Introduction to Sets & Maps

C++

C++ contains two versions of sets and maps; one using sorting and the other using hashing.

Java

Java contains two versions of sets and maps; one using sorting and the other using hashing.

Python

Python has built-in sets and maps, however they do not store elements in sorted order.

C++

Unordered Sets

In unordered sets, elements are stored in an arbitrary order through hashing. Insertions, deletions, and searches are all $\mathcal{O}(1)$ (with a high constant factor). Unordered sets are implemented by std:unordered_set in the <unordered_set> header.

Some operations on an std::unordered_set named s include:

• s.insert(x), which adds the element x to s if not already present.
• s.erase(x), which removes the element x from s if present.
• s.count(x), which returns 1 if s contains x and 0 if it doesn't.

Unordered sets work with primitive types, but require a custom hash function for structures/classes like vectors and pairs.

Copy
unordered_set<int> s;
s.insert(1);                 // {1}
s.insert(4);                 // {1, 4}
s.insert(2);                 // {1, 4, 2}
s.insert(1);                 // does nothing because 1's already in the set
cout << s.count(1) << endl;  // 1
s.erase(1);                  // {2, 4}
cout << s.count(5) << endl;  // 0
s.erase(0);                  // does nothing because 0 wasn't in the set

Sorted Sets

In sorted sets, the elements are sorted in order of element. Insertions, deletions, and searches are all $\mathcal{O}(\log N)$, where $N$ is the number of elements in the set. Sorted sets are implemented by std:set in the <set> header.

std::set includes all the essential operations that std::unordered_set has (including insertion, deletion, searches, etc.), but also some additional ones. Refer to the More Operations on Sorted Sets module for more detail.

You can iterate through a set in sorted order using a for-each loop.

Copy
set<int> s;
s.insert(1);  // [1]
s.insert(4);  // [1, 4]
s.insert(2);  // [1, 2, 4]

// Outputs 1, 2, and 4 on separate lines
for (int element : s) { cout << element << endl; }

Java

Unordered Sets

In unordered sets, elements are stored in an arbitrary order. Insertions, deletions, and searches are all $\mathcal{O}(1)$ (with a high constant factor). Unordered sets are implemented in Java by the HashSet class (which comes from the java.util library).

Some operations on a HashSet named set include:

• set.add(x), which adds the element x to set if not already present.
• set.remove(x), which removes the element x from set if present.
• set.contains(x), which checks whether set contains the element x.

Unordered sets work with primitive types and their object wrappers, but require a custom hash function for custom classes.

Copy
Set<Integer> set = new HashSet<>();
set.add(1);  // {1}
set.add(4);  // {1, 4}
set.add(2);  // {1, 4, 2}
set.add(1);  // {1, 4, 2}
// the add method did nothing because 1 was already in the set
System.out.println(set.contains(1));  // true
set.remove(1);                        // {4, 2}
System.out.println(set.contains(5));  // false
set.remove(0);  // does nothing because 0 wasn't in the set

Sorted Sets

In sorted sets, the elements are stored in order. Insertions, deletions, and searches are all $\mathcal{O}(\log N)$, where $N$ is the number of elements in the set. Sorted sets are implemented in Java by the TreeSet class.

TreeSet includes all the operations that HashSet has, but also includes some additional ones. Refer to the More Operations on Sorted Sets module for more detail.

You can iterate through a TreeSet in sorted order using a for-each loop.

Copy
Set<Integer> set = new TreeSet<>();
set.add(1);  // {1}
set.add(4);  // {1, 4}
set.add(2);  // {1, 2, 4}

// Outputs 1, 2, and 4 on separate lines
for (int element : set) { System.out.println(element); }

Python

Python's built-in set uses hashing to support $\mathcal{O}(1)$ insertion, deletion, and searches. Some operations on a Python set named s include:

• s.add(x): adds element x to s if not already present
• s.remove(x): removes an element x from set if present
• x in s: checks whether s contains the element x

Copy
s = set()
s.add(1)  # {1}
s.add(4)  # {1, 4}
s.add(2)  # {1, 4, 2}
s.add(1)  # {1, 4, 2}
# the add method did nothing because 1 was already in the set
print(1 in s)  # True
s.remove(1)  # {4, 2}
print(5 in s)  # False
s.remove(0)  # {4, 2}
# if the element to be removed does not exist, nothing happens

C++

Copy
#include <bits/stdc++.h>

using namespace std;

int main() {
	int n;
	cin >> n;

	set<int> distinctNumbers;
	for (int i = 0; i < n; i++) {

Java

Copy
// Source: Daniel

import java.io.*;
import java.util.*;

public class DistinctNumbers {
	public static void main(String[] args) throws IOException {
		Kattio io = new Kattio();
		int n = io.nextInt();
		Set<Integer> set = new HashSet<>();

