|
dev
newsgroups
|
|||||||||||||||||||||||
|
|||||||||||||||||||||||
HashTable stores whole object as Key NOT the result of calling GetHashCode on the object when you doI was testing my understanding of Hashtables and I thought that if I override GetHashCode then i could add an object to hashTable like so: Dim ht As New Hashtable ht.Add(s1, s1) ht.Add(s2, s2) ht.Add(s3, s3) Where s* are instance of a student class that overrides GetHashCode to return the StudentID (code at end of post). I shouldn't add it like: ht.Add(s1.GetHashCode(), s1) as the HashTable calls GetHashCode on the Key for me. This should then store the Hash (The ID returned from GHC) as the Key and the student as the object. However if you loop through the HashTable with: Dim de As DictionaryEntry For Each de In ht Console.WriteLine("Key = {0}, Value = {1}", de.Key, de.Value) Next It shows that the student is stored as both the Key and the Value. Does this mean that you should just add objects to the HashTable like: Dim ht As New Hashtable ht.Add(s1, Nothing) ht.Add(s2, Nothing) ht.Add(s3, Nothing) Seems to me I have misunderstood something here and thw whole point of a HashTable is to use a key to get to a value, was hoping somebody could clear this up for me. Cheers John Student Code Class Student Private _Name As String Private _ID As Integer Private _Age As Integer Public Sub New(ByVal Name As String, ByVal ID As Integer, ByVal age As Integer) _Name = Name _ID = ID _Age = age End Sub Public Overrides Function ToString() As String Return String.Format("{0} is {1} years old and is student no: {2}", _Name, _Age, _ID) End Function Public Overrides Function GetHashCode() As Integer Return _ID End Function End Class John Davis wrote:
Show quote > Hi All The purpose of a hashtable is to provide a fast lookup based on the> > I was testing my understanding of Hashtables and I thought that if I > override GetHashCode then i could add an object to hashTable like so: > > Dim ht As New Hashtable > ht.Add(s1, s1) > ht.Add(s2, s2) > ht.Add(s3, s3) > > Where s* are instance of a student class that overrides GetHashCode to > return the StudentID (code at end of post). > > I shouldn't add it like: > > ht.Add(s1.GetHashCode(), s1) > > as the HashTable calls GetHashCode on the Key for me. > > This should then store the Hash (The ID returned from GHC) as the Key and > the student as the object. However if you loop through the HashTable with: > > Dim de As DictionaryEntry > For Each de In ht > Console.WriteLine("Key = {0}, Value = {1}", de.Key, de.Value) > Next > > It shows that the student is stored as both the Key and the Value. Does this > mean that you should just add objects to the HashTable like: > > Dim ht As New Hashtable > ht.Add(s1, Nothing) > ht.Add(s2, Nothing) > ht.Add(s3, Nothing) > > Seems to me I have misunderstood something here and thw whole point of a > HashTable is to use a key to get to a value, was hoping somebody could clear > this up for me. > key. It achieves this lookup using the hash of the key value. But remember, two keys may have the same hash value, so it has to store the entire key to be sure it's located the correct item. The best way to understand how it works is to use Reflector. Hashtable is implemented in mscorlib, and the interesting things to look at are Insert() and InitHash/GetHash. (Also look at ContainsKey() to understand how the lookups work) In your case, you're own objects can serve as their key, but in many cases they will not - for instance, a hashtable containing a large object relating to pending requests for a client may be keyed on the ClientID. In your case, you could have chosen not to override GetHashCode and instead called ht.Add(s1.StudentID,s1). This will allow you to find an item using their StudentID, whereas in your example, you need to have an Student object before you can search for... a Student object. About the only thing you can do with your sample is, I believe, use your hash table to test for membership (is the student in the hash table), whereas with my example, I can determine membership and locate the larger object. Hope this helps, Damien Typically you would use Add(s1.StudentId,s1) as this allows you to look up a
