C# program to merge two hash table collections

Hash table collection in C# stores key-value pairs. Each element or item in the collection is a key-value pair, that is, the collection is a two-element collection. Key is a unique, non-empty key used to access elements in a hash table.

The hash table collection is immutable and cannot have duplicate elements. This means that key-value combinations should be unique. However, these values can be empty or repeated. .Net Framework provides a HashTable class to implement a hash table collection and contains the functionality required to implement a hash table without any additional coding.

Each element in a hash table collection is a DictionaryEntry object with two properties: a key element and a value element. When an element is added to a hash table, a hash code is automatically generated. This hash code is internal and hidden. The elements in a hash table collection are ordered by the hidden hash code. Therefore, hash table elements are considered randomly selected.

With a brief introduction to hash table collections, let’s look at how to merge two hash table collections.

How to merge two hash table collections?

The Hashtable class is provided by System. The collection namespace contains only base class libraries that can be used to construct hash table objects and perform operations such as adding/removing elements, counting the number of elements, etc. There is no method/function provided that can be used to merge two hash tables together.

We have to design our own way to merge two hash tables. We know that the capacity or size of a hash table is the number of elements that the hash table holds. As elements are inserted into the hash table, the size of the hash table grows automatically through reallocation.

So when we merge two hash tables together, we add elements of one hash table to the other. As we add elements, the size of this hash table will be adjusted accordingly.

method

• Create two hash table objects.

• Use the Add method to populate both tables with elements.

• Traverse the second hash table using the key, and if the current item (the key being traversed) does not already exist in the first hash table, add each of its key-value pairs to the first in the hash table.

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• Print the generated hash table.

Note: Before adding a key, we explicitly check if the key exists in the hash table, because hash tables do not allow duplicate keys to be added.

Example

Convert the above method into a C# program as shown below.

using System; using System. Collections; class MyHashTable { static public void Main() { Hashtable indianNumberSystem = new Hashtable(); indianNumberSystem.Add(1,"Ones"); indianNumberSystem.Add(10,"Tens"); indianNumberSystem.Add(100,"Hundred"); indianNumberSystem.Add(1000,"Thousand"); Console.WriteLine("Contents of indianNumberSystem hashtable:"); foreach(DictionaryEntry ele1 in indianNumberSystem){ Console.WriteLine("{0} ({1}) ", ele1.Key, ele1.Value); } Hashtable langCodes = new Hashtable(); langCodes.Add("C++","CPlusPlus"); langCodes.Add("C#","CSharp"); langCodes.Add("Java","Java"); langCodes.Add("PL","Perl"); Console.WriteLine("Contents of langCodes Hashtable:"); foreach(DictionaryEntry ele1 in indianNumberSystem){ Console.WriteLine("{0} ({1}) ", ele1.Key, ele1.Value); } foreach (DictionaryEntry entry in langCodes) { if(!indianNumberSystem.ContainsKey(entry.Key)) { indianNumberSystem.Add(entry.Key, entry.Value); }} Console.WriteLine("Key, Value pairs after merging langCodes to indianNumberSystem:"); foreach(DictionaryEntry ele1 in indianNumberSystem){ Console.WriteLine("{0} ({1}) ", ele1.Key, ele1.Value); } } }

Here we have two hash tables, indianNumberSystem and langCodes.

The hash table indianNumberSystem has the following data,

The hash table langCodes has the following data.

We first display the contents of these two tables. We then iterate over it using the keys of the langCodes hash table. In the traversal loop, we first check if the hash table indianNumberSystem has the same key. If the key does not exist, we add the langCodes element pointed to by the current key to the indianNumberSystem hash table.

Output

Finally, we display the merged table.

Contents of indianNumberSystem hashtable: 1000 (Thousand) 10 (Tens) 100 (Hundred) 1 (Ones) Contents of langCodes Hashtable: 1000 (Thousand) 10 (Tens) 100 (Hundred) 1 (Ones) Key, Value pairs after merging langCodes to indianNumberSystem: 100 (Hundred) 1000 (Thousand) PL (Perl) 10 (Tens) C# (CSharp) Java (Java) C++ (CPlusPlus) 1 (Ones)

From the generated output we can see that both tables are merged correctly.

Example

Now let us consider another example, the C# program given below.

using System; using System. Collections; using System.Collections.Generic; class MyHashTable { static public void Main() { Hashtable indianNumberSystem = new Hashtable(); indianNumberSystem.Add(1,"Ones"); indianNumberSystem.Add(10,"Tens"); indianNumberSystem.Add(100,"Hundred"); indianNumberSystem.Add(1000,"Thousand"); Console.WriteLine("Contents of indianNumberSystem hashtable:"); foreach(DictionaryEntry ele1 in indianNumberSystem){ Console.WriteLine("{0} ({1}) ", ele1.Key, ele1.Value); } Hashtable NumberNames = new Hashtable(); NumberNames.Add(1,"One"); NumberNames.Add(2,"Two"); NumberNames.Add(3,"Three"); NumberNames.Add(4,"Four"); Console.WriteLine("Contents of NumberNames Hashtable:"); foreach(DictionaryEntry ele1 in NumberNames){ Console.WriteLine("{0} ({1}) ", ele1.Key, ele1.Value); } foreach (DictionaryEntry entry in NumberNames) { if(!indianNumberSystem.ContainsKey(entry.Key)) { indianNumberSystem.Add(entry.Key, entry.Value); }} Console.WriteLine("Key, Value pairs after merging NumberNames to indianNumberSystem:"); foreach(DictionaryEntry ele1 in indianNumberSystem){ Console.WriteLine("{0} ({1}) ", ele1.Key, ele1.Value); } } }

This program is the same as the previous one except that we replace the langCodes hash table with the NumberNames hash table. The NumberNames hash table has the following elements.

Output

As we can see, the hash tables indianNumberSystem and NumberNames have common data. Now let's execute this program to check how the merge happens.

Contents of indianNumberSystem hashtable: 1000 (Thousand) 10 (Tens) 100 (Hundred) 1 (Ones) Contents of NumberNames Hashtable: 4 (Four) 3 (Three) 2 (Two) 1 (One) Key, Value pairs after merging NumberNames to indianNumberSystem: 100 (Hundred) 1000 (Thousand) 10 (Tens) 4 (Four) 3 (Three) 2 (Two) 1 (Ones)

As you can see from the above output, the data element (key=1) in NumberNames is not added to the indianNumberSystem hash table. This is because duplication is not allowed.

in conclusion

Therefore, we can merge two hash table collections by copying or adding the data of one hash table to another hash table collection. Whenever a common key exists in both hash tables, duplicate keys are not added. But programmers must make sure to check when adding data to a hash table to avoid accidentally adding data, leading to unpredictable results.

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