I want to take any kind of values inside a function (r/lvalue) and I also want to ensure that the value will not be mutated in the scope of the function, even if the value itself is not a const.
struct Tree(T) {
T item;
Tree!T* parent, left, right;
this(T item) {
this.item = item;
}
Tree!T* searchTree(const ref T item) {
if (&this is null)
return null;
if (this.item == item)
return &this;
return (this.item < item) ? this.right.searchTree(item) : this.right.searchTree(item);
}
}
unittest {
auto text1 = "Hello", text2 = "World";
auto tree2 = Tree!string(text1);
assert(tree2.searchTree(text2) is null);
assert(tree2.searchTree(text1) !is null);
}
This works with ref parameters however if I give int literals to the function, it fails:
auto tree1 = Tree!int(4);
assert(tree1.searchTree(5) is null);
assert(tree1.searchTree(4) !is null);
Templates to the rescue! D has a feature called auto ref that automatically generates the overloads for ref and non-ref parameters. The only requirement is that the function must be a template.
For your searchTree function, that means it needs to have this signature:
Tree!T* searchTree()(const auto ref T item)
With that simple change, your code should compile and do The Right Thing™.
there is currently an experimental compiler switch for DMD that allows this.
try adding "-preview=rvaluerefparam" to your compiler switches.
Related
I am trying to solve a puzzle, and it has been suggested that I use backtracking - I did not know the term so did some investigation, and found the following in Wikipedia:
In order to apply backtracking to a specific class of problems, one must provide the data P for the particular instance of the problem that is to be solved, and six procedural parameters, root, reject, accept, first, next, and output. These procedures should take the instance data P as a parameter and should do the following:
root(P): return the partial candidate at the root of the search tree.
reject(P,c): return true only if the partial candidate c is not worth completing.
accept(P,c): return true if c is a solution of P, and false otherwise.
first(P,c): generate the first extension of candidate c.
next(P,s): generate the next alternative extension of a candidate, after the extension s.
output(P,c): use the solution c of P, as appropriate to the application.
The backtracking algorithm reduces the problem to the call backtrack(root(P)), where backtrack is the following recursive procedure:
procedure backtrack(c) is
if reject(P, c) then return
if accept(P, c) then output(P, c)
s ← first(P, c)
while s ≠ NULL do
backtrack(s)
s ← next(P, s)
I have attempted to use this method for my solution, but after the method finds a rejected candidate it just starts again and finds the same route, rather than the next possible one.
I now don't think I have used the next(P,s) correctly, because I don't really understand the wording 'after the extension s'.
I've tried 2 methods:
(a) in the first() function, generating all possible extensions, storing them in a list, then using the first. The next() function then uses the other extensions from the list in turn. But this maybe can't work because of the calls to backtrack() in between the calls to next().
(b) adding a counter to the data (i.e. the class that includes all the grid info) and incrementing this for each call of next(). But can't work out where to reset this counter to zero.
Here's the relevant bit of code for method (a):
private PotentialSolution tryFirstTrack(PotentialSolution ps)
{
possibleTracks = new List<PotentialSolution>();
for (Track trytrack = Track.Empty + 1; trytrack < Track.MaxVal; trytrack++)
{
if (validMove(ps.nextSide, trytrack))
{
ps.SetCell(trytrack);
possibleTracks.Add(ps);
}
}
return tryNextTrack(ps);
}
private PotentialSolution tryNextTrack(PotentialSolution ps)
{
if (possibleTracks.Count == 0)
{
ps.SetCell(Track.Empty);
return null;
}
ps = possibleTracks.First();
// don't use same one again
possibleTracks.Remove(ps);
return ps;
}
private bool backtrackTracks(PotentialSolution ps)
{
if (canExit)
{
return true;
}
if (checkOccupiedCells(ps))
{
ps = tryFirstTrack(ps);
while (ps != null)
{
// 'testCells' is a copy of the grid for use with graphics - no need to include graphics in the backtrack stack
testCells[ps.h, ps.w].DrawTrack(g, ps.GetCell());
if (ps.TestForExit(endColumn, ref canExit) != Track.MaxVal)
{
drawRowColTotals(ps);
return true;
}
ps.nextSide = findNextSide(ps.nextSide, ps.GetCell(), ref ps.h, ref ps.w);
if (ps.h >= 0 && ps.h < cellsPerSide && ps.w >= 0 && ps.w < cellsPerSide)
{
backtrackTracks(ps);
ps = tryNextTrack(ps);
}
else
return false;
}
return false;
}
return false;
}
and here's some code using random choices. This works fine, so I conclude that the methods checkOccupiedCells() and findNextSide() are working correctly.