student given ONLY the student ID [without doing something stupid like creating a student from the ID for the sole purpose of lookup] Whilst it is a good idea to override GetHashCode to return student ID, the hash code is not generally required to be unique so writing Add(s1.GetHashCode(),s1) would still be bad as it would confuse people. Once you have your student you could then have a different hashtable to look up some other stuff to do with the student such as Add(s1,degreeName). In this case the hash table would indeed keep a REFERENCE to the student and call the GetHashCode method. This is often useful as it allows you to find all the students in the relationship in one step rather than mapping from ID to degreeName which would require you to get all the IDs and then lookup them up in the "ID to student" table. To keep the code nice and deal with the asymetric access you will want 2 methods (or equivalent): void Add(Student s) { ht.Add(s.StudentId,s); } Student GetStudent(int id) { return ht[id]; } Show quote "Damien" <Damien_The_Unbelie***@hotmail.com> wrote in message news:1144856001.378762.213920@z34g2000cwc.googlegroups.com... > John Davis wrote: >> Hi All >> >> I was testing my understanding of Hashtables and I thought that if I >> override GetHashCode then i could add an object to hashTable like so: >> >> Dim ht As New Hashtable >> ht.Add(s1, s1) >> ht.Add(s2, s2) >> ht.Add(s3, s3) >> >> Where s* are instance of a student class that overrides GetHashCode to >> return the StudentID (code at end of post). >> >> I shouldn't add it like: >> >> ht.Add(s1.GetHashCode(), s1) >> >> as the HashTable calls GetHashCode on the Key for me. >> >> This should then store the Hash (The ID returned from GHC) as the Key and >> the student as the object. However if you loop through the HashTable >> with: >> >> Dim de As DictionaryEntry >> For Each de In ht >> Console.WriteLine("Key = {0}, Value = {1}", de.Key, de.Value) >> Next >> >> It shows that the student is stored as both the Key and the Value. Does >> this >> mean that you should just add objects to the HashTable like: >> >> Dim ht As New Hashtable >> ht.Add(s1, Nothing) >> ht.Add(s2, Nothing) >> ht.Add(s3, Nothing) >> >> Seems to me I have misunderstood something here and thw whole point of a >> HashTable is to use a key to get to a value, was hoping somebody could >> clear >> this up for me. >> > The purpose of a hashtable is to provide a fast lookup based on the > key. It achieves this lookup using the hash of the key value. But > remember, two keys may have the same hash value, so it has to store the > entire key to be sure it's located the correct item. > > The best way to understand how it works is to use Reflector. Hashtable > is implemented in mscorlib, and the interesting things to look at are > Insert() and InitHash/GetHash. (Also look at ContainsKey() to > understand how the lookups work) > > In your case, you're own objects can serve as their key, but in many > cases they will not - for instance, a hashtable containing a large > object relating to pending requests for a client may be keyed on the > ClientID. In your case, you could have chosen not to override > GetHashCode and instead called ht.Add(s1.StudentID,s1). This will allow > you to find an item using their StudentID, whereas in your example, you > need to have an Student object before you can search for... a Student > object. About the only thing you can do with your sample is, I believe, > use your hash table to test for membership (is the student in the hash > table), whereas with my example, I can determine membership and locate > the larger object. > > Hope this helps, > > Damien > Hi Guys
Thanks for your input, I had a look using Reflector and it is a lot clearer now. Regarding your suggestion to add a student using ht.Add(s1.StudentID, s1) This is obviously a good approach if there is an obvious property or public field to use as the hash but I just wondered if a class designer has overriden GetHashCode then this would mean that the user of said object wouldn't have to know which field property to use and could just call. ht.Add(s1.GetHashCode(), s1) Having said this the user of the object would need to know what GetHashCode actually returned in order to do a lookup so they might as well just add it using that property/field in the first place. I