I built a function that needs to add a node to a binary search tree that is sorted by its ID (moviecode = id) but it doesn't work right. can you help me figure out whats wrong with the code?
Node *buildtree(Node *dataTree, char *name, int id, float rating, int numvote) {
if ((dataTree == NULL)) {
dataTree= make_node();
strcpy(dataTree->data.movieName,name);
dataTree->data.meanRaiting = rating;
dataTree->data.numOfVoters = numvote;
dataTree->data.movieCode = id;
return;
}
else if (dataTree->data.movieCode > id)
(buildtree (dataTree->left, name, id, rating, numvote));
else
buildtree (dataTree->right, name, id, rating, numvote);
return dataTree;
}
At first glance, it looks to me like dataTree is getting instantiated, but it's not attached to anything. You should have a pointer point to the new object or it'll be lost at the end of the method.
What most people do in situations like this is pass a double pointer. When you need to modify the pointer, you simply do *dataTree = make_node();
Does that help?
Related
I'm trying to update a record given the customer Id, the row Id, and a dynamic column name.
Thus far I have the following, with the trouble spot marked by ***:
public void UpdateRecord(int Id, string rval, string column, string value)
{
var rId = GetRvalId(rval);
var entry = _context.Customers
.Where(x => x.Id == Id && x.RVals.Id == rId && x.***column?*** == column).First();
entry = value;
}
I haven't been able to find a good example of how to do this.
Addition after comments at the end
The reason you couldn't find examples is because it is not a good design.
Your method is very error prone, difficult to test and horrible to maintain. What if someone types the incorrect column name? What if you try to assign a string to the customer's birthday? And even if you would implement some string checking for column names and proposed values, then your program wouldn't work anymore after someone changes the names or the types of the columns.
So let's redesign!
Apparently you have a Customer with an Id and a property Rvals. This property Rvals also has a property Id.
You also have a function GetRValId that can convert a string rval to an int rvalId.
What you want, is given an Id and a string rval, you want to update one of the columns of the first Customer with this Idand rValId.
Side questions: Can there be more than one Customer with Id? In that case: are you sure Id is an ID? What do you want if there are more matching Customers? Update all customers or update only the first one? Which customer do you define as the first customer?
Leaving the side questions aside. We want a function signature that reports errors at compile time if you use non-existing customer properties, or if you try to assign a string to a Birthday. Something like this perhaps?
Update the name of the customer:
int customerId = ...
string rval = ...
string proposedName = "John Doe";
UpdateCustomerRecord(id, rval, customer => customer.Name = proposedName);
Update the Birthday of the customer:
DateTime proposedBirthday = ...
UpdateCustomerRecord(id, rval, customer => customer.Birthday = proposedBirthday)
This way you can't use any column that does not exist, and you can't assign a string to a DateTime.
You want to change two values in one call? Go ahead:
UpdateCustomerRecord(id, rval, customer =>
{
customer.Name = ...;
customer.Birthday = ...;
});
Convinced? Let's write the function:
public void UpdateCustomerRecord(int customerId, string rval, Action<Customer> action)
{
// the beginning is as in your function:
var rId = GetRvalId(rval);
// get the customer that you want to update:
using (var _Context = ...)
{
// get the customer you want to update:
var customerToUpdate = _Context.Customers
.Where(customer => customer.Id == Id
&& customer.RVals.Id == rId)
.FirstOrDefault();
// TODO: exception if there is no customerToUpdate
// perform the action and save the changes
action(customerToUpdate);
_context.SaveChanges();
}
Simple comme bonjour!
Addition after comments
So what does this function do? As long as you don't call it, it does nothing. But when you call it, it fetches a customer, performs the Action on the Customer you provided in the call, and finally calls SaveChanges.
It doesn't do this with every Customer, no it does this only with the Customer with Id equal to the provided Id and customer.RVals.Id == ... (are you still certain there is more than one customer with this Id? If there is only one, why check for RVals.Id?)
