i am starting to think through an example program i want to try. the premis is simple in that it lists me a large list of names with their gender.
for now i started to have a Person Class which has name and gender as properties. after that i build up a dictionary with the alphabetic letters as keys and an array of persons underneath each other. that works fine as in the table methods i can easily get the persons for each alphabet letter.
now i want to implement that i can switch of male or female.
the problem i'm running into is that all tableview controller methods would suddenly get much more complex. as an instance the numberOfRowsInSection method. before i just got the array for the letter key and returned the count. now i would have to parse through everything and count up the male Persons or female.
i feel like there is maybe a better way to approach this. but i'm not certain.
any ideas ? thanks in advance
You either have to do it dynamically as you suggest which could become really slow if there are a lot of people in the list.
The other approach is to separate things into 3 arrays: one that contains everyone, one that contains males, and one that contains females.
Then you have a pointer that references one of the 3 arrays and you change this pointer when the user changes their preferences.
Related
I am writing a program that tells the user which juice taste to buy. I store the avaible juices in an array and the previously bought in another array. Based on how many times a taste has been bought and the time since it was last bought, each taste gets a score. The higher score the more likely it should be for that taste to be selected.
When I arrive at the step to randomly select a juice taste (that is, selecting an index in the array) I have two instances of NSMutableArray, one for the avaible tastes, one for the scores. Both arrays have 17 slots.
It might looks something like this: (only three different tastes)
#[#"Apple juice", #"Orange juice", #"Grape juice"]; // Avaible tastes array
#[19.21, 45.83, 24.84]; // Scores array
How do I select one taste randomly from this?
Initially, I thought that each taste could have a range, starting from the previous taste's score going to the previous score + the taste's own score. Then a random number between 0 and the total score would be generated. The taste who's range matches the random number would be the one that was going to be selected, but this feels like a overly complex solution.
Is there an easier way?
Edit
By the way, it is written in Objective-C.
On way to solve your problem is what is know as roulette wheel selection. There are a few example implementations in this other question Roulette Selection in Genetic Algorithms
Simple option, copy each item into an array multiple times, based on the weighting, and then pick a random item from the array.
I am building a "Survey" type application. The user answers a set of questions with pre-vetted answers.
Question: Where do you live?
Answers: England, Finland, Spain, France, Monrovia
The answers in this case would be in a DropDownList.
Once the user has completed the basic responses (location, age, sex etc) I would like to be able to prevent them accessing the rest of the survey based on their answers.
So for example, if they live anywhere but England I want to direct them to a page which says "Thanks, but Monrovian's can't complete this survey". I need my filtering to be user configurable (Table based) and I need to be able to have ANDs and ORs.
So one filter being the user MUST earn 100k+ a year.
Another being they must either live in Spain, or be female AND like model trains - "100k+ && (Spain || (Female && Trains))"
I would usually use Enums and bitmasking for this, but as my country list is 200+ items long, I can't think of a sensible way to store the filtering.
Hopefully I have made some sense and someone has a decent solution :)
I don't know if I can answer your question completely, but I'll try...
So, we have a bunch of Views that are only visible to the user if she previously chose some answers, like, she will see view#3 if she is older that 30, and view#4 if she is younger than 30 AND from China, and view#5 if she is older than 40 AND from Spain OR Italy, and so on...I want also to introduce the notion of **step**, and for each step we could have 1, 2, or more corresponding views. Each view should have a set of rules (like the ones above) that define if it is displayed or not.
How to create these rules? These rules could be simple instances of a Filter class/interface that, when asked, should return true/false. Like this:
interface Filter {
boolean apply();
}
Then you can create Filters like 'older than 30', 'from Spain', whatever. Remember that each view is configured with a set of rules (Filters) so it can answer yes/no if asked if it can display itself.
Next, how to apply these filters?
We could have a controller object that only knows about **steps** (each step can have one or more corresponding views, as I said), and, after the user pressed **next** at the current step, it should collect the answers and apply them against the rules attached to each view. Like, take the answers from step one, and apply them to all views from step two, and see which one matches (returns true). For example, at step two, you can have two separate views, one for young people, other for old people, and you apply the rules from each view to decide if you show the old or young view.
I could give you one code example, and you could also do research on your own, since I know nothing about your technical environment. I have used Google Guava's predicates on a similar problem and here it is: suppose we are dealing with Witch objects, and each of them has name(string), age(int) and spells(collection) attributes. If I have a list of witches and I need to sort them based on specific criteria, I can do:
// first I want to sort witches by age(natural ordering) then by spells,
// and then by name lexicographically
Ordering.natural()
.compound(new BySpellsWitchOrdering())
.compound(new ByNameWitchOrdering())
.sortedCopy(witchList);
The above line of code is going to take the witch list and return a list of sorted witches according to the criteria. Your situation is pretty similar.
Next, how to create the answers? For each view(page), you have possible answers, like, for view#1, you can have : age, sex, race, country. You can construct some answers, in the form of strings, ints, enums, and pass them to the controller, which in turn is going to apply them to each view corresponding to the next step.
As for how to store the rules in the database, as an example, you could have a column defining rule name (like, OLDER THAN) and one column for value, say, 30. Again, I do not know that much about your environment, and it is a really general issue, so I will stop here...
