Explain the principle of operation of a hash table and its main purpose.

A hash table is a data structure that provides fast access to elements by key. It works as follows: the key is passed through a hash function, which transforms it into an array index (hash code). The value associated with the key is stored at this index.

The main purpose of a hash table is to provide efficient search, insertion, and deletion of data with an amortized complexity of about O(1).

Example in Dart (Flutter):

var map = <String, int>{};
map['apple'] = 3;
print(map['apple']); // 3

Here, Map is an implementation of a hash table, where the key is a string and the value is a number.