Python program to Convert a list into a dictionary with index as key

Given a python list, the task is to write a Python program to convert the list into a dictionary with indexes as keys.

Example:

Input – [‘Ramesh’, ‘Mahesh’, ‘Kamlesh’, ‘Suresh’]
Output – {0: ‘Ramesh’, 1: ‘Mahesh’, 2: ‘Kamlesh’, 3: ‘Suresh’}

Input – [‘Carrot’, ‘Raddish’, ‘Brinjal’, ‘Potato’]
Output – {0: ‘Carrot’, 1: ‘Raddish’, 2: ‘Brinjal’, 3: ‘Potato’}

Method 1: Using Dictionary Comprehensions

In this method without using any extra function, we will convert a list into a dictionary.

Output:

{0: 'Carrot', 1: 'Raddish', 2: 'Brinjal', 3: 'Potato'}

Method 2: Using enumerates and a for loop.

In this method, we will enumerate the provided list and then use the index as a key and assign it to the corresponding value.

Python3

employee_names = ['Ramesh', 'Mahesh', 'Kamlesh', 'Suresh']

names_dict = {}
for i, name in enumerate(employee_names):
  names_dict[i] = name

print(names_dict)

Output

{0: 'Ramesh', 1: 'Mahesh', 2: 'Kamlesh', 3: 'Suresh'}

Time complexity: O(n), where n is the number of elements in the employee_names list. 
Auxiliary space: O(n), as the names_dict dictionary has to store one key-value pair for each element in the employee_names.

The above task can be done without using the for loop as shown below:

Python3

vegetables = ['Carrot', 'Raddish', 'Brinjal', 'Potato']

veg_dict = dict(enumerate(vegetables))
print(veg_dict)

Output

{0: 'Carrot', 1: 'Raddish', 2: 'Brinjal', 3: 'Potato'}

Method 3: Using the zip and range function

In this method, basically, we use lists of the values and the list of indexes that are created by using the range() function.

Python3

vegetables = ['Carrot', 'Raddish', 'Brinjal', 'Potato']

veg_dict = dict(zip(range(len(vegetables)), vegetables))
print(veg_dict)

Output

{0: 'Carrot', 1: 'Raddish', 2: 'Brinjal', 3: 'Potato'}

Time complexity: O(n), where n is the length of the list ‘vegetables’.
Auxiliary space: O(n), as the dictionary ‘veg_dict’ created will have n key-value pairs, where n is the length of the list ‘vegetables’.

Method 4: Use yield

Another approach that could be used to convert a list into a dictionary with indexes as keys is to use the dict function and pass it a generator function that generates the tuples. This can be done using a generator function and the yield function.

Python3

def index_tuple_generator(lst):
    for i, x in enumerate(lst):
        yield (i, x)

lst = ['Ramesh', 'Mahesh', 'Kamlesh', 'Suresh']

dct = dict(index_tuple_generator(lst))
print(dct)
#This code is contributed by Edula Vinay Kumar Reddy

Output

{0: 'Ramesh', 1: 'Mahesh', 2: 'Kamlesh', 3: 'Suresh'}

Time complexity: O(n) because it involves a single loop that iterates through all the elements in the input list.
Auxiliary Space: O(1) because it does not use any additional data structures and the space it uses is independent of the input size. The yield statement is used to generate the tuples one at a time, so the function does not need to store all the tuples in memory at once.

Method 5: Using list comprehension and the enumerate function.

Use a list comprehension and the built-in enumerate function to generate a list of tuples containing the index and element of each item in the input list. Then, the built-in dict function is used to convert the list of tuples into a dictionary.

Follow the below steps to implement the above idea:

• Use a list comprehension and the enumerate function to create a list of tuples, where each tuple contains the index and element of each item in the input list. The syntax for the list comprehension is [(i, x) for i, x in enumerate(lst)].
• Use the built-in dict function to convert the list of tuples into a dictionary.
• Return the dictionary.

Below is the implementation of the above approach:

Python3

def index_tuple_generator(lst):
    return dict([(i, x) for i, x in enumerate(lst)])

lst = ['Ramesh', 'Mahesh', 'Kamlesh', 'Suresh']
dct = index_tuple_generator(lst)
print(dct)

Output

{0: 'Ramesh', 1: 'Mahesh', 2: 'Kamlesh', 3: 'Suresh'}

Time Complexity: O(n), where n is the length of the input list.
Auxiliary Space: O(n), where n is the length of the input list.