Background: it is a Linked List (ordering task for delivery), which involves different shopping carts (Node). i need to write a function to check if the carts are sorted according to the totalprice in the Class ShoppingCart.
My questions are:
Compiler does not have error message, but my function is false. May I know how or where should i change the code?
Any articles about debugging are recommendable for the beginners?
Thanks for the kindness.
Code as below:
1.
class OrderProcessing {
var first: OrderNode? = null
// Node for linked List
data class OrderNode(val order: Order, var next: OrderNode?)
....
fun isSorted(): Boolean {
var run = first
var sort = false
while (run != null) {
if(run.order.shoppingCart.totalPrice > run.next!!.order.shoppingCart.totalPrice)
run = run.next
sort = true
}
return sort
}
2.
Class ShoppingCart () {
private val shoppingList = mutableListOf<Pair<Product, Int>>
// totalprice = sum of item price in each cart
val totalprice : double
... }
3.
data class Order(
val shoppingCart: ShoppingCart,
val address: Address
)
4.
fun main(){
// while loop until "exit", Menu with functions to add goods to cart, clear cart, show shopping list
I think in your case you need to start debugging your code and step by step check your program flow.
You can read this article - https://www.jetbrains.com/help/idea/debugging-code.html, it's about java but anyway it's a good starting point.
Related
As below code showed, I got the list(type: Flow<List<T>>),
how can I use it to update lazyColumn's item, so when the room data changed, lazyColumn updates items accordingly?
Many thanks.
#Composable
fun SomeContent(context: Context) {
// get the view model ref
val viewModel: SomeViewModel =
viewModel(factory = SomeViewModelFactory(Db.getInstance(context)))
// get the list from room, it is a Flow list
val list = viewModel.sumDao.getAllRows()
LazyColumn(
) {
// I need to use the list in below, but got errors, how to do?
items(list.size) {
SomeListItem(list[it])
}
}
}
You must collect your FlowList as follows
val list = viewModel.sumDao.getAllRows().collectAsState(initial = emptyList())
Instead of items, use itemsIndexed
itemsIndexed(list) {idx, row -> SomeListItem(row)}
Let me know if the above works.
if using vn1gam's solution, might need to do
itemsIndexed(list.value) {idx, row -> SomeListItem(row)}
I have a set of domain object, deriving from a base, where I've overridden Equals, IEquatable<T>.Equals and equality operators. I've successfully used Contains, but now I am trying to use Distinct differently. Here's look at a sample code:
var a = new Test { Id = 1 };
var a2 = new Test { Id = 1 };
var list = new List<Test> { a, a2 };
var distinct = list.Distinct().ToList(); // both objects, Equal implementations not called
var containsA = list.Contains(a); // true, Equal implementations called
var containsA2 = list.Contains(a); // true
var containsNewObjectWithSameId = list.Contains(new Test { Id = 1 }); // true
public class Test : IEquatable<Test>
{
public int Id { get; init; }
public bool Equals(Test other)
{
if (ReferenceEquals(null, other))
return false;
if (ReferenceEquals(this, other))
return true;
if (this.GetType() != other.GetType())
return false;
return this.Id == other.Id;
}
public override int GetHashCode() => base.GetHashCode + this.Id;
}
Contains finds matches, but Distinct is feeling very inclusive and keeps them both. From MS docs:
The first search does not specify any equality comparer, which means FindFirst uses
EqualityComparer.Default to determine equality of boxes. That in turn uses the implementation
of the IEquatable.Equals method in the Box class.
What am I missing?
Thanks #JonSkeet for your insight in the comments.
The problem in this case is the way I wrote my GetHashCode method. It has nothing to do with LINQ, as I originally thought.
Explanation
GetHashCode has to be identical for objects that compare equally. In my case - since the base implementation of object.Equals only checks for reference equality and I am comparing two separate objects - a and b, their base.GetHashCode would result in different values, which in turn would render those two objects as not equal.
Solution
In this case, simply returning the Id value is enough as is shown in MS docs:
One of the simplest ways to compute a hash code for a numeric value that has the same or a smaller range than the Int32 type is to simply return that value.
