Python | Initializing dictionary with list index-values

While working with Python we might need to perform tasks in which we need to assign a dictionary with list values as dictionary values and index as dictionary keys. This type of problem is quite common in cases we need to perform data-type conversion. Let’s discuss certain ways in which this task can be performed. 

Method #1 : Using dictionary comprehension + len() This task can be performed using the combination of above functions in which we perform the construction of dictionary using the dictionary comprehension and indexing the limited using the len function. 

Python3

# Python3 code to demonstrate working of
# Initializing dictionary with list index-values
# Using dictionary comprehension + len()

# initializing list
test_list = ['Gfg', 'is', 'best']

# printing original list
print("The original list is : " + str(test_list))

# Initializing dictionary with list index-values
# Using dictionary comprehension + len()
res = {x : test_list[x] for x in range(len(test_list))}

# printing result
print("The dictionary indexed as list is : " + str(res))

Output

The original list is : ['Gfg', 'is', 'best']
The dictionary indexed as list is : {0: 'Gfg', 1: 'is', 2: 'best'}

Time complexity: O(n), where n is the length of the input list, as the code needs to traverse the entire input list to create the dictionary.
Auxiliary space: O(n), as the size of the output dictionary is proportional to the size of the input list.

Method #2 : Using dict() + enumerate() The combination of these methods can also be used to perform this task. In this we use the quality of enumerate function to get the indices and dict() is used to convert the list to dictionary. 

Python3

# Python3 code to demonstrate working of
# Initializing dictionary with list index-values
# Using dict() + enumerate()

# initializing list
test_list = ['Gfg', 'is', 'best']

# printing original list
print("The original list is : " + str(test_list))

# Initializing dictionary with list index-values
# Using dict() + enumerate()
res = dict(enumerate(test_list))

# printing result
print("The dictionary indexed as list is : " + str(res))

Output

The original list is : ['Gfg', 'is', 'best']
The dictionary indexed as list is : {0: 'Gfg', 1: 'is', 2: 'best'}

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. 

Method #3: Using zip() and dict()

This approach uses the zip() function to combine the list elements with their indices and the dict() function to convert the resulting tuples to a dictionary.

Python3

# Python3 code to demonstrate working of
# Initializing dictionary with list index-values
# Using zip() and dict()

# initializing list
test_list = ['Gfg', 'is', 'best']

# printing original list
print("The original list is : " + str(test_list))

# Initializing dictionary with list index-values
# Using zip() and dict()
res = dict(zip(range(len(test_list)), test_list))

# printing result
print("The dictionary indexed as list is :  " + str(res))
#This code is contributed by Edula Vinay Kumar Reddy

Output

The original list is : ['Gfg', 'is', 'best']
The dictionary indexed as list is :  {0: 'Gfg', 1: 'is', 2: 'best'}

Time complexity: O(n)
Auxiliary space: O(n) 

Method #4: Using a for loop

Step-by-Step Approach:

Initialize the list ‘test_list’ with some values.
Print the original list using print() function.
Initialize an empty dictionary ‘res’.
Use a for loop to iterate over the range of length of ‘test_list’.
Inside the for loop, use the index ‘i’ to access the elements of ‘test_list’ at index i and assign it to the corresponding key i in ‘res’.
Print the final dictionary using print() function.

Python3

# initializing list
test_list = ['Gfg', 'is', 'best']

# printing original list
print("The original list is : " + str(test_list))

# Initializing dictionary with list index-values
# Using for loop
res = {}
for i in range(len(test_list)):
    res[i] = test_list[i]

# printing result
print("The dictionary indexed as list is :  " + str(res))

Output

The original list is : ['Gfg', 'is', 'best']
The dictionary indexed as list is :  {0: 'Gfg', 1: 'is', 2: 'best'}

Time Complexity: O(n) – since we are iterating over the list only once.
Auxiliary Space: O(n) – since we are creating a new dictionary with n key-value pairs.