There are some collections - let's say each collection is set of programming languages a developer knows.
{"Alice" => Set["Java", "Python", "C++"], "Bob" => Set["Ruby"], "Charlie" => Set["Ruby", "C++"]}.
I want to group these objects by collections they belong to - in this case getting a mapping from sets of developers sharing knowledge of certain languages to sets of such languages. Every language present in the input will occur exactly once here:
{Set["Alice"] => Set["Java", "Python"], Set["Alice", "Charlie"] => Set["C++"], Set["Bob", "Charlie"] => Set["Ruby"]}
Type of this operation would be Hash[A, Set[B]] => Hash[Set[A], Set[B]]. (in practice plain arrays would most likely be used instead of sets, I'm using sets here to say that order doesn't matter and there are no duplicates)
I'm not asking how to code this operation (of course if you know a particularly elegant way, feel free to share) - I'm wondering if it has a name. It seems common enough that it should, but I cannot think of anything.
"Reverse Mapping"?
Related
I've been flirting with Redis for a while now.
I've watched these series some time ago and they were awesome. I've been through some of the documentation and the mentioning of the Time complexity of the queries blew me away, this is something that's rarely mentioned in web materials but is of huge importance for app building.
Anyhow I'm trying to make my app use the Redis on the consumer end so the users can fetch the data as fast as possible.
So I'm trying to save some objects to hash as:
$redis->hmset("taxi_car", array(
"brand" => "Toyota",
"model" => "Yaris",
"license number" => "RO-01-PHP",
"year of fabrication" => 2010,
"nr_stats" => 0)
as found here and this works nicely.
However I can't find a way to delete the whole entry anywhere.
Did I get this hash thing wrong?
Following this example I would like to delete the entry with given licence number. All I could find is how to delete the licence number from the object:
$redis->hdel("taxi_car", "license number");
and can't figure out how to delete the whole hash row (please do correct with proper word for row here).
Another problem here is that it seems this only allows me to save a single taxi_car in the Redis. How do I set the UUID so I can have multiple Taxi cars?
I'm going to play with this a bit, any help is welcome. Thanks!
To delete a key of any type, Hash included, call the Redis DEL command.
To have multiple keys, give them different names, e.g. taxi_car:1, taxi_car:2 etc.
Well, I'm a bit new to Prolog, so my question is on Prolog pattern/logic.
I have an relationship called tablet. It has many parameters, such as name, operationSystem, ramCapacity, etc. I have many objects/predicates of this relationship, like
tablet(
name("tablet1"),
operatingSystem("ios"),
ramCapacity(1024),
screen(
type("IPS"),
resolution(1024,2048)
)
).
tablet(
name("tablet2"),
operatingSystem("android"),
ramCapacity(2048),
screen(
type("IPS"),
resolution(1024,2048),
protected(yes)
),
isSupported(yes)
).
And some others similar relationships, BUT with different amounts of parameters. Some of attributes in different objects I do not need OR I have created some tablets, and one day add one more field and started to use it in new tablets.
There are two questions:
I need to use the most flexible structure as possible in prolog. Some of the tablets have attributes/innerPredicates and some do not, but They are all tablets.
I need to access data the easiest way, for example find all tablets that have ramCapacity(1024), not include ones that do not have this attributes.
I do need to change some attributes' values in the easiest way. For example query - change ramCapacity to 2048 for tablet that has name "tablet1".
If it's possible it should be pretty to read in a word editor :)
Is this structure flexible? Should I use another one? Do I need additional rules to manipulate this structure? Is this structure easy to change with query?(I keep this structure in a file).
Since the number of attributes is not fixed and needs to be so flexible, consider to represent these items as in option lists, like this:
tablet([name=tablet1,
operating_system=ios,
ram_capacity=1024,
screen=screen([type="IPS",
resolution = res(1024,2048)])]).
tablet([name=tablet2,
operating_system=android,
ram_capacity=2048,
screen=screen([type="IPS",
resolution = res(1024,2048)]),
is_supported=yes]).
You can easily query and arbitrarily extend such lists. Example:
?- tablet(Ts), member(name=tablet2, Ts).
Ts = [name=tablet2, operating_system=android, ram_capacity=2048, screen=screen([type="IPS", resolution=res(..., ...)]), is_supported=yes] ;
false.