guess like you say Nick, GetHashCode is only useful if your are storing the student as the key in order to store a different object (such as there exams results/degree name) in the value. Cheers again for your help on this matter. John Show quote "Nick Hounsome" <nh***@nickhounsome.me.uk> wrote in message news:hmo%f.43180$8Q3.43128@fe1.news.blueyonder.co.uk... > Typically you would use Add(s1.StudentId,s1) as this allows you to look up > a student given ONLY the student ID [without doing something stupid like > creating a student from the ID for the sole purpose of lookup] > > Whilst it is a good idea to override GetHashCode to return student ID, the > hash code is not generally required to be unique so writing > Add(s1.GetHashCode(),s1) would still be bad as it would confuse people. > > Once you have your student you could then have a different hashtable to > look up some other stuff to do with the student such as > Add(s1,degreeName). In this case the hash table would indeed keep a > REFERENCE to the student and call the GetHashCode method. This is often > useful as it allows you to find all the students in the relationship in > one step rather than mapping from ID to degreeName which would require you > to get all the IDs and then lookup them up in the "ID to student" table. > > To keep the code nice and deal with the asymetric access you will want 2 > methods (or equivalent): > > void Add(Student s) { ht.Add(s.StudentId,s); } > Student GetStudent(int id) { return ht[id]; } > > > "Damien" <Damien_The_Unbelie***@hotmail.com> wrote in message > news:1144856001.378762.213920@z34g2000cwc.googlegroups.com... >> John Davis wrote: >>> Hi All >>> >>> I was testing my understanding of Hashtables and I thought that if I >>> override GetHashCode then i could add an object to hashTable like so: >>> >>> Dim ht As New Hashtable >>> ht.Add(s1, s1) >>> ht.Add(s2, s2) >>> ht.Add(s3, s3) >>> >>> Where s* are instance of a student class that overrides GetHashCode to >>> return the StudentID (code at end of post). >>> >>> I shouldn't add it like: >>> >>> ht.Add(s1.GetHashCode(), s1) >>> >>> as the HashTable calls GetHashCode on the Key for me. >>> >>> This should then store the Hash (The ID returned from GHC) as the Key >>> and >>> the student as the object. However if you loop through the HashTable >>> with: >>> >>> Dim de As DictionaryEntry >>> For Each de In ht >>> Console.WriteLine("Key = {0}, Value = {1}", de.Key, >>> de.Value) >>> Next >>> >>> It shows that the student is stored as both the Key and the Value. Does >>> this >>> mean that you should just add objects to the HashTable like: >>> >>> Dim ht As New Hashtable >>> ht.Add(s1, Nothing) >>> ht.Add(s2, Nothing) >>> ht.Add(s3, Nothing) >>> >>> Seems to me I have misunderstood something here and thw whole point of a >>> HashTable is to use a key to get to a value, was hoping somebody could >>> clear >>> this up for me. >>> >> The purpose of a hashtable is to provide a fast lookup based on the >> key. It achieves this lookup using the hash of the key value. But >> remember, two keys may have the same hash value, so it has to store the >> entire key to be sure it's located the correct item. >> >> The best way to understand how it works is to use Reflector. Hashtable >> is implemented in mscorlib, and the interesting things to look at are >> Insert() and InitHash/GetHash. (Also look at ContainsKey() to >> understand how the lookups work) >> >> In your case, you're own objects can serve as their key, but in many >> cases they will not - for instance, a hashtable containing a large >> object relating to pending requests for a client may be keyed on the >> ClientID. In your case, you could have chosen not to override >> GetHashCode and instead called ht.Add(s1.StudentID,s1). This will allow >> you to find an item using their StudentID, whereas in your example, you >> need to have an Student object before you can search for... a Student >> object. About the only thing you can do with your sample is, I believe, >> use your hash table to test for membership (is the student in the hash >> table), whereas with my example, I can determine membership and locate >> the larger object. >> >> Hope this helps, >> >> Damien >> > >  |
|||||||||||||||||||||||
Other interesting topics