I have a model that looks like this:
and want to display it in a UITableView using an NSFetchedResultsController. I want to list all of the Objects for a given Owner, grouped by Group. For example, if we were to view the groups for some Owner Owner A the table view might look like this:
Group A
Object A
Object B
Object C
Group B
Object A
Object D
Group C
Group D
Object C
Object E
It's also important that the Groups be ordered by their name attribute (not shown in the schema above) and that NSFetchedResultsController delegate methods get called whenever Objects are modified or added/removed from a Group.
Given those requirements, I have set up the NSFetchedResultsController to fetch a bunch of Objects with a predicate like [NSPredicate predicateWithFormat:#"ANY groups.owner = %#", someOwner], which throws an NSInvalidArgumentException exception: "to-many key not allowed here". I have tried a few other predicates, but am stuck.
Any suggestions? Is this not something I should be using an NSFetchedResultsController for? Or is there a better way to model my data?
Thanks!
Edit: I actually got the predicate working with the above code, my mistake was in my sectionNameKeyPath argument. I was trying to pass in groups.name, which was what was producing the error. I can see why that way wouldn't work, but am struggling to find a different way to achieve the desired results. Perhaps a join object?
Edit 2: This works somewhat well with a join object like this:
There are two downsides that I see now. The first is that I have to enforce uniqueness myself using code. That's easy enough, but a bit of a nuisance. The second downside, that I don't yet see a way around, is that the NSFetchedResultsController will not call its delegate for updates to Objects anymore. I can live with that downside for now, but am happy to hear better suggestions.
I think the problem you're going to see is that if you have a fetched results controller searching for Objects then it's going to find each Object only exactly once. So each object will appear exactly once in your table. Whereas what you sort of want to do is invert things and find all the Groups, then displaying all the relevant contained Objects. In terms of the fetched results controller, rather than finding rows and thereby being able to divide into sections you want to find sections and thereby figure out what to supply as rows.
The easiest thing, I think, would be to create a fetched results controller on Group and to add an intermediary object of your own that remaps those to sections and supplies group.objects (with a suitably deterministic sorting applied) as the rows per section.
If you want the rows to be dynamic then I guess the easiest thing is to create a fetched results controller per section based on the feedback of the Group controller.
If that's all getting a bit painful and you want to just write your own collection logic then you'll probably want to catch NSManagedObjectContextDidSaveNotification and rerun your logic whenever that occurs. For memory efficiency reasons you probably want to keep hold only of the objectIDs and to get the appropriate objects only when the table view requests them via the existingObjectWithID:error: method on the context. Core Data has a built-in caching mechanism that responds appropriately to memory warnings so that's all written for you.
You shouldn't need a join object to get at data in Many to Many relationships
See this answer: How to deal with many to many relationships with NSFetchedResultsController?
And example project: https://github.com/dmathewwws/Many-to-Many-CoreData
Good afternoon everyone!
I'm studying NHibernate, and decided to make some changes. Among them, I noticed that some fields are unnecessary. So I bring my doubt:
I have a list, let's call it Class_List within each study class, I can have N students for each class. Within the list Class_List, I also have other properties as simple as the name of the class.
How I see it is unnecessary to store how many students I have in the database, I would, in a single query, how many records I have. This, using NHibernate.
Is this possible? How?
Best regards,
Gustavo.
Edit: I've forgot to say one thing... I want to return this number of record, as a column. But this column is not mapped in my .hbm.xml file.
If students are mapped as a collection on Class, you can try using something like this:
var numberOfStudents = session.CreateCriteria<Class>()
.Add(Restrictions.IdEq(1))
.CreateCriteria("_students", "students")
.SetProjection(Projections.RowCount())
.UniqueResult<Int32>();
Where '1' is the id of the class (you can use other property) and '_students' is the name of the students collection.
I have a problem naming the elements in my application's data model.
In the application, the user has the possibility to create his own metamodel. He does so by creating entity types and a type defines which properties an entity has. However, there are three kinds of entity types:
There is always exactly one instance of the type.
For instance, I want to model the company I am working for. It has a name, a share price and a number of employees. These values change over time, but there is always exactly one company.
There are different instances of the type, each is unique.
Example: Cities. A city has a name and a population count, there are different cities and each city exists exactly once.
Each instance of the type defines multiple entities.
Example: Cars. A car has a color and a manufacturer. But there is not only one red mercedes. And even though they are similar, red mercedes #1 is different from red mercedes #2.
So lets say you are a user of this tool and you understood the concept of these three flavors. You want to create a new entity type and are prompted to choose between option 1, 2 and 3. How would you name these options?
Edit:
Documentation and help is available to the user. Also the user can be expecteted to have a technical/programming background, so understanding these three concepts should be no problem.
First of all let me make sure I understand the problem,
Here's what you have (correct me if I'm wrong):
#of instances , is/are Unique
(1,true)
(n,true)
(n,false)
If so,
for #of instances I would use single \ plural
for is\are unique (\ not unique) I would use unique \ ununique.
so you'll get:
singleUnique
pluralUnique
pluralUnunique
That's the best I could think of.. I don't know exactly who are your users and what is the environment, But if you have an option of adding tips (or documentation) that should be used for sure.