Explain the concept of a dictionary in programming and its main functions.

A dictionary in programming is a data structure that stores pairs of 'key-value'. Each key is unique and associated with a specific value, allowing quick data retrieval by key.

Main functions of a dictionary:

• Fast search for a value by key.
• Adding new key-value pairs.
• Removing elements by key.
• Traversing all pairs for processing or output.

In Swift, a dictionary is represented by the type Dictionary<Key, Value>. Example:

var dict: [String: Int] = ["apple": 3, "banana": 5]
dict["orange"] = 7 // adding
let count = dict["apple"] // retrieving value by key