Python

Copy
n = int(input())  # unused
nums = [int(x) for x in input().split()]
distinct_nums = set(nums)
print(len(distinct_nums))

We can do this more efficiently by skipping the creation of the list, and use a set comprehension directly:

Copy
n = int(input())  # unused
distinct_nums = {int(x) for x in input().split()}
print(len(distinct_nums))

Copy
n = int(input())  # unused
distinct_nums = set(input().split())
print(len(distinct_nums))

C++

In sorted maps, the pairs are sorted in order of key. Insertions, deletions, and searches are all $\mathcal{O}(\log N)$, where $N$ is the number of pairs in the map. In unordered maps, the pairs aren't kept in sorted order and all insertions, deletions, and searches are all $\mathcal{O}(1)$. Sorted maps are implemented with std::map and unordered maps are implemented with std::unordered_map.

Some operations on an std::map and std::unordered_map named m include:

• m[key], which returns a reference to the value associated with the key key.

  • If key is not present in the map, then the value associated with key is constructed using the default constructor of the value type. For example, if the value type is int, then calling m[key] for a key not within the map sets the value associated with that key to 0. As another example, if the value type is std::string, then calling m[key] for a key not within the map sets the value associated with that key to the empty string. More discussion regarding what happens in this case can be found here.
  • Alternatively, m.at(key) behaves the same as m[key] if key is contained within m but throws an exception otherwise.
  • m[key] = value will assign the value value to the key key.
• m.count(key), which returns the number of times the key is in the map (either one or zero), and therefore checks whether a key exists in the map.
• m.erase(key), which removes the map entry associated with the specified key if the key was present in the map.

Copy
map<int, int> m;
m[1] = 5;                    // [(1, 5)]
m[3] = 14;                   // [(1, 5); (3, 14)]
m[2] = 7;                    // [(1, 5); (2, 7); (3, 14)]
m[0] = -1;                   // [(0, -1); (1, 5); (2, 7); (3, 14)]
m.erase(2);                  // [(0, -1); (1, 5); (3, 14)]
cout << m[1] << endl;        // 5
cout << m.count(7) << endl;  // 0
cout << m.count(1) << endl;  // 1
cout << m[2] << endl;        // 0

Java

In sorted maps, the pairs are sorted in order of key. Insertions, deletions, and searches are all $\mathcal{O}(\log N)$, where $N$ is the number of pairs in the map.

In unordered maps, the pairs aren't kept in sorted order and all insertions, deletions, and searches are all $\mathcal{O}(1)$. Sorted maps are implemented with TreeMap and unordered maps are implemented with HashMap.

In both TreeMap and HashMap, the put(key, value) method assigns a value to a key and places the key and value pair into the map. The get(key) method returns the value associated with the key. The containsKey(key) method checks whether a key exists in the map. Lastly, remove(key) removes the map entry associated with the specified key.

Copy
Map<Integer, Integer> map = new TreeMap<Integer, Integer>();
map.put(1, 5);                           // [(1, 5)]
map.put(3, 14);                          // [(1, 5); (3, 14)]
map.put(2, 7);                           // [(1, 5); (2, 7); (3, 14)]
map.remove(2);                           // [(1, 5); (3, 14)]
System.out.println(map.get(1));          // 5
System.out.println(map.containsKey(7));  // false
System.out.println(map.containsKey(1));  // true

Python

Colloquially, maps are referred to as dicts in python. They act as hash maps, so they have $\mathcal{O}(1)$ insertion, deletion, and searches.

Copy
d = {}
d[1] = 5  # {1: 5}
d[3] = 14  # {1: 5, 3: 14}
d[2] = 7  # {1: 5, 2: 7, 3: 14}
del d[2]  # {1: 5, 3: 14}
print(d[1])  # 5
print(7 in d)  # False
print(1 in d)  # True

C++

An std::map stores entries as pairs in the form {key, value}. To iterate over maps, you can use a for loop. The auto keyword suffices to iterate over any type of pair (here, auto substitutes for pair<int, int>).

Copy
// Both of these output the same thing
for (const auto &x : m) { cout << x.first << " " << x.second << endl; }
for (auto x : m) { cout << x.first << " " << x.second << endl; }

The first method (iterating over const references) is generally preferred over the second because the second will make a copy of each element that it iterates over. Additionally, you can pass by reference when iterating over a map, allowing you to modify the values (but not the keys) of the pairs stored in the map:

Copy
for (auto &x : m) {
	x.second = 1234;  // Change all values to 1234
}

Java

To iterate over maps, you can use a for-each loop over the keys:

Copy
for (int k : m.keySet()) { System.out.println(k + " " + m.get(k)); }

You can also use a for-each loop over the entries:

