I have been working mainly in JAVA but now I need to program some small things in kotlin. Among other things I am trying to convert the result of a database query into a list of array. The result of the database query has 4 columns, the number of rows I can not predict.
I have tried the following:
var output: mutableList<List<String>>
var output = mutableListOf<String>()
var output = mutableListOf<ArrayList>
List<List<String>> listOfLists = new ArrayList<List<String>>()
What I would like to do is this:
output.add(arrayOf("Field1", "Filed2", "Field3", "Field4"))
It can't be that hard, can it?
A list of arrays can be expressed as List<Array<T>>.
So if you want a mutable list to which you can add arrays of strings, simply do:
var output = mutableListOf<Array<String>>()
output.add(arrayOf("Field1", "Filed2", "Field3", "Field4"))
That being said, why do you want to use arrays? It's generally more convenient to work with lists, unless you're constrained by another API.
In kotlin you should use lists where you can. What you are trying to create is:
val output = mutableListOf<List<String>>()
output.add(listOf("Field1", "Filed2", "Field3", "Field4"))
If you are iterating through some other list to create your data for output you could do something like:
val otherList = listOf<String>("a", "b", "v")
val output = otherList.map { otherListData ->
listOf(otherListData + 1, otherListData + 2, otherListData + 3, otherListData + 4)
}
In which case you would only have immutable lists.
Related
Teaching myself swift, so complete noob here, but I'm far into a project and just know there must be an easier way to achieve something.
I have a 2D array:
var shopArray = [
["theme":"default","price":0,"owned":true,"active":true,"image":UIImage(named: "defaultImage")!,"title":"BUY NOW"],
["theme":"red","price":1000,"owned":false,"active":false,"image":UIImage(named: "redImage")!,"title":"BUY NOW"],
["theme":"blue","price":2000,"owned":false,"active":false,"image":UIImage(named: "blueImage")!,"title":"BUY NOW"],
["theme":"pool","price":3000,"owned":true,"active":false,"image":UIImage(named: "blueImage")!,"title":"BUY NOW"],
["theme":"line","price":4000,"owned":false,"active":false,"image":UIImage(named: "lineImage")!,"title":"BUY NOW"],
["theme":"neon","price":5000,"owned":false,"active":false,"image":UIImage(named: "lineImage")!,"title":"BUY NOW"]]
Where I simply want to create a function that runs and search for all the "owned" keys and make them all "false".
How do you search and replace in Arrays / 2D Arrays. More specifiaclly, what should the func look like?
Thank you!
You don't have a 2D array, you have an Array of Dictionaries.
You can set all of the values for the owned keys by iterating the indices of the Array and updating the values:
shopArray.indices.forEach { shopArray[$0]["owned"] = false }
That is the functional way to do it. You could also do the same operation with a for loop:
for idx in shopArray.indices {
shopArray[idx]["owned"] = false
}
You could do something like this to loopthrough the array replacing the approriate element.
var i = 0
for x in shopArray {
var y = x
y["owned"] = false
shopArray.remove(at: i)
shopArray.insert(y, at: i)
i = i + 1
}
or you could use a while loop to do the same with less code lines.
var y = 0
while y < shopArray.count {
shopArray[y].updateValue(false, forKey: "owned")
y += 1
}
There is proably somthing doable with .contains, but I'm not sure you need that toachive the result you mention above. Play around in a play ground in xcode and try a few different options without doing anything that might cause issues in your project.
I have been trying to randomize the values in an ordered array (ex:[0,1,2,3]) in Godot. There is supposed to be a shuffle() method for arrays, but it seems to be broken and always returns "null". I have found a workaround that uses a Fisher-Yates shuffle, but the resulting array is considered "unsorted" by the engine, and therefore when I try to use methods such as bsearch() to find a value by it's position, the results are unreliable at best.
My solution was to create a dictionary, comprised of an array containing the random values I have obtained, merged with a second array of equal length with (sorted) numbers (in numerical order) which I can then use as keys to access specific array positions when needed.
Question made simple...
In GDScript, how would you take 2 arrays..
ex: ARRAY1 = [0,1,2,3]
ARRAY2 = [a,b,c,d]
..and merge them to form a dictionary that looks like this:
MergedDictionary = {0:a, 1:b, 2:c, 3:d}
Any help would be greatly appreciated.
Godot does not support "zip" methodology for merging arrays such as Python does, so I am stuck merging them manually. However... there is little to no documentation about how to do this in GDScript, despite my many hours of searching.
Try this:
var a = [1, 2, 3]
var b = ["a", "b", "c"]
var c = {}
if a.size() == b.size():
var i = 0
for element in a:
c[element] = b[i]
i += 1
print("Dictionary c: ", c)
If you want to add elements to a dictionary, you can assign values to the keys like existing keys.
