hashmap - Hash Map in Python

Question

I want to implement a HashMap in Python. I want to ask a user for an input. depending on his input I am retrieving some information from the HashMap. If the user enters a key of the HashMap, I would like to retrieve the corresponding value.

How do I implement this functionality in Python?

HashMap<String,String> streetno=new HashMap<String,String>();
   streetno.put("1", "Sachin Tendulkar");
   streetno.put("2", "Dravid");
   streetno.put("3","Sehwag");
   streetno.put("4","Laxman");
   streetno.put("5","Kohli")

Answer 1

Python dictionary is a built-in type that supports key-value pairs.

streetno = {"1": "Sachin Tendulkar", "2": "Dravid", "3": "Sehwag", "4": "Laxman", "5": "Kohli"}

as well as using the dict keyword:

streetno = dict({"1": "Sachin Tendulkar", "2": "Dravid"}) 

or:

streetno = {}
streetno["1"] = "Sachin Tendulkar" 

Answer 2

All you wanted (at the time the question was originally asked) was a hint. Here's a hint: In Python, you can use dictionaries.

Answer 3

It's built-in for Python. See dictionaries.

Based on your example:

streetno = {"1": "Sachine Tendulkar",
            "2": "Dravid",
            "3": "Sehwag",
            "4": "Laxman",
            "5": "Kohli" }

You could then access it like so:

sachine = streetno["1"]

Also worth mentioning: it can use any non-mutable data type as a key. That is, it can use a tuple, boolean, or string as a key.

Answer 4

streetno = { 1 : "Sachin Tendulkar",
            2 : "Dravid",
            3 : "Sehwag",
            4 : "Laxman",
            5 : "Kohli" }

And to retrieve values:

name = streetno.get(3, "default value")

Or

name = streetno[3]

That's using number as keys, put quotes around the numbers to use strings as keys.

Answer 5

Hash maps are built-in in Python, they're called dictionaries:

streetno = {}                        #create a dictionary called streetno
streetno["1"] = "Sachin Tendulkar"   #assign value to key "1"

Usage:

"1" in streetno                      #check if key "1" is in streetno
streetno["1"]                        #get the value from key "1"

See the documentation for more information, e.g. built-in methods and so on. They're great, and very common in Python programs (unsurprisingly).

Answer 6

Here is the implementation of the Hash Map using python For the simplicity hash map is of a fixed size 16. This can be changed easily. Rehashing is out of scope of this code.

class Node:
    def __init__(self, key, value):
        self.key = key
        self.value = value
        self.next = None

class HashMap:
    def __init__(self):
        self.store = [None for _ in range(16)]
    def get(self, key):
        index = hash(key) & 15
        if self.store[index] is None:
            return None
        n = self.store[index]
        while True:
            if n.key == key:
                return n.value
            else:
                if n.next:
                    n = n.next
                else:
                    return None
    def put(self, key, value):
        nd = Node(key, value)
        index = hash(key) & 15
        n = self.store[index]
        if n is None:
            self.store[index] = nd
        else:
            if n.key == key:
                n.value = value
            else:
                while n.next:
                    if n.key == key:
                        n.value = value
                        return
                    else:
                        n = n.next
                n.next = nd

hm = HashMap()
hm.put("1", "sachin")
hm.put("2", "sehwag")
hm.put("3", "ganguly")
hm.put("4", "srinath")
hm.put("5", "kumble")
hm.put("6", "dhoni")
hm.put("7", "kohli")
hm.put("8", "pandya")
hm.put("9", "rohit")
hm.put("10", "dhawan")
hm.put("11", "shastri")
hm.put("12", "manjarekar")
hm.put("13", "gupta")
hm.put("14", "agarkar")
hm.put("15", "nehra")
hm.put("16", "gawaskar")
hm.put("17", "vengsarkar")
print(hm.get("1"))
print(hm.get("2"))
print(hm.get("3"))
print(hm.get("4"))
print(hm.get("5"))
print(hm.get("6"))
print(hm.get("7"))
print(hm.get("8"))
print(hm.get("9"))
print(hm.get("10"))
print(hm.get("11"))
print(hm.get("12"))
print(hm.get("13"))
print(hm.get("14"))
print(hm.get("15"))
print(hm.get("16"))
print(hm.get("17"))

Output:

sachin
sehwag
ganguly
srinath
kumble
dhoni
kohli
pandya
rohit
dhawan
shastri
manjarekar
gupta
agarkar
nehra
gawaskar
vengsarkar

Answer 7

class HashMap:
    def __init__(self):
        self.size = 64
        self.map = [None] * self.size

    def _get_hash(self, key):
        hash = 0

        for char in str(key):
            hash += ord(char)
        return hash % self.size

    def add(self, key, value):
        key_hash = self._get_hash(key)
        key_value = [key, value]

        if self.map[key_hash] is None:
            self.map[key_hash] = list([key_value])
            return True
        else:
            for pair in self.map[key_hash]:
                if pair[0] == key:
                    pair[1] = value
                    return True
                else:
                    self.map[key_hash].append(list([key_value]))
                    return True

    def get(self, key):
        key_hash = self._get_hash(key)
        if self.map[key_hash] is not None:
            for pair in self.map[key_hash]: 
                if pair[0] == key:
                    return pair[1]
        return None

    def delete(self, key):
        key_hash = self._get_hash(key)

        if self.map[key_hash] is None :
            return False
        for i in range(0, len(self.map[key_hash])):
            if self.map[key_hash][i][0] == key:
                self.map[key_hash].pop(i)
                return True

    def print(self):

        print('---Phonebook---')
        for item in self.map:
            if item is not None:
                print(str(item))

h = HashMap()

Answer 8

Python Counter is also a good option in this case:

from collections import Counter

counter = Counter(["Sachin Tendulkar", "Sachin Tendulkar", "other things"])

print(counter)

This returns a dict with the count of each element in the list:

Counter({'Sachin Tendulkar': 2, 'other things': 1})

Answer 9

In python you would use a dictionary.

It is a very important type in python and often used.

You can create one easily by

name = {}

Dictionaries have many methods:

# add entries:
>>> name['first'] = 'John'
>>> name['second'] = 'Doe'
>>> name
{'first': 'John', 'second': 'Doe'}

# you can store all objects and datatypes as value in a dictionary
# as key you can use all objects and datatypes that are hashable
>>> name['list'] = ['list', 'inside', 'dict']
>>> name[1] = 1
>>> name
{'first': 'John', 'second': 'Doe', 1: 1, 'list': ['list', 'inside', 'dict']}

You can not influence the order of a dict.

Answer 10

A dictionary in Python is the best way to implement this. We can create the following dictionary using the given <key,value> pairs:

d = {"1": "Sachin Tendulkar", "2": "Dravid", "3": "Sehwag", "4": "Laxman", "5": "Kohli"}

To extract the value of a particular key, we can directly use d[key]:

name = d["1"] # The value of name would be "Sachin Tendulkar" here