5.6. Unordered Sets

An unordered_set is an unordered collection of zero or more unique C++ data values of a particular type. To use unordered_sets, you import unordered_set from the Standard template library with #include <unorderd_set>.

Unordered_sets allow for fast retrieval of individual elements based on their value. In an unordered_set, the value of an element is at the same time its key, that identifies it uniquely. Keys are immutable, therefore, the elements in an unordered_set cannot be modified once in the container - However, they can be inserted and removed.

Unordered sets do not allow duplicates and are initialized using comma-delimited values enclosed in curly braces. The collection can be assigned to a variable as shown below.

set<int> mySet = {3, 6, 4, 78, 10}

Unordered sets support a number of methods that should be familiar to those who have worked with sets in a mathematics setting. Table 6 provides a summary. Examples of their use follow.

The code below is an example of a program that can detect if a specific char is in an unordered set.

// Function that checks to see if a char // is in the unorderd set #include <iostream> #include <unordered_set> using namespace std; void checker(unordered_set<char> set, char letter){ if(set.find(letter) == set.end()){ cout << "letter " << letter << " is not in the set." << endl; } else{ cout << "letter " << letter << " is in the set." << endl; } } int main(){ unordered_set<char> charSet = {'d', 'c', 'b', 'a'}; char letter = 'e'; checker(charSet, letter); charSet.insert('e'); checker(charSet, letter); return 0; }

the find method used for a conditional in Checker compares each item in the set with the given parameter until there is a match. the set.find(letter) == set.end() section means that if find cannot find the letter before reaching the end of the set, then letter is not contained in the set.

5.6.1. Matching

Q-2: Match the Unordered Sets operations with their corresponding explination. Feedback shows incorrect matches.
• union
• Returns a new set with all elements from both sets.
• intersection
• Returns a new set with only those elements common to both sets.
• difference
• Returns a new set with all items from first set not in second.
• add
• Adds item to the set.
• remove
• erases item from the set.
• clear
• Removes all elements from the set.

5.7. Summary

1. A statically allocated C++ array is an ordered collection of one or more C++ data values of identical type stored in contiguous memory.

2. A vector is a dynamically allocated array with many useful methods. It is more similar to the Python list than the array.

3. C++ strings are a sequential collection of zero or more characters. They are very similar to Python strings.

4. A hash table is used to store keys-value pairs. It applies a related hash function to the key in order to compute the location of the associated value. Look-up is typically very fast.

5. A set is an unordered collection of unique values.

5.8. Check Yourself

Q-3: Which C++ structure is the best choice for a group of ordered data of a fixed length?

• array
• Correct!
• hash table
• No. hash tables are not ordered.
• string
• A string would only work for character data. Try again.
• vector
• There is a better choice given that the group is fixed length
• more than one of the above
• Only of the above is best.

Q-4: Drag each data type to its' corresponding C++ initialization syntax. Feedback shows incorrect matches.
• Array
• {“What”, “am”, “I”, "am"}
• Set
• {“What”, “am”, “I”}
• String
• “What am I”
• Hash Table
• {​{“What”, “am I”}​}

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