I have a LinkedHashSet which was created from a Seq. I used a LinkedHashSet because I need to keep the order of the Seq, but also ensure uniqueness, like a Set. I need to check this LinkedHashSet against another sequence to verify that various properties within them are the same. I assumed that I could loop through using an index, i, but it appears not. Here is an example of what I would like to accomplish.
var s: Seq[Int] = { 1 to mySeq.size }
return s.forall { i =>
myLHS.indexOf(i).something == mySeq.indexOf(i).something &&
myLHS.indexOf(i).somethingelse == mySeq.indexOf(i).somethingelse
}
So how do I access individual elements of the LHS?
Consider using the zip method on collections to create a collection of pairs (Tuples). The specifics of this depend on your specifics. You may want to do mySeq.zip(myLHS) or myLHS.zip(mySeq), which will create different structures. You probably want mySeq.zip(myLHS), but I'm guessing. Also, if the collections are very large, you may want to take a view first, e.g. mySeq.view.zip(myLHS) so that the pair collection is also non-strict.
Once you have this combined collection, you can use a for-comprehension (or directly, myZip.foreach) to traverse it.
A LinkedHashSet is not necessary in this situation. Since I made it from a Seq, it is already ordered. I do not have to convert it to a LHS in order to also make it unique. Apparently, Seq has the distinct method which will remove duplicates from the sequence. From there, I can access the items via their indexes.
Related
Not sure if this is science fiction, but would it be possible to create a type that represents an Array that matches a certain condition, such as being always sorted?
Or a 2-tuple where the first element is always bigger than the second?
What you're describing is called a dependent type (https://en.wikipedia.org/wiki/Dependent_type). Swift does not have these, and I'm not aware of any mainstream (non-research) language that does. You can of course create a special kind of collection that is indexed like an array and sorts itself whenever it is modified, and you can crate a struct with greater and lessor properties that always reorders itself. But these criteria cannot be attached to the existing Array or tuple types.
I am calculating average length of identifiers with CQLinq in NDepend, and I want to get the length of the names of classes, fields and methods. I walked through this page of CQlinq: http://www.ndepend.com/docs/cqlinq-syntax, and I have code like:
let id_m = Methods.Select(m => new { m.SimpleName, m.SimpleName.Length })
let id_f = Fields.Select(f => new { f.Name, f.Name.Length })
select id_m.Union(id_f)
It doesn't work, one error says:
'System.Collections.Generic.IEnumerable' does not
contain a definition for 'Union'...
The other one is:
cannot convert from
'System.Collections.Generic.IEnumerable' to
'System.Collections.Generic.HashSet'
However, according to MSDN, IEnumerable Interface defines Union() and Concat() methods.
It seems to me that I cannot use CQLinq exactly the same way as Linq. Anyway, is there a way to get the information from Types, Methods and Fields domains within a singe query?
Thanks a lot.
is there a way to get the information from Types, Methods and Fields domains within a singe query?
Not for now, because a CQLinq query can only match a sequence of types, or a sequence of methods or a sequence of field, so you need 3 distinct code queries.
For next version CQLinq, will be improved a lot and indeed you'll be able to write things like:
from codeElement in Application.TypesAndMembers
select new { codeElement, codeElement.Name.Length }
Next version will be available before the end of the year 2016.
Is there a way the define a Set data-structure in PowerShell?
In computer science, a set is an abstract data type that can store certain values, without any particular order, and no repeated values. It is a computer implementation of the mathematical concept of a finite set. Unlike most other collection types, rather than retrieving a specific element from a set, one typically tests a value for membership in a set.
I need to use a data structure as keystore that:
assures no-repetitions;
minimizes the computational effort in order to retrieve and remove an element.
You can use the .NET HashSet class that is found under System.Collections.Generic:
$set = New-Object System.Collections.Generic.HashSet[int]
The collection guarantees unique items and the Add, Remove, and Contains methods all operate with O(1) complexity on average.