So changing the above code sample like this:
public override int GetHashCode() => this.Id;
would solve the issue. Please keep in mind that if the value of Id is not unique, this will cause ill behavior. In such cases you'll need another property to check and you will have to compose GetHashCode from ALL those properties. For further info refer to MS docs
I am starting to use Classroom API to enhance local apps in our school. In order to make a report for a class, I want to list all student assignments and gradings. I use loops to go through all courses for a student, then all coursework for every course, and then all submissions for every coursework. Here is the piece of code that I use:
function fListWorkStudent(idStudent)
{
// Variables
var pageToken = null;
var optionalArgs =
{
pageToken: pageToken,
courseStates: 'ACTIVE',
studentId: idStudent,
pageSize: 0
};
var optionalArgs2 =
{
pageToken: pageToken,
userId: idStudent,
pageSize: 0
};
// Courses for a student
var response = Classroom.Courses.list(optionalArgs);
var sCourses = response.courses;
if (sCourses.length === 0)
Logger.log("No courses");
else
{
for (course in sCourses)
{
var idCourse=sCourses[course].id;
var nomprof=getUserName(sCourses[course].ownerId);
// Coursework for every course
var responseW = Classroom.Courses.CourseWork.list(idCourse);
var works = responseW.courseWork;
if (works && (works.length > 0))
{
for work in works)
{
var idWork=works[work].id;
// Submissions for every coursework
var responseS = Classroom.Courses.CourseWork.StudentSubmissions.list(idCourse, idWork, optionalArgs2);
var submissions = responseS.studentSubmissions;
if (submissions && submissions.length >0)
{
for (submission in submissions)
{
// Prepare report here
}
}
}
}
}
}
}
The problem with this code is that when I call Classroom.Courses.CourseWork.StudentSubmissions.list(idCourse, idWork, optionalArgs2) to get the submissions filtered of selected student, and the loop reaches a coursework not assigned to that student, the call fails with error 'classroom.courses.courseWork.studentSubmissions.list; error: Requested entity was not found.'
I could solve it by checking in the loop if the coursework is not assigned to that student before calling the API function, or maybe using a try..catch clause to catch the possible error, but I would like to know if there is a smarter solution to this issue.
Regards
Rafael
Unfortunately the API does not give you an endpoint to list directly all assignment / submissions of a given student
However, you are not alone with this problem, there is already a feature request for this functionality on Google's Public Issue Tracker.
I recommend you to give it a "star" in order to increase visibility.
In the mean time, indeed you either need to implement a try...catch statement, or a conditonal statement, something like:
if(works[work].assigneeMode == "ALL_STUDENTS" || (works[work].assigneeMode == "INDIVIDUAL_STUDENTS" && works[work].individualStudentsOptions.studentIds.indexOf(idStudent)!=-1))
{
var responseS = Classroom.Courses.CourseWork.StudentSubmissions.list(idCourse, idWork, optionalArgs2);
...
}
I red a lot about sorting a CellTable. I also went trough the ColumnSorting with AsyncDataProvider. But my CellTable does not sort.
Here is my code:
public class EventTable extends CellTable<Event> {
public EventTable() {
EventsDataProvider dataProvider = new EventsDataProvider(this);
dataProvider.addDataDisplay(this);
SimplePager.Resources pagerResources = GWT.create(SimplePager.Resources.class);
SimplePager pager = new SimplePager(TextLocation.CENTER, pagerResources, false, 5, true);
pager.setDisplay(this);
[...]
TextColumn<Event> nameCol = new TextColumn<Event>() {
#Override
public String getValue(Event event) {
return event.getName();
}
};
nameCol.setSortable(true);
AsyncHandler columnSortHandler = new AsyncHandler(this);
addColumnSortHandler(columnSortHandler);
addColumn(nameCol, "Name");
getColumnSortList().push(endCol);
}
}
public class EventsDataProvider extends AsyncDataProvider<Event> {
private final EventTable eventTable;
public EventsDataProvider(EventTable eventTable) {
this.eventTable = eventTable;
}
#Override
protected void onRangeChanged(HasData<Event> display) {
int start = display.getVisibleRange().getStart();
int length = display.getVisibleRange().getLength();
// check false values
if (start < 0 || length < 0) return;
// check Cache before making a rpc
if (pageCached(start, length)) return;
// get Events async
getEvents(start, length);
}
}
I do now know, if all the methods are need here. If so, I will add them. But in short:
pageCached calls a method in my PageCache Class which holds a map and a list. Before making a rpc call, the cache is checked if the events where already taken and then displayed.
getEvents just makes an rpc call via asynccallback which updates the rowdata via updateRowData() on success.
