Operations on Hashtable in C#

Question

Using Hashtable I want to have multiple objects mapped to the same key. E.g. Key is 'Age' and Values are ojects of struct 'Student'.

I suppose perceptually there will be a sort of linked list with Key acting as a 'Head' 25->obj1->obj2->obj3

Here are my questions:

Is the above representation correct? If not, which data structure can be used for achieving the same?

Can I look up for a specific field on the above data representation. e.g. One I reach the key 25, I look for the name 'Scott' in the row. Will I be able to stop/ get a pointer to the object containing the field Scott?

Thanks!

I noticed a webpage which made me think the way I did: math.grin.edu/~walker/courses/153.sp02/… Here, the Hash Table seem to show the kind of concept I am referring to. Can can i achieve something like this? — Manav Sharma
– Manav Sharma, Commented Aug 9, 2010 at 15:29

Justin Niessner · Accepted Answer · 2010-08-09 15:21:44Z

7

If you're using C#, the best option would probably be (which, essentially, is what you're proposing):

var collection = new Dictionary<Age, List<Student>>();

Assuming Age and Student are both types.

answered Aug 9, 2010 at 15:21

Justin Niessner

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3 Comments

Hamish Grubijan Over a year ago

Why not use an ISet instead of List?

Justin Niessner Over a year ago

@Hamish - In this case, I'm using var to hold the value. I need concrete types on the right side of the assignment.

Woot4Moo Over a year ago

@Hamish it would depend completely on his implementation of student. For instance 2 Students can have the same name. On the converse he could use the ID of the student and compare the 2 values before inserting them.

thelost · Accepted Answer · 2010-08-09 15:21:27Z

3

Only one value can be stored for a key. So how about Hashtable<int, List<Student>> ?

answered Aug 9, 2010 at 15:21

thelost

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2 Comments

Josh Sterling Over a year ago

Hashtable isn't generic, Dictionary<key, List<value>> does the same and saves you some casting.

Joshua Over a year ago

I have a type that I call Index that is essentially a wrapped version of this.

Ani · Accepted Answer · 2010-08-10 13:30:20Z

1

If you already have a collection of all the students at the time you want to create this data-structure, you could do

var lookUp = Students.ToLookUp (student => student.Age);

The disadvantage is that the Lookup <K,V> data-structure is immutable. If this is not an option, a Dictionary<int, List<Student>> structure might be more appropriate as others have mentioned.

edited Aug 10, 2010 at 13:30

answered Aug 9, 2010 at 15:29

Ani

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Comments

casperOne · Accepted Answer · 2010-08-09 15:25:15Z

0

There is no wrong data structure to use here, just some structures to store will be better suited for your needs depending on what you want to do with the results that are keyed on the age.

You indicated you wanted the ability to perform a lookup on the items based on name once you get the students that are a particular age. In this case, your would want to have an IDictionary<string, Student> where the key is the first name of the Student.

Note, that this is a highly specialized structure here. If it keeps expanding to where you need to search on other facets, you might want to take a look at other options, as the code required would grow pretty unmaintainable as your selection criteria grew.

answered Aug 9, 2010 at 15:25

casperOne

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