Notice also the common Prolog naming_convention_to_use_underscores_for_readability instead of mixingCasesAndMakingEverythingExtremelyHardToRead.
is it possible to create some unique ID for articles on rails?
For example, first article will get ID - aa-001,
second - aa-002
...
article #999 - aa-999,
article #1000 - ab-001 and so on?
Thanks in advance for your help!
The following method gives the next id in the sequence, given the one before:
def next_id(id, limit = 3, seperator = '-')
if id[/[0-9]+\z/] == ?9 * limit
"#{id[/\A[a-z]+/i].next}#{seperator}#{?0 * (limit - 1)}1"
else
id.next
end
end
> next_id("aa-009")
=> "aa-010"
> next_id("aa-999")
=> "ab-001"
The limit parameter specifies the number of digits. You can use as many prefix characters as you want.
Which means you could use it like this in your application:
> Post.last.special_id
=> "bc-999"
next_id(Post.last.special_id)
=> "bd-001"
However, I'm not sure I'd advice you to do it like this. Databases have smart methods to avoid race conditions for creating ids when entries are created concurrently. In Postgres, for example, it doesn't guarantee gapless ids.
This approach has no such mechanism, which could potentially lead to race conditions. However, if this is extremely unlikely to happen such in a case where you are the only one writing articles, you could do it anyway. I'm not exactly sure what you want to use this for, but you might want to look into to_param.
You may want to look into the FriendlyId gem. There’s also a Railscast on this topic which covers a manual approach as well as the usage of FriendlyId.
I am building a system that keeps track of many counters in real time in Redis. Each counter is basically the impression, conversion details for ad keywords shown on a specific url.
ie. if 10 keywords are shown on a specific url, I need to update a count for each of those keywords for both impressions and conversions. And on each impression of a url, possibly a different set of 10 keywords can be shown.
ie. the basic data model I need is something like
> url=>
k1 =>
impression => 2
conversion => 1
k2 =><br>
impression => 100
conversion => 8
.
.
k100 (max around 100)</li>
I understand Redis doesnt have nested hashes so I cant store a 2 level hash as I have shown above.
What is the best way to solve this problem?
I thought of combining k1-impression and k1 conversion and making it one single field
ie like
url =>
k1-impression => 100
k1-conversion => 3
.<br>
. so on</li>
But the problem is the lengths of 'k1', 'k2' etc is significant ( 120-150 bytes) and I dont want to replicate that data, if possible, to save on memory.
How would I go about solving this problem?
Any help will be appreciated.
If your keywords are of significant enough length that you're worried about it, you should normalize them. Make a hash of keyword -> id, and a hash of id -> keyword, for encoding and decoding them. Then you can have per-url hashes of the form url => {kw_id:impressions => 1123, kw_id:conversions => 28}. This will also serve you well when you start needing to make indexes of the key words, which you will as soon as you get a requirement to show the top 10 best performing key words across all urls, for example.
I need to be able to lookup based on the full key or part of the key..
e.g. I might store keys like 10,20,30,40 11,12,30,40, 12,20,30,40
I want to be able to search for 10,20,30,40 or 20,30,40
What is the best data structure for achieving this..best for time.
our programming language is Java..any pointers for open source projects will be appreciated..
Thanks in advance..
If those were the actual numbers I'd be working with, I'd use an array where a given index contains an array of all records that contain the index. If the actual numbers were larger, I'd use a hash table employed the same way.
So the structure would look like (empty indexes elided, in the case of the array implementation):
10 => ((10,20,30,40)),
11 => ((11,12,30,40)),
12 => ((11,12,30,40), (12,20,30,40)),
20 => ((10,20,30,40), (12,20,30,40)),
30 => ((10,20,30,40), (11,12,30,40), (12,20,30,40)),
40 => ((10,20,30,40), (11,12,30,40), (12,20,30,40)),
It's not clear to me whether your searches are inclusive (OR-based) or exclusive (AND-based), but either way you look up the record groups for each element of the search set; for the inclusive search you find their union, and for the exclusive search you find their intersection.
Since you seen to care about retrieval time over other concerns (such as space), I suggest you use a hashtable and you enter your items several times, once per subkey. So you'd put("10,20,30,40",mydata), then put("20,30,40",mydata) and so on (of course this would be a method, you're not going to manually call put so many times).
Use a tree structure. Here is an open source project that might help ... written in Java :-)
http://suggesttree.sourceforge.net/