Hash Tables

Can we implement a Dictionary such that all operations: insertion, deletion, and searching, are O(1)?

Yes. Use a hash table.

Hash tables are fundamental data structures. They’re also analogous to the Dictionary ADT, aka associative arrays. Most dictionaries are implemented with a hash table. You really should learn them!

I assume you already know about the Dictionary ADT and hashing.



# Naive (and Wrong) Implementation
# but it communicates the concept
class HashTable:

  def __init__(self):
    self.array = []
  def set(self, key, value):
    index = hash(key) % len(self.array)
    self.array[index] = value

  def get(self, key):
    index = hash(key) % len(self.array)
    return self.array[index]

# but...

Two Fundamental Problems:

  1. collisions: “Someone’s already in here!”
  2. array sizing: “There’s no more space!”

Collision Resolution:

Array Resizing:


You may see some hash tables or maps described as weak. This means that hash table uses a weak reference for its values. Once there are no strong (regular) references to the value the garbage collector is free to eliminate it.

Implementations differ:


Python’s dict:

# Example Python's dict
secrets = {}
secrets["You"] = "idk"
secrets["HGPA"] = "doesn't like watermelon"
secrets["Tony"] = "has a WWF belt"
secrets["Tyler"] = "listen's to Bjork"
del secrets["You"]
for k in secrets:
  print("{} {}").format(k, secrets[k])
for k,v in d.iteritems(): # Python 2
  print("{} {}").format(k, v)



C++’s unordered_map:

#include <iostream>
#include <unordered_map>
using namespace std;
int main() {
  unordered_map<string, string> secrets;
  secrets["You"] = "idk";
  secrets["HGPA"] = "doesn't like watermelon";
  secrets["Tony"] = "has a WWF belt";
  secrets["Tyler"] = "listen's to Bjork";
  delete secrets["You"];
  unordered_map<string,string>::iterator i;
  // actually iterator across pair<string, string>
  for (i = secrets.begin(); i != secrets.end(); i++) {
    cout << i->first << "  " << i->second << endl;


As usual Java has various implementations.

Java’s HashMap<K,V>:

// Example Java's HashMap
import java.util.HashMap;
import java.util.Map;
import java.util.Iterator;
import java.util.Set;
public class Example {
  public static void main(String args[]) {
    HashMap<String, String> secrets = new HashMap<String, String>();
    // setting
    secrets.put("You", "idk");
    secrets.put("HGPA", "doesn't like watermelon");
    secrets.put("Tony", "has a WWF belt");
    secrets.put("Tyler", "listen's to Bjork");
    // getting
    // removing
    // traverse -- also example why I don't like java
    Set set = secrets.entrySet();
    Iterator iterator = set.iterator();
    while (iterator.hasNext()) {
       Map.Entry entry = (Map.Entry) iterator.next();
       System.out.print(entry.getKey() + " " + entry.getValue());


Hash table. Wikipedia.

The Mighty Dictionary. Brandon Craig Rhodes. PyCon 2010. An optimized explanation of Python’s Dictionary implementation using a hash table.

How std::unordered_map is implemented?. Stack Overflow.


A Proposal to Add Hash Tables to the Standard Library (revision 4). Matthew Austern. 2003-04-03.

How does a HashMap work in JAVA.

C++ Tutorial: Intro to Hash Tables.

Hash Tables (C). Eternally Confuzzled.