Copy
for (Map.Entry entry : m.entrySet()) {
	System.out.println(entry.getKey() + " " + entry.getValue());
}

It's also possible to change the values while iterating over the keys (or over the values themselves, if they're mutable):

Copy
for (int k : m.keySet()) {
	m.put(k, 1234);  // Change all values to 1234
}

Python

To iterate over dicts, there are three options, all of which involve for loops. Dicts will be returned in the same order of insertion in Python 3.6+. You can iterate over the keys:

Copy
for key in d:
	print(key)

Over the values:

Copy
for value in d.values():
	print(value)

And even over key-value pairs:

Copy
for key, value in d.items():
	print(key, value)

It's also possible to change the values while iterating over the keys (or over the values themselves, if they're mutable):

Copy
for key in d:
	d[key] = 1234  # Change all values to 1234

Python

For example, the following code attempts to remove every entry from a map, but results in a runtime error.

Copy
d = {i: i for i in range(10)}

for i in d:
	del d[i]

Traceback (most recent call last):
  File "test.py", line 3, in <module>
    for i in d:
RuntimeError: dictionary changed size during iteration

One way is to get around this is to create a new map.

Copy
d = {i: i for i in range(10)}

# only includes every third element
d_new = dict(item for i, item in enumerate(d.items()) if i % 3 == 0)

print("new dict:", d_new)  # new dict: {0: 0, 3: 3, 6: 6, 9: 9}

Another is to maintain a list of all the keys you want to remove and remove them after the iteration finishes:

Copy
d = {i: i for i in range(10)}

# removes every third element
to_remove = {key for i, key in enumerate(d) if i % 3 == 0}

for key in to_remove:
	del d[key]

print("new dict:", d)  # new dict: {1: 1, 2: 2, 4: 4, 5: 5, 7: 7, 8: 8}

C++

For example, the following code attempts to remove every entry from a map, but results in a segmentation fault.

Copy
map<int, int> m;

for (int i = 0; i < 10; ++i) m[i] = i;

for (auto &it : m) {
	cout << "Current Key: " << it.first << endl;

	m.erase(it.first);
}

The reason is due to "iterators, pointers and references referring to elements removed by the function [being] invalidated" (as stated in the documentation for erase), though iterators are beyond the scope of this module.

One way to get around this is to just create a new map instead of removing from the old one.

Copy
map<int, int> m, M;

for (int i = 0; i < 10; ++i) m[i] = i;

int current_iteration = 0;

for (const auto &it : m) {
	// only includes every third element
	if (current_iteration % 3 == 0) { M[it.first] = it.second; }

Another is to maintain a list of all the keys you want to erase and erase them after the iteration finishes.

Copy
map<int, int> m;
for (int i = 0; i < 10; ++i) { m[i] = i; }

vector<int> to_erase;
int current_iteration = 0;

for (const auto &it : m) {
	// removes every third element
	if (current_iteration % 3 == 0) { to_erase.push_back(it.first); }

Java

Modifying a Collection (Set, Map, etc.) in the middle of a for-each loop will cause a ConcurrentModificationException. See the following snippet for an example:

Copy
Map<Integer, Integer> m = new TreeMap<>();
// m starts as {0: 0, 1: 1, 2: 2}
m.put(0, 0);
m.put(1, 1);
m.put(2, 2);

for (int key : m.keySet()) {
	m.remove(key);  // ConcurrentModificationException thrown!!
}

One work-around is to use Iterator and the .remove() method to remove elements while looping over them, like in the next code snippet:

Copy
Map<Integer, Integer> m = new TreeMap<>();
// m starts as {0: 0, 1: 1, 2: 2}
m.put(0, 0);
m.put(1, 1);
m.put(2, 2);

Iterator<Map.Entry<Integer, Integer>> iter = m.entrySet().iterator();
while (iter.hasNext()) {
	int key = iter.next().getKey();
	if (key == 0 || key == 2) { iter.remove(); }

However, Iterator is outside the scope of this module.

The easiest option (in most cases) if you want to remove/insert mutiple entries at once is to use your Container's .addAll(c) or .removeAll(c) methods. That means that you should put all the elements you want to remove (or add) in a new Collection, and then use that new Collection as the parameter of the .addAll(c) or .removeAll(c) method that you call on your original Collection. See the following code snippet for an example (it works equivalently to the code above):

Copy
Map<Integer, Integer> m = new TreeMap<>();
// m starts as {0: 0, 1: 1, 2: 2}
m.put(0, 0);
m.put(1, 1);
m.put(2, 2);

Set<Integer> keysToRemove = new TreeSet<>();
for (Map.Entry<Integer, Integer> entry : m.entrySet()) {
	int key = entry.getKey();
	if (key == 0 || key == 2) { keysToRemove.add(key); }

C++

Java

Python