So the caller not only has to provide the Id, and the RVal, which define the Customer to update, but he also has to define what must be done with this customer.
This definition takes the form of:
customer =>
{
customer.Name = X;
customer.BirthDay = Y;
}
Well if you want, you can use other identifiers than customer, but it means the same:
x => {x.Name = X; x.BirthDay = Y;}
Because you put it on the place of the Action parameter in the call to UpdateCustomerRecord, I know that x is of type Customer.
The Acton statement means: given a customer that must be updated, what must we do with the customer? You can read it as if it was a Function:
void Action(Customer customer)
{
customer.Name = ...
customer.BirthDay = ...
}
In the end it will do something like:
Customer customerToUpdate = ...
customerToUpdate.Name = X;
customerToUpdate.BirthDay = Y;
SaveChanges();
So in the third parameter, called Action you can type anything you want, even call functions that have nothing to do with Customers (probably not wise). You have an input parameter of which you are certain that it is a Customer.
See my earlier examples of calling UpdateCustomerRecord, one final example:
UpdateCustomerRecord( GetCustomerId(), GetCustomerRVal,
// 3rd parameter: the actions to perform once we got the customerToUpdate:
customer =>
{
DateTime minDate = GetEarliestBirthDay();
if (customer.BirthDay < minDate)
{ // this Customer is old
customer.DoThingsThatOldPeopleDo();
}
else
{ // this Customer is young
customer.DoThingsThatYoungPeopleDo();
}
}
}
So the Action parameter is just a simpler way to say: "once you've got the Customer that must be updated, please perform this function with the Customer
So if you only want to update a given property of the customer write something like:
UpdateCustomerRecord(... , customer =>
{
Customer.PropertyThatMustBeUpdated = NewValueOfProperty;
}
Of course this only works if you know which property must be updated. But since you wrote "I am trying to update a specific cell." I assume you know which property the cells in this column represent.
It is not possible to pass the column name as the string value in LINQ. Alternate way to do it, if you have the limited number of the column name which can be passed then it can be achieved as below:
public void UpdateRecord(int Id, string rval, string column, string value)
{
var rId = GetRvalId(rval);
var entry = _context.Customers
.Where(x => x.Id == Id &&
x.RVals.Id == rId &&
(x.column1 == value || column == column1) &&
(x.column2 == value || column == column2) &&
(x.column3 == value || column == column3) &&
(x.column4 == value || column == column4) &&
(x.column5 == value || column == column5) &&
)).First();
entry = value;
}
UpdateRecord(5, "rval", "column1", "value");
UpdateRecord(5, "rval", "column2", "value");
UpdateRecord(5, "rval", "column3", "value");
Here, suppose you have the 5 columns that can be passed while calling the funcion UpdateRecord then you can add the 5 clauses in the WHERE as above.
Other way to do it dynamic LINQ
var entry = db.Customers.Where(column + " = " + value).Select(...);
I want to use DocumentDB to store roughly 200.000 documents of the same type. The documents each get an integer id field and I would like to retrieve them paged, in reverse order (highest id first).
So recently I found out there is no sorting for DocumentDB (see also DocumentDB - query result order). Perhaps it is better to go for a different database (such as RavenDB) however, time is pressing and I want to avoid the cost of switching to another database.
The question:
I have been looking at implementing my own sorted index of the documents on the client side (ASP Web API 2). I was thinking of creating a SortedList of key(id) and value(document.selflink). Then I could create a Getter with parameters for count, offset and a predicate to filter the documents. Below I added a quick example.
