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 <tab_setmethods>{.interpreted-text role=”ref”} provides a summary. Examples of their use follow.
- ::: {#tab_setmethods}
- +——————+————————+————————+
- | 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 |
- +——————+————————+————————+
- | >
insert| >aset.insert(item)| > Adds item to the set | - +——————+————————+————————+
- | >
erase| >aset.erase(item)| > Removes item from | - | | | > the set |
- +——————+————————+————————+
- | >
clear| >aset.clear()| > Removes all elements | - | | | > from the set |
- +——————+————————+————————+
- | >
find| > [`aset.f | ` Find item in the | - | | ind(item)]{.title-ref} | set |
- +——————+————————+————————+
-
Table 6: Methods Provided by Sets in C++ :::
The code below is an example of a program that can detect if a specific char is in an unordered set.
::: {.activecode language=”cpp”} Unordered_set_example
// 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.
Matching
::: {.dragndrop feedback=”Feedback shows incorrect matches.” match_1=”union|||Returns a new set with all elements from both sets.” match_2=”intersection|||Returns a new set with only those elements common to both sets.” match_3=”difference||| Returns a new set with all items from first set not in second.” match_4=”add|||Adds item to the set.” match_5=”remove|||erases item from the set.” match_6=”clear|||Removes all elements from the set.”} matching_us
Match the Unordered Sets operations with their corresponding explination. :::
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.
Check Yourself
::: {.mchoice answer_a=”array” answer_b=”hash table” answer_c=”string” answer_d=”vector” answer_e=”more than one of the above” correct=”a” feedback_a=”Correct!” feedback_b=”No. hash tables are not ordered.” feedback_c=”A string would only work for character data. Try again.” feedback_d=”There is a better choice given that the group is fixed length” feedback_e=”Only of the above is best.”} mc_fixed
Which C++ structure is the best choice for a group of ordered data of a fixed length? :::
::: {.dragndrop feedback=”Feedback shows incorrect matches.” match_1=”Array|||{“What”, “am”, “I”, "am"}” match_2=”Set|||{“What”, “am”, “I”}” match_3=”String|||“What am I”” match_4=”Hash Table|||{{“What”, “am I”}}”} collect_data_types
Drag each data type to its' corresponding C++ initialization syntax. :::