Background:
Hey all, I am experimenting with external APIs and am trying to pull in all of the followers of a User from a site and apply some sorting.
I have refactored a lot of the code, HOWEVER, there is one part that is giving me a really tough time. I am convinced there is an easier way to implement this than what I have included and would be really grateful on any tips to do this in a much more eloquent way.
My goal is simple. I want to collapse an array of arrays of hashes (I hope that is the correct way to explain it) into one array of hashes.
Problem Description:
I have an array named f_collectionswhich has 5 elements. Each element is an array of size 200. Each sub-element of these arrays is a hash of about 10 key-value pairs. My best representation of this is as follows:
f_collections = [ collection1, collection2, ..., collection5 ]
collection1 = [ hash1, hash2, ..., hash200]
hash1 = { user_id: 1, user_name: "bob", ...}
I am trying to collapse this multi-dimensional array into one array of hashes. Since there are five collection arrays, this means the results array would have 1000 elements - all of which would be hashes.
followers = [hash1, hash2, ..., hash1000]
Code (i.e. my attempt which I do not want to keep):
I have gotten this to work with a very ugly piece of code (see below), with nested if statements, blocks, for loops, etc... This thing is a nightmare to read and I have tried my hardest to research ways to do this in a simpler way, I just cannot figure out how. I have tried flatten but it doesn't seem to work.
I am mostly just including this code to show I have tried very hard to solve this problem, and while yes I solved it, there must be a better way!
Note: I have simplified some variables to integers in the code below to make it more readable.
for n in 1..5 do
if n < 5
(0..199).each do |j|
if n == 1
nj = j
else
nj = (n - 1) * 200 + j
end
#followers[nj] = #f_collections[n-1].collection[j]
end
else
(0..199).each do |jj|
njj = (4) * 200 + jj
#followers[njj] = #f_collections[n-1].collection[jj]
end
end
end
Oh... so It is not an array objects that hold collections of hashes. Kind of. Lets give it another try:
flat = f_collection.map do |col|
col.collection
end.flatten
which can be shortened (and is more performant) to:
flat = f_collection.flat_map do |col|
col.collection
end
This works because the items in the f_collection array are objects that have a collection attribute, which in turn is an array.
So it is "array of things that have an array that contains hashes"
Old Answer follows below. I leave it here for documentation purpose. It was based on the assumption that the data structure is an array of array of hashes.
Just use #flatten (or #flatten! if you want this to be "inline")
flat = f_collections.flatten
Example
sub1 = [{a: 1}, {a: 2}]
sub2 = [{a: 3}, {a: 4}]
collection = [sub1, sub2]
flat = collection.flatten # returns a new collection
puts flat #> [{:a=>1}, {:a=>2}, {:a=>3}, {:a=>4}]
# or use the "inplace"/"destructive" version
collection.flatten! # modifies existing collection
puts collection #> [{:a=>1}, {:a=>2}, {:a=>3}, {:a=>4}]
Some recommendations for your existing code:
Do not use for n in 1..5, use Ruby-Style enumeration:
["some", "values"].each do |value|
puts value
end
Like this you do not need to hardcode the length (5) of the array (did not realize you removed the variables that specify these magic numbers). If you you want to detect the last iteration you can use each_with_index:
a = ["some", "home", "rome"]
a.each_with_index do |value, index|
if index == a.length - 1
puts "Last value is #{value}"
else
puts "Values before last: #{value}"
end
end
While #flatten will solve your problem you might want to see how DIY-solution could look like:
def flatten_recursive(collection, target = [])
collection.each do |item|
if item.is_a?(Array)
flatten_recursive(item, target)
else
target << item
end
end
target
end
Or an iterative solution (that is limited to two levels):
def flatten_iterative(collection)
target = []
collection.each do |sub|
sub.each do |item|
target << item
end
end
target
end
I have been porting over an algorithm I've been using in Java (Android) to Swift (iOS), and have run into some issues with speed on the Swift version.