I just have the feeling this is a bad idea; either slow, costing too many resources or can be better done another way. So I am open for implementation suggestions...
public class SortableDocumentDbRepository
{
private SortedList _sorted = new SortedList();
private readonly string _sortedPropertyName;
private DocumentCollection ReadOrCreateCollection(string databaseLink) {
DocumentCollection col = base.ReadOrCreateCollection(databaseLink);
var docs = Client.CreateDocumentQuery(Collection.DocumentsLink)
.AsEnumerable();
lock (_sorted.SyncRoot) {
foreach (Document doc in docs) {
var propVal = doc.GetPropertyValue<string>(_sortedPropertyName);
if (propVal != null) {
_sorted.Add(propVal, doc.SelfLink);
}
}
}
return col;
}
public List<T> GetItems<T>(int count, int offset, Expression<Func<T, bool>> predicate) {
List<T> result = new List<T>();
lock (_sorted.SyncRoot) {
var values = _sorted.GetValueList();
for (int i = offset; i < _sorted.Count; i++) {
var queryable = predicate != null ?
Client.CreateDocumentQuery<T>(values[i].ToString()).Where(predicate) :
Client.CreateDocumentQuery<T>(values[i].ToString());
T item = queryable.AsEnumerable().FirstOrDefault();
if (item == null || item.Equals(default(T))) continue;
result.Add(item);
if (result.Count >= count) return result;
}
}
return result;
}
}
Microsoft has implemented Sorting:
https://learn.microsoft.com/en-us/azure/cosmos-db/sql-api-sql-query-reference#bk_orderby_clause
Example: SELECT * FROM c ORDER BY c._ts DESC
As you mentioned, order by unfortunately isn't implemented yet.
Your approach looks reasonable to me.
I see you are using a predicate to narrow the query result set (pulling 200,000 records for any DB will be costly).
Since it looks like you are looking to order by id - you can also look in to setting up a range index on id allowing you to perform range queries (e.g. < and >) on the id and further narrow the query result set. There is also a range index included by default on the _ts (timestamp) system property on documents that may also be helpful in this context.
See: http://azure.microsoft.com/en-us/documentation/articles/documentdb-indexing-policies/
My query is as follows:
int ID = db.Q_Table.Find(item.PassedInID).ID;
I already found a solution for my issue, however i am wondering why i must write it like so
1.
Nullable(int) ID = db.Q_Table.Find(item.PassedInID).ID;
2.
db.Q_Table.Where(w => w.PassedInID== item.PassedInID).Select(s => s.ID ).SingleOrDefault();
It wouldn't let me put int int he < in the above code -.-...
I am curious why i have to code it to a nullable int? I really didn't want to code it like 2nd solution because its more code :). Yes i have made sure there are values in the database and from the below image you can see my database doesn't accept nulls.
Thanks for any answers
There is a difference between int and Nullable<int> or int?, you can't directly assign a Nullable<int> to int (Nullable<T>), Consider:
int? x = 123;
int y = x; //This would be an error
But you can use null-coalescing operator
int? x = 123;
int y = x ?? 0;
Now for your case, your ID seems to be a mapped to column in database which allows null. That will map to C# Nullable<int>, if you want to assign the result to an int you can do:
int ID = db.Q_Table.Find(item.PassedInID).ID ?? 0;
That will give your variable the value of ID or 0 if it is null.
ID is not an int but rather a Nullable<int> (or short form int?).
This is typically the case when the underlying database column is nullable.
There is also no < operator defined on an int?. If you want to do that, you have to check for the presence of a value first:
db.Q_Table.Where(w => w.PassedInID.HasValue && w.PassedInID.Value == item.PassedInID)
.Select(s => s.ID ).SingleOrDefault();
that's probably because db.Q_Table.Find returns a nullible int.
you can probably also do the following if you wanted to
int? ID = db.Q_Table.Find(item.PassedInID).ID;
I guess you could do something like the following, it is a little less code:
int? t;
int ID = (t = db.Q_Table.Find(item.PassedInID).ID) == null ? -1 : t;
Of course, now you will have to handle -1 ID as a special case later in your code. I expect you just want a nullable int.
I am slow today
There is a request
"Take random child and put it into another garden."
I changed the code, but error in the last line of code "Does not contain a definition…and no extension method":
var query = db.Child.Where(x => x.Garden != null);
int count = query.Count();
int index = new Random().Next(count);
var ch = db.Child.OrderBy(x => query.Skip(index).FirstOrDefault());
ch.Garden_Id = "1";
What am I doing wrong?