If you prefer to stick with native PowerShell types, you can use HashTable and just ignore the key values:
# Initialize the set
$set = #{}
# Add an item
$set.Add("foo", $true)
# Or, if you prefer add/update semantics
$set["foo"] = $true
# Check if item exists
if ($set.Contains("foo"))
{
echo "exists"
}
# Remove item
$set.Remove("foo")
For more information see: https://powershellexplained.com/2016-11-06-powershell-hashtable-everything-you-wanted-to-know-about/#removing-and-clearing-keys
Hashset is what you are looking for if you want to store only unique values in an array with relatively faster add, remove and find operations. It can be created as -
$set = [System.Collections.Generic.HashSet[int]]#()
I'm trying to sort an array of integers with a custom ordering.
E.g.
quickSort[Int](indices)(Ordering.by[Int, Double](value(_)))
Basically, I'm trying to sort indices of rows by the values of a particular column. I end up with a stackoverflow error when I run this on a fairly large data. If I use a more direct approach (e.g. sorting Tuple), this is not a problem.
Is there a problem if you try to extend the default Ordering[Int]?
You can reproduce this like this:
val indices = (0 to 99999).toArray
val values = Array.fill[Double](100000)(math.random)
scala.util.Sorting.quickSort[Int](indices)(Ordering.by[Int, Double](values(_))) // Works
val values2 = Array.fill[Double](100000)(0.0)
scala.util.Sorting.quickSort[Int](indices)(Ordering.by[Int, Double](values2(_))) // Fails
Update:
I think that I found out what the problem is (am answering my own question). It seems that I've created a paradoxical situation by changing the ordering definition of integers.
Within the quickSort algorithm itself, array positions are also integers, and there are certain statements comparing positions of arrays. This position comparison should be following the standard integer ordering.
But because of the new definition, now these position comparators are also following the indexed value comparator and things are getting really messed up.
I suppose that at least for the time being, I shouldn't be changing these default value type ordering as library might depend on default value type ordering.
Update2
It turns out that the above is in fact not the problem and there's a bug in quickSort when used together with Ordering. When a new Ordering is defined, the equality operator among Ordering is 'equiv', however the quickSort uses '=='. This results in the indices being compared, rather than indexed values being compared.
Am just mulling over what's the best way i.e. data structure to store a data that has several rows and columns. Shoudl I store it as :
1. an array of arrays?
2. NSDictionary?
or is there any grid-like data structure in iOS where I can easily fetch any row/column with ease from the data structure? For example, I must be able to fetch the value in 3rd column in row 5. Currently, say, I store each row as an array and the store these arrays in another array (so an array of arrays, say), then to fetch the value in column 3 in row 5, I need to fetch the 5th row in the array of arrays, and then in the resulting array, I need to fetch the 3rd object. Is there a better way to do this? Thoughts please?
then to fetch the value in column 3 in row 5, I need to fetch the 5th
row in the array of arrays, and then in the resulting array, I need to
fetch the 3rd object. Is there a better way to do this?
An array of arrays is fine for the implementation, and the collection subscripting that was recently added to Objective-C makes this easier -- you can use an expression like
NSString *s = myData[m][n];
to get the string at the nth column of the mth row.
That said, it may still be a good idea to create a separate class for your data structure, so that the rest of your code is protected from needing to know about how the data is stored. That would also simplify the process of changing the implementation from, say, an array of arrays to a SQLite table or something else.
Your data storage class doesn't need to be fancy or complicated. Here's a first pass:
#interface DataTable
- (id)objectAtRow:(NSInteger)row column:(NSInteger)column;
- (void)setObjectAtRow:(NSInteger)row column:(NSInteger)column;
#end
I'm sure you can see how to implement those in terms of an array of arrays. You'll have to do a little work to add rows and/or columns when the caller tries to set a value outside the current bounds. You might also want to add support for things like fast enumeration and writing to and reading from property lists, but that can come later.
There are other ways of doing it, but there's nothing wrong with the method you are using. You could use an NSDictionary with a key of type NSIndexPath, for example, or even a string key of the form "row,col", but I don't see any advantage in those except for sparse matrices.
You can either use an array of arrays, as you're doing, or an array of dictionaries. Either is fine, and I don't think there's any preference for one over the other. It all depends on which way is most convenient for you to set up the data structure in the first place. Accessing the data for the table view is equally easy using either method.