comparison {#836 ▶}
-orderings: array:2 [▶]
-firstResult: null
-maxResults: null
}
-count: 1
#collection: null
#initialized: false
}
As soon as I access an element of the returned array, I get an error
ORMException::unrecognizedField(0)
in vendor/doctrine/orm/lib/Doctrine/ORM/Persisters/Entity/BasicEntityPersister.php (line 1193)
But as soon as I want to access the elements (e.g. loop and dump) I get some error like An exception has been thrown during the rendering of a template ("Unrecognized field: 0").
As far as I have studied the code, the problem is that the query results have not been fetched from the database. Only count() works. How can I trigger this fetch?
Does it matter that my entity has #ORM\InheritanceType("JOINED")?
This code (circumventing the use of Criteria) does give correct results, but I'd like to use Criteria:
$riskPositions = $this->getDoctrine()->getRepository(DataCategory::class)
->createQueryBuilder('p')
->where('p.position > 0')
->orderBy('p.position', 'ASC')
->getQuery()
->execute();
The issue is caused by line:
->orderBy(['riskPosition', Criteria::ASC]);
Doctrine\Common\Collections\Criteria `s orderBy accepts an array argument where
Keys are field and values are the order, being either ASC or DESC.
github link
Apparently, there is a mistake at doctrine s documentation.
So doctrine thinks that "0", which is the 1st key of the array argument, is the field to sort by, but cannot find it.
To solve, change the above line to:
->orderBy(['riskPosition' => Criteria::ASC]);
tl;dr
How can I derive a keyword from a number in ClojureScript:
(keyword 22)
;;=> :22 but in fact returns nil.
Background
In my ClojureScript/Hoplon application I make HTTP requests via cljs-http. Parts of the response I receive look like this:
{:companies
{:22 {:description ... } ; A company.
:64 {:description ... }
... }
{:offers
[{:description ... } ; An offer.
{:description ... }
... ]
Each offer within the vector behind :offers has a :companyId which represents a key in :companies. As soon as I receive the response, I reset! a cell (similar to an atom) query.
Now, I'd like to iterate over each offer and call a function offer-tpl that creates the corresponding HTML. In order to do so, offer-tpl needs the offer itself as well as the related company:
(for [offer (:offers #query)]
(offer-tpl offer (get-in #query [:companies (keyword (:companyId offer))]))))))
Despite the fact that this surely can be done more elegant (suggestions very appreciated), the get-in doesn't work. (:companyId offer) returns a number (e.g. 22) but (keyword (:companyId offer)) returns nil. Calling (keyword (str (:companyId offer))) does the trick, but aren't there any other ways to do this?
(keyword "22") or (keyword (str 22)) returns :22
The reason you are getting :22 is likely because of the keywordize-keys option of a JSON translation. For example:
cljs-http defaults to keywordize-keys for jsonp:
https://github.com/r0man/cljs-http/blob/1fb899d3f9c5728521786432b5f6c36d1d7a1452/src/cljs_http/core.cljs#L115
But you can (and should) in this case pass in a flag to disable keywordization.
Not all keys in JSON are appropriate for Clojure keywordization. For example spaces in a JSON key are valid, but not in Clojure.
Please be aware that numeric keywords are probably incorrect.
https://clojuredocs.org/clojure.core/keyword#example-542692cec026201cdc326d70
It seems like that caveat has been removed from the current Clojure website, so perhaps that means something but I'm not sure what.
http://clojure.org/reference/reader Currently states that
Keywords - Keywords are like symbols, except: They can and must begin
with a colon, e.g. :fred. They cannot contain '.' or name classes.
Like symbols, they can contain a namespace, :person/name A keyword
that begins with two colons is resolved in the current namespace: In
the user namespace, ::rect is read as :user/rect
and that
Symbols begin with a non-numeric character and can contain
alphanumeric.
This definition of a keyword excludes :22 and :with spaces
The keyword function returns a result for invalid input, but this is not an endorsement, it is simply because checking for incorrect input would be a performance overhead in a core part of Clojure.
In short, not all JSON keys translate to keywords, so you should avoid keywordize-keys unless you know the keyspace and/or doing so provides some conveniences.
I have a list of tuples that I must change the values for in a map that contains those tuples. So if I have a list such as List((0,2), (0,3)) with a map that looks like this: Map((0,2) => List(1,2,3), (0,3) => List(1,2)), I need to access the matching map tuples with the tuples listed in the list, then remove a number from the mapping.
So in the example above, if I wanted to remove 2 from the mapping, I would get Map((0,2) => List(1,3), (0,3) => List(1)).
Design wise, I was thinking of pattern matching the map, but I've read some answers that said that may not be the best way. The tough part for me is that it has to be immutable, so I was thinking of pattern matching the list, getting the map value, change the value, then recreate the map and recursively call the function again. What do you think of this implementation?
This could be a way to remove 2 from your Map:
val newMap = oldMap.mapValues(list => list.filter(_ != 2))
Or more generally:
def filterInMap(element: Int, oldMap: Map[(Int,Int),List[Int]]) =
oldMap.mapValues(list => list.filter(_ != element))
This way there's no need to mutate anything at all. mapValues transforms just the values of your Map and returns a copy of the original without mutating it at all. filter then gets the job done by only allowing elements that don't match the element we would like to remove.
Bonus: even more generally:
def filterInMap[A](element: A, oldMap: Map[(A,A),List[A]]) =
oldMap.mapValues(list => list.filter(_ != element))
How come the Hashtbl remove restores the previous binding.
Hashtbl.add t key1
Hashtbl.remove t key1
Hashtbl.remove t key1 => This should do anything but not restore the key1 !
Anyway, how come I can remove something making sure and if it was deleted before then proper flow shall be followed?
val remove : ('a, 'b) t -> 'a -> unit
Hashtbl.remove tbl x removes the current binding of x in tbl, restoring the previous binding if it exists. It does nothing if x is not bound in tbl.
There are two legitimate mode of uses of Hashtbl: always using Hashtbl.replace, which ensures that each key only has one binding in the table, or using the table as a multi-mapping (each key pointing to a list of values) with Hasthbl.add, Hashtbl.find and Hashtbl.find_all.
Please make sure that you understand which mode of use you're interested in. There is no point in adding several bindings to the same key if you don't want to keep old bindings (this can result in performance issues, memory leaks and stack overflows); in that case you should use Hashtbl.replace instead of Hashtbl.add, and Hashtbl.remove will do exactly what you expect.
If you are using the hashtable as a multi-mapping, and want a function that remove all bindings for a key, you can implement it yourslef (code untested):
let rec remove_all tbl key =
if Hashtbl.mem tbl key then begin
Hashtbl.remove tbl key;
remove_all tbl key
end
Edit: I just understood that another way to read your (hard to understand) question is "how can I make sure that there is a key to remove in the table, instead of silently doing nothing when remove is called?". cago provides a code snippet for that, in essence you can use Hashtbl.mem to check that the binding exists when you assume it should exist.
If you use Hashtbl.replace instead of Hashtbl.add you'll replace the current binding of the key in t. So the function Hashtbl.remove will not restore anything.
You can also write your own remove function :
let remove tbl key =
if Hashtbl.mem tbl key then Hashtbl.remove tbl key
else raise Nothing_to_remove_in_the_hashtbl
Hashtbl.replace t key1 value;;
remove t key1;;
remove t key1;; (* raise Nothing_to_remove_in_the_hashtbl *)