private bool backtrackTracks(PotentialSolution ps)
{
if (canExit)
{
return true;
}
if (checkOccupiedCells(ps))
{
Track track = createRandomTrack(ps);
if (canExit)
return true;
if (track == Track.MaxVal)
return false;
ps.SetCell(track);
ps.nextSide = findNextSide(ps.nextSide, track, ref ps.h, ref ps.w);
if (ps.h >= 0 && ps.h < cellsPerSide && ps.w >= 0 && ps.w < cellsPerSide)
backtrackTracks(ps);
else
return false;
}
}
If it helps, there's more background info in the puzzle itself here
Is it equal in flutter null safety?
if(widget.mapPickerController != null){
widget.mapPickerController!.mapMoving = mapMoving;
widget.mapPickerController!.mapFinishedMoving = mapFinishedMoving;
}
widget.mapPickerController?.mapMoving = mapMoving;
widget.mapPickerController?.mapFinishedMoving = mapFinishedMoving;
Both versions should end up doing the same thing. When null-safety enabled, ?. short-circuits the rest of the expression, so if the receiver is null, the rest of the expression (including the right-hand-side of the assignment) won't be evaluated. You can observe that in the following example:
class Foo {
int value = 0;
}
int bar() {
print('Called bar');
return 42;
}
void main() {
Foo? foo = null;
foo?.value = bar();
}
where running it generates no errors and shows that bar() is never called.
That said, the second version using ?. is much less clear (which is evident from the apparent confusion in the several deleted answers) and therefore probably should be avoided.
The best way would be to introduce a local variable:
final mapPickerController = widget.mapPickerController;
if (mapPickerController != null){
mapPickerController.mapMoving = mapMoving;
mapPickerController.mapFinishedMoving = mapFinishedMoving;
}
and then you don't need to use the null-assertion operator at all, and it's slightly more efficient since you check for null only once.
I got a warning on the following piece of code, and I don't know why.
List<Map<String, int>> callNoInfo = [];
int getCallNo(String phoneNo) {
callNoInfo.forEach((item) {
if (item.containsKey(phoneNo)) {
return item[phoneNo];
}
});
return 0;
}
The warning is:
This function has a return type of 'int', but doesn't end with a return statement. (missing_return at [project_name] lib\helper\call_no.dart:35)
Can anyone tell me why this happens? thanks in advance
In the forEach method, you are creating a lambda function without explicitly defining the return type, so Dart is attempting to infer it from the return statements. If we pull the function out of the forEach method, it might help to see what I mean:
...
(item) {
if (item.containsKey(phoneNo)) {
return item[phoneNo];
}
}
...
The function includes a return statement that returns item[phoneNo], which is an int value. Using this, Dart infers that the return type of this lambda function is int. However, now that it knows this, it also notices that if the code execution does not enter the if block, there is no return statement to match the else side of the if condition. If the item object does not contain the key phoneNo, what is the method going to return?
(The answer is that the method will implicitly return null which is why the message is only a warning and not a compiler error, but the warning appears because this probably wasn't intentional by you the developer and also as a nudge to help you make your code less reliant on invisible Dart runtime magicks.)
To fix this, there needs to be another return outside of the if block:
...
(item) {
if (item.containsKey(phoneNo)) {
return item[phoneNo];
}
return 0;
}
...