The basic idea is there are objects with depths (comment tree), and I can hide and show replies from the dataset by matching against a list of hidden objects. Below is a visualization
Top
- Reply 1
- - Reply 2
- - Reply 3
- Reply 4
and after hiding from the dataset
Top
- Reply 1
- Reply 4
The relevant methods I've converted from Java are as follows
//Gets the "real" position of the index provided in the "position" variable. The comments array contains all the used data, and the hidden array is an array of strings that represent items in the dataset that should be skipped over.
func getRealPosition(position: Int)-> Int{
let hElements = getHiddenCountUpTo(location: position)
var diff = 0
var i = 0
while i < hElements {
diff += 1
if(comments.count > position + diff && hidden.contains(comments[(position + diff)].getId())){
i -= 1
}
i += 1
}
return position + diff
}
func getHiddenCountUpTo(location: Int) -> Int{
var count = 0
var i = 0
repeat {
if (comments.count > i && hidden.contains(comments[i].getId())) {
count += 1
}
i += 1
} while(i <= location && i < comments.count)
return count
}
This is used with a UITableViewController to display comments as a tree.
In Java, using array.contains was quick enough to not cause any lag, but the Swift version calls the getRealPosition function many times when calling heightForRowAt and when populating the cell, leading to increasing lag as more comment ids are added to the "hidden" array.
Is there any way I can improve on the speed of the array "contains" lookups (possibly with a different type of collection)? I did profiling on the application and "contains" was the method that took up the most time.
Thank you
Both Java and Swift have to go through all elements contained in the array. This gets slower and slower as the array gets larger.
There is no a priori reason for Java to fare better, as they both use the exact same algorithm. However, strings are implemented very differently in each language, so that could make string comparisons more expensive in Swift.
In any case, if string comparison slows you down, then you must avoid it.
Easy fix: use a Set
If you want a simple performance boost, you can replace an array of strings with a set of strings. A set in Swift is implemented with a hash table, meaning that you have expected constant time query. In practice, this means that for large sets, you will see better performance.
var hiddenset Set<String> = {}
for item in hidden {
strset.insert(item)
}
For best performance: use a BitSet
But you should be able to do a whole lot better than even a set can do. Let us look at your code
hidden.contains(comments[i].getId()))
If you are always accessing hidden in this manner, then it means that what you have is a map from integers (i) to Boolean values (true or false).
Then you should do the following...
import Bitset;
let hidden = Bitset ();
// replace hidden.append(comments[i].getId())) by this:
hidden.add(i)
// replace hidden.contains(comments[i].getId())) by this:
hidden.contains(i)
Then your code will really fly!
To use a fast BitSet implementation in Swift, include the following in Package.swift (it is free software):
import PackageDescription
let package = Package(
name: "fun",
dependencies: [
.Package(url: "https://github.com/lemire/SwiftBitset.git", majorVersion: 0)
]
)
i think you need the realPosition to link from a tap on a row in the tableview to the source array?
1) make a second array with data only for the tableViewDataSource
copy all visible elements to this new array. create a special ViewModel as class or better struct which only has the nessesary data to display in the tableview. save in this new ViewModel the realdataposition also as value. now you have a backlink to the source array
2) then populate this TableView only from the new datasource
3) look more into the functional programming in swift - there you can nicer go over arrays for example:
var array1 = ["a", "b", "c", "d", "e"]
let array2 = ["a", "c", "d"]
array1 = array1.filter { !array2.contains($0) }
or in your case:
let newArray = comments.filter{ !hidden.contains($0.getId()) }
or enumerated to create the viewmodel
struct CommentViewModel {
var id: Int
var text: String
var realPosition: Int
}
let visibleComments: [CommentViewModel] = comments
.enumerated()
.map { (index, element) in
return CommentViewModel(id: element.getId(), text: element.getText(), realPosition: index)
}
.filter{ !hidden.contains($0.id) }
I'm having a horrible time understanding a few things about Swift.
I am attempting to do scripting in Swift to see if it might be useful. One of the major runs of scripting file is how easy the language gets data in from text-files and the hash/dictionary/array parsing syntax.
In trying to parse huge csv files, I'm trying to create a matrix Dictionary that mimics a perl method ${$hash}{$var1}{$var2}{$var3} = $value; where #house_hash is the actual hash and each of the keys are dynamically created and added if not present already.
in ObjectiveC, this was simple enough, using NSMutableDictionary, atleast the retrieval part of it.
Swift seems pretty exciting, i wonder with how much ease can this be done?
Till now, all I have is:
var hash = [Int : [Int : [String : AnyObject]]] ()
let number = hash[var1]?[var2]?["var3"]? .hashValue
var d3 = [String : AnyObject]()
var d2 = [Int : [String : AnyObject]] ()
d3["var3"] = var3 //var3 is a float
if(number == nil)
{
d2[var2] = d3
hash[var1] = d2
}
else
{
hash[var1]![var2] = d3
}
Absolutely no idea what I'm doing. There are no good examples if this dynamic nature of Swift dictionaries that house unknown object types (could be arrays, Ints, Strings, another Dictionary)..
UPDATE: I found this SO answer. But it does not seem to work in Swift Playground.