It's hard to tell what you're doing wrong, because you didn't say why the results you're getting does not satisfy you.
But I can see two possible mistakes.
You're counting items with x.Garden != null condition, but taking from all children.
Take returns IEnumerable<T> even when you specify it to return only 1 item, you should probably use First instead.
I think your k should be
var k = db.Child.Where(x => x.Garden != null).Skip(rnd.Next(0,q)).First();
I am attempting to get unique values in a list of similar value distinguished only by a one element in a pipe delimited string... I keep getting at least one object must implement Icomparable. I don't understand why I keep getting that. I am able to groupBy that value... Why can't I find the max... I guess it is looking for something to compare it with. If I get the integer version will it stop yelling at me? This is the last time I am going to try using LINQ...
var queryResults = PatientList.GroupBy(x => x.Value.Split('|')[1]).Select(x => x.Max());
I know I can get the unique values some other way. I am just having a hard time figuring it out. In that List I know that the string with the highest value amongst its similar brethren is the one that I want to add to the list. How can I do that? I am totally drawing a blank because I have been trying to get this to work in linq for the last few days with no luck...
foreach (XmlNode node in nodeList)
{
XmlDocument xDoc = new XmlDocument();
xDoc.LoadXml(node.OuterXml);
string popPatInfo = xDoc.SelectSingleNode("./template/elements/element[#name=\"FirstName\"]").Attributes["value"].Value + ", " + xDoc.SelectSingleNode("./template/elements/element[#name=\"LastName\"]").Attributes["value"].Value + " | " + DateTime.Parse(xDoc.SelectSingleNode("./template/elements/element[#name=\"DateOfBirth\"]").Attributes["value"].Value.Split('T')[0]).ToString("dd-MMM-yyyy");
string patientInfo = xDoc.SelectSingleNode("./template/elements/element[#name=\"PatientId\"]").Attributes["value"].Value + "|" + xDoc.SelectSingleNode("./template/elements/element[#name=\"PopulationPatientID\"]").Attributes["enc"].Value;// +"|" + xDoc.SelectSingleNode("./template/elements/element[#name=\"AdminDate\"]").Attributes["value"].Value;
int enc = Int32.Parse(patientInfo.Split('|')[1]);
if (enc > temp)
{
lastEncounter.Add(enc, patientInfo);
temp = enc;
}
//lastEncounter.Add(Int32.Parse(patientInfo.Split('|')[1]));
PatientList.Add( new SelectListItem { Text = popPatInfo, Value = patientInfo });
}
I was thinking about using some kind of temp variable to find out what is the highest value and then add that string to the List. I am totally drawing a blank however...
Here I get the IDs in an anonymous type to make it readable.
var patientEncounters= from patient in PatientList
let PatientID=Int32.Parse(patient.Value.Split('|')[0])
let EncounterID=Int32.Parse(patient.Value.Split('|')[1])
select new { PatientID, EncounterID };
Then we group by UserID and get the last encounter
var lastEncounterForEachUser=from pe in patientEncounters
group pe by pe.PatientID into grouped
select new
{
PatientID=grouped.Key,
LastEncounterID=grouped.Max(g=>g.EncounterID)
};
Linq doesn't know how to compare 2 Patient objects, so it can't determine which one is the "greatest". You need to make the Patient class implement IComparable<Patient>, to define how Patient objects are compared.
// Compare objets by Id value
public int CompareTo(Patient other)
{
return this.Id.CompareTo(other.Id);
}
Another option is to use the MaxBy extension method available in Jon Skeet's MoreLinq project:
var queryResults = PatientList.GroupBy(x => x.Value.Split('|')[1])
.Select(x => x.MaxBy(p => p.Id));
EDIT: I assumed there was a Patient class, but reading your code again, I realize it's not the case. PatientList is actually a collection of SelectListItem, so you need to implement IComparable in that class.