However, now there's a different problem. The forEach method on lists has the following signature:
forEach(void f(E element)) → void
In fact, there are two problems. First, the method passed as the parameter needs to have a return type of void, and the forEach method itself also has a return type of void. This means that you cannot return values from within the forEach method at all.
The thing about the forEach method is that it is intended to iterate over the collection and process each of the values within it. It's not meant to (and can't) search for a value and return it once it's found. Furthermore, the iteration is exhaustive, meaning once you start it, the method cannot be stopped until each and every element in the collection has been iterated over.
Which is why, as the other answers have pointed out, what you really should be doing is using a for or for in loop:
List<Map<String, int>> callNoInfo = [];
int getCallNo(String phoneNo) {
for(var item in callNoInfo) {
if (item.containsKey(phoneNo)) {
return item[phoneNo];
}
}
return 0;
}
(I'm not sure why you don't get a compiler error for assigning a lambda function with a return value of int to the forEach method which clearly is requesting a one with a void return type. But if I had to guess, I'd say the Dart runtime treats them as compatible and reconciles the difference in return type by simply discarding the return value of the lambda function.)
You don't have an else case for the if statement inside the forEach loop. Even though you might have one at the end, it still is expecting a case everywhere.
List<Map<String, int>> callNoInfo = [];
int getCallNo(String phoneNo) {
callNoInfo.forEach((item) {
if (item.containsKey(phoneNo)) {
return item[phoneNo];
}
// you don't have a case for here since it's in a callback
});
return 0;
}
However, you could do this which uses a for in loop:
List<Map<String, int>> callNoInfo = [];
int getCallNo(String phoneNo) {
for (var item in callNoInfo) {
if (item.containsKey(phoneNo)) {
return item[phoneNo];
}
}
return 0;
}
I am new to RxJava2.
I am trying to get a list of Transaction object both from cache and from server.
I want to compare the server value to cache value and if the server value is the same, then ignore it.
I was able to do it easily using .scan() because we can return null and when null is returned from the .scan() the value got ignored(filtered).
RxJava 1
private Observable<List<Transaction>> getTransactionsFromCacheAndServer() {
return Observable.concat(
getTransactionsFromCache(),
getTransactionsFromServer()
)
.scan((p1, p2) -> {
if (p1 == null && p2 != null) {
return p2;
} else if (p1 != null && !isListSame(p1, p2)) {
return p2;
} else {
return null;
}
});
}
With RxJava 2, since I cannot return null anymore, things are not easy.
RxJava 2
private Observable<List<Transaction>> getTransactionsFromCacheAndServer() {
return Observable.concat(
getTransactionsFromCache(),
getTransactionsFromServer()
)
.map(FilterObject::new)
.scan((filterObject1, filterObject2) -> {
List<Transaction> p1 = (List<Transaction>)filterObject1.value;
List<Transaction> p2 = (List<Transaction>)filterObject2.value;
if (p1.size() == 0 && p2.size() > 0) {
return filterObject2;
} else if (!isListSame(p1, p2)) {
return filterObject2;
} else {
filterObject2.filter = true;
return filterObject2;
}
})
.filter(filterObject -> !filterObject.filter)
.map(filterObject -> (List<Transaction>)filterObject.value);
}
Where FilterObject is:
public class FilterObject {
public Object value;
public boolean filter;
public FilterObject(Object value) {
this.value = value;
}
}
Even though I can achieve the same thing using above method, it seems very ugly. Also I had to include two maps which might not be so performance friendly.
Is there a simple/clean way to achieve what I want?
I don't think there is a generic solution to this problem, since an empty list and a list that needs to be filtered (which happens to be empty in all cases) are two different things (the output of the scan) and needs to be handled differently.
However, in your particular case you never emit an empty list, except maybe for the first output.
(I am using String instead Transaction, shouldn't matter)
private Observable<List<String>> getTransactionsFromCacheAndServer() {
return Observable.concat(
getTransactionsFromCache(),
getTransactionsFromServer()
)
.filter(list -> !list.isEmpty())
// If you prefer a consistent empty list over the first
// empty list emission getting filtered
.startWith((List<String>) Collections.EMPTY_LIST)
// Newly emitted value cannot be empty, it only depends only on the comparison
.distinctUntilChanged(this::isListSame);
}
That's the closest I could get with as few operators as possible. Hope it solves your problem.
