The University of Iowa
The College of Liberal Arts and Sciences
Department of Computer Science

Programming Languages and Tools:

CS:3210:0001

Lecture/Lab #21

Programming with C++

Associative containers: std::set, sets of objects, set vs map

Warm up

How would you use std::map to maintain a registry of students with unique emails?

std::set

https://repl.it/@agurtovoy/lab21-set

A collection of unique keys

auto unqiue_words( std::istream& in )
{
    std::set<std::string> result;
    std::string word;
    while ( in >> word )
        result.insert( word );
    return result;
}

  • std::set is an ordered associative container of unique elements, similar to Python's set / Java's TreeSet

  • std::set<K>  contains a sorted set of unique objects of type K

  • Given std::set<K> s; definition, s.insert(key) adds and s.erase(key) removes the key to the set, and s.count(key) checks if the key is present in the set

  • By default elements are ordered using operator <

std::set (cont.)

set<K> s = { k1, k2, ..., kn };

Initializes s with provided elements.

s.empty()

Returns true if s is empty.

s.insert( k )

Adds k to s if it's not already there.

s.size()

Returns number of elements in s.

s.count( k )

Returns the number of elements with key k.

for ( auto k : s ) ...

Iterates through s's elements in the order defined by s's comparison function (by default operator<).

s.clear()

Erases all elements from s.

s.erase( k )

Erases an element with the key k, if any.

auto iter = s.find( k );

Finds an element with key equivalent to k.

auto iter = s.find( x );

Finds an element with key equivalent to value x.

std::set vs Python's set

people = set([ "jack", "sape" ]) 

people.add( "guido" )
for k in people:
    print(k)

people.update(set([
    "bob", 
    "alice"
]))

people.discard( "sape" )
print( "sape" in people )

set<string> people{ "jack", "sape" };

people.insert( "guido" );
for ( auto k : people )
    std::cout << k << "\n";

people.insert({
    "bob", 
    "alice"
});

people.erase( "sape" );
std::cout << people.count( "sape" ) 
    << "\n";

Python

C++

Sets of objects

https://repl.it/@agurtovoy/lab21-set-of-objects

  • Object type should define operator<

  • Alternatively, we can provide a custom comparison as a second argument to std::set

  • To relax the comparison rules, use std::less<> or a custom comparison type with a nested is_transparent type alias.

set<student> vs. map<string,student>

https://repl.it/@agurtovoy/lab21-set-vs-map

  • Either one is idiomatic, the choice depends on the use case and personal preferences

Programming with C++, Spring 2020, Lecture #21

By Aleksey Gurtovoy

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Programming with C++, Spring 2020, Lecture #21

Associative containers: std::set, sets of objects, set vs map

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