Introduction

 

In this article, I am going to teach you about the dictionary data structure python. 

 

Creation

A dictionary is an ordered data structure in python. It holds key-value pairs. A dictionary is a python implementation of a “has a table”. You can access key-value pairs in constant time or O(1). 

 

You can declare a dictionary  like this

my_dic = {}

The {} tells the python interpreter that you are declaring a list 

my_dic = dict()

You can use the built-in python dict() function to declare a dictionary. 

#Syntax

my_dic = {key:value}

Let’s add some key-value pairs to your dictionary. 

my_dic = {1: "cat", 2: "rat", 3:"mat", 4: "sat"}

Each item in our dictionary has to have a key along with a value, and we separate the items by using a comma. 

Accessing An Item

Let's say we want to access the cat item from our dictionary, we would have to pass the key 1 in order to access the value cat. 

print(my_dic[1]) # output “cat”

We can access the other items in the same way 

print(my_dic[2]) # output "rat"

print(my_dic[3]) # output "mat"

The key does not have to be an integer it can be any immutable datatype such as a string, tuple, float, and int,  but you cannot use a list because it's a mutual datatype that can change.

Let's define a dictionary thats has keys with strings

person={"first_name": "Mark", "age":38, "job": "doctor"}

We can access  each value using the same syntax as before but now we pass in a string as key

print(person["first_name"]) # output "mark"
print(person["age"]) # output 38
print(person["job"]) # output doctor

Adding Items to Dictionary 

Let's say we want to add a last name to the person dictionary we created earlier. We would use the following syntax

Person["last_name"] = "Johnson"

If we print out the Person dictionary you will see last_name is added to the dictionary 

print(person) #output {"first_name": "Mark",
                      # "age":38, 
                       #"job": "doctor",
                       # Last_name: "Johnson"} 

Overwriting values

Let's say we want to change last_name from “Johnson” to “Smith” we would write

Person["last_name"] = "Smith"

print(person) #output {"first_name": "Mark",
                      # "age":38, 
                       #"job": "doctor",
                       # Last_name: "Smith"}

Removing Items From Dictionary

You can remove an element from a dictionary by using the .pop() method

If you call .pop on your person dictionary it will remove the last item in the dictionary

person={"first_name": "Mark", "age":38, "job": "doctor"}
person.pop() # remove last item 
#output {"first_name": "Mark", "age":38}

The .pop() method can also take in a key of the item you want to remove. 

#Syntax .pop(key)

person={"first_name": "Mark", "age":38}
person.pop("age") # remove last item 
#output {"first_name": "Mark"}

Iterating over a dictionary

The first we to iterate over a dictionary is

for x in person:
     print(x)

Second why to do this

for x in person.items():
        print(x) 

If we just want the keys from the dictionary

for x in person.keys():
        print(x) 

Or just want the values

for x in person.values():
        print(x) 

.key() method of dictionary returns a list of key so we can access a key by index. 

my_keys = person.keys()
print(my_keys[0]) # output first_name

You do the same for values as well. The .value() method also returns a list

my_values = person.values()
print(my_values[0]) # output mark

Check If Value Is In Dictionary 

If we want to check if a key exist in a dictionary we write the following code. 

person={"first_name": "Mark", "age":38} 

if "first_name" in person:
     print("yes")
 else 
    print("no") 

#output yes
 
if "middle_name" in person:
     print("yes")
 else 
    print("no") 

#output no