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.

**Table 6: Methods Provided by Sets in C++**¶
Method Name	Use	Explanation
`union`	`set_union()`	Returns a new set with all elements from both sets
`intersection`	`set_intersection()`	Returns a new set with only those elements common to both sets
`difference`	`set_difference()`	Returns a new set with all items from first set not in second
`add`	`aset.insert(item)`	Adds item to the set
`remove`	`aset.erase(item)`	Removes item from the set
`clear`	`aset.clear()`	Removes all elements from the set

5.7. Summary¶

A statically allocated C++ array is an ordered collection of one or more C++ data values of identical type stored in contiguous memory.
A vector is a dynamically allocated array with many useful methods. It is more similar to the Python list than the array.
C++ strings are a sequential collection of zero or more characters. They are very similar to Python strings.
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.
A set is an unordered collection of unique values.

5.8. Check Yourself¶

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.

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”}}