My Table is displayed fast with currently around 500 entries (could be more, depends on the customer). No missing data and the paging works fine.
I just cannot get the sorting work. As far as I know, AsyncHandler will fire a setVisibleRangeAndClearData() and then an onRangeChanged(). onRangeChanged is never fired. As for the setVisibleRangeAndClearData() I do not know. But the sortindicator (arrow next to the columnname) does change on every click.
I do not want to let the server sort the list. I have my own Comparators. It is enough, if the current visible page of the table is sorted. I do now want to sort the whole list.
Edit:
I changed following code in the EventTable constructor:
public EventTable() {
[...]
addColumnSortHandler(new ColumnSortEvent.AsyncHandler(this) {
public void onColumnSort(ColumnSortEvent event) {
super.onColumnSort(event);
MyTextColumn<Event> myTextColumn;
if (event.getColumn() instanceof MyTextColumn) {
// Compiler Warning here: Safetytype unchecked cast
myTextColumn = (MyTextColumn<Event>) event.getColumn();
MyLogger.log(this.getClass().getName(), "asc " + event.isSortAscending() + " " + myTextColumn.getName(), Level.INFO);
}
List<Event> list = dataProvider.getCurrentEventList();
if (list == null) return;
if (event.isSortAscending()) Collections.sort(list, EventsComparator.getComparator(EventsComparator.NAME_SORT));
else Collections.sort(list, EventsComparator.descending(EventsComparator.getComparator(EventsComparator.NAME_SORT)));
}
});
addColumn(nameCol, "Name");
getColumnSortList().push(endCol);
}
I had to write my own TextColumn to determine the Name of the column. Otherwise how should I know, which column was clicked? The page gets sorted now but I have to click twice on the column. After then, the sorting is done with every click but in the wrong order.
This solution does need polishing and it seems kinda hacky to me. Any better ideas?
The tutorial, that you linked to, states:
This sorting code is here so the example works. In practice, you would
sort on the server.
Async provider is used to display data that is too big to be loaded in a single call. When a user clicks on any column to sort it, there is simply not enough objects on the client side to display "first 20 evens by name" or whatever sorting was applied. You have to go back to your server and request these first 20 events sorted by name in ascending order. And when a user reverses sorting, you have to go to the server again to get first 20 events sorted by name in a descending order, etc.
If you can load all data in a single call, then you can use regular DataProvider, and all sorting can happen on the client side.
EDIT:
The problem in the posted code was in the constructor of EventsDataProvider. Now it calls onRangeChanged, and the app can load a new sorted list of events from the server.
Hard question to understand perhaps, but let me explain. I have a List of Channel-objects, that all have a ChannelId property (int). I also have a different List (int) - SelectedChannelIds, that contains a subset of the ChannelId-s.
I want to select (through LINQ?) all the Channel-objects that has a ChannelId-property matching one in the second List.
in other words, I have the following structure:
public class Lists
{
public List<Channel> AllChannels = ChannelController.GetAllChannels();
public List<int> SelectedChannelIds = ChannelController.GetSelectedChannels();
public List<Channel> SelectedChannels; // = ?????
}
public class Channel
{
// ...
public int ChannelId { get; set; }
// ...
}
Any ideas on what that LINQ query would look like? Or is there a more effective way? I'm coding for the Windows Phone 7, fyi.
You can use List.Contains in a Where clause:
public Lists()
{
SelectedChannels = AllChannels
.Where(channel => SelectedChannelIds.Contains(channel.ChannelId))
.ToList();
}
Note that it would be more efficient if you used a HashSet<int> instead of a List<int> for the SelectedChannelIds. Changing to a HashSet will improve the performance from O(n2) to O(n), though if your list is always quite small this may not be a significant issue.
SelectedChannels = new List<Channel>(AllChannels.Where(c => SelectedChannelIds.Contains(c.ChannelId)));