Based on andras' answer, I modified little bit to achieve what I want.
private Observable<List<String>> getTransactionsFromCacheAndServer() {
return Observable.concat(
getTransactionsFromCache(),
getTransactionsFromServer()
)
.filter(list -> !list.isEmpty())
.distinctUntilChanged(this::isListSame)
.switchIfEmpty(Observable.just(new ArrayList<>()));
}
Andreas' answer will always receive an empty list and then a real data.
My solution above will receive:
1. Data from cache (and then data from server if different)
2. Empty list if both cache and server returns Empty list.
Most of the times all I have to do with JavaScript is just add some dynamics to simple HTML. Recently, however, after discovering CoffeeScript, I got interested in *Object Oriented JavaScript". Here is some code in CoffeeScript.
class MyClass
constructor: (title, purpose)->
#title = typeof title is undefined ? "My Class" : title
#purpose = typeof purpose is undefined ? "None" : purpose
#myMethod()
myMethod: ->
_getTitle = #getTitle
_getPurpose = #getPurpose
$(window).click ->
_getTitle()
_getPurpose()
return
return
getTitle: ->
_title = #title
window.console.log "Title of the class this object belongs to is: #{_title}"
return
getPurpose: ->
_purpose = #purpose
window.console.log "Purpose of creating this class is: #{_purpose}"
return
title = ""
purpose = ""
myObject = new MyClass("Testbed", "to test Object Oriented JavaScript")
For those who prefer JavaScript, here is the compiled (?) JavaScript.
var MyClass, myObject;
MyClass = (function() {
var purpose, title;
function MyClass(title, purpose) {
var _ref, _ref1;
this.title = (_ref = typeof title === void 0) != null ? _ref : {
"My Class": title
};
this.purpose = (_ref1 = typeof purpose === void 0) != null ? _ref1 : {
"None": purpose
};
this.myMethod();
}
MyClass.prototype.myMethod = function() {
var _getPurpose, _getTitle;
_getTitle = this.getTitle;
_getPurpose = this.getPurpose;
$(window).click(function() {
_getTitle();
_getPurpose();
});
};
MyClass.prototype.getTitle = function() {
var _title;
_title = this.title;
window.console.log("Title of the class this object belongs to is: " + _title);
};
MyClass.prototype.getPurpose = function() {
var _purpose;
_purpose = this.purpose;
window.console.log("Purpose of creating this class is: " + _purpose);
};
title = "";
purpose = "";
return MyClass;
})();
myObject = new MyClass("Testbed", "to test Object Oriented JavaScript");
Sorry about the long code. I had to try to keep it interesting. The thing is, this code outputs:
Title of the class this object belongs to is: undefined
Purpose of creating this class is: undefined
whereas I was expecting it to output:
Title of the class this object belongs to is: Testbed
Purpose of creating this class is: to test Object Oriented JavaScript
And I could've sworn this was how it worked when I last tinkered with it (around six months ago). I learnt that in a method that is part of the prototype of an object, this refers to the prototype itself. And this.something would actually point to object.something. Whereas in this example, inside myObject.myMethod(), this behaves as it should and this.getTitle() refers to myObject.getTitle(). Inside myObject.getTitle(), however, this refers to window. Why?
Is it because getTitle() was called inside a $(window).click() handler? But why would that change the context? getTitle() is still a property of myObject.
Also, you see what I am trying to accomplish here. How could I accomplish that?
There are several problems.
1) You never return anything from .getPurpose or .getTitle
2) You should create a reference to this in myMethod. i.e. var me = this and then inside the event listener call me.getTitle() and me.getPurpose(). This is needed because inside the event listener (window onclick), this refers to the window and not the object.
3) It looks like your ternary expressions are always evaluating to false. You need to rethink them.
P.S. I looked at the straight JS version