Python – Convert Index Dictionary to List

Sometimes, while working with Python dictionaries, we can have a problem in which we have keys mapped with values, where keys represent list index where value has to be placed. This kind of problem can have application in all data domains such as web development. Let’s discuss certain ways in which this task can be performed.

Input : test_dict = { 1 : ‘Gfg’, 3 : ‘is’, 5 : ‘Best’ } 
Output : [0, ‘Gfg’, 0, ‘is’, 0, ‘Best’, 0, 0, 0, 0, 0] 

Input : test_dict = { 2 : ‘Gfg’, 6 : ‘Best’ } 
Output : [0, 0, ‘Gfg’, 0, 0, 0, ‘Best’, 0, 0, 0, 0]

Method #1: Using list comprehension + keys() The combination of above functions can be used to solve this problem. In this, we perform the task of assigning values to list checking for index, by extracting keys of dictionary for lookup. 

Python3

# Python3 code to demonstrate working of
# Convert Index Dictionary to List
# Using list comprehension + keys()

# initializing dictionary
test_dict = { 2 : 'Gfg', 4 : 'is', 6 : 'Best' }

# printing original dictionary
print("The original dictionary is : " + str(test_dict))

# Convert Index Dictionary to List
# Using list comprehension + keys()
res = [test_dict[key] if key in test_dict.keys() else 0 for key in range(10)]

# printing result
print("The converted list : " + str(res))

Output : 

The original dictionary is : {2: 'Gfg', 4: 'is', 6: 'Best'}
The converted list : [0, 0, 'Gfg', 0, 'is', 0, 'Best', 0, 0, 0]

Time Complexity: O(n), where n is the length of the list test_dict
Auxiliary Space: O(n) additional space of size n is created where n is the number of elements in the res list

Method #2: Using list comprehension + get() The combination of above functions can be used to solve this problem. In this, we extract element using get(), so as to assign the default values harnessing default value initializing property of get(). 

Python3

# Python3 code to demonstrate working of
# Convert Index Dictionary to List
# Using list comprehension + get()

# initializing dictionary
test_dict = { 2 : 'Gfg', 4 : 'is', 6 : 'Best' }

# printing original dictionary
print("The original dictionary is : " + str(test_dict))

# Convert Index Dictionary to List
# Using list comprehension + get()
res = [test_dict.get(ele, 0) for ele in range(10)]

# printing result
print("The converted list : " + str(res))

Output : 

The original dictionary is : {2: 'Gfg', 4: 'is', 6: 'Best'}
The converted list : [0, 0, 'Gfg', 0, 'is', 0, 'Best', 0, 0, 0]

Method #3 : Using * operator and insert() method

• Create a list of 0’s
• Later on access dictionary and insert  value at key index in list
• Display list

Python3

# Python3 code to demonstrate working of
# Convert Index Dictionary to List


# initializing dictionary
test_dict = {2: 'Gfg', 4: 'is', 6: 'Best'}

# printing original dictionary
print("The original dictionary is : " + str(test_dict))

# Convert Index Dictionary to List
x = [0]*10
for i in list(test_dict.keys()):
    x.insert(i, test_dict[i])
# printing result
print("The converted list : " + str(x))

Output

The original dictionary is : {2: 'Gfg', 4: 'is', 6: 'Best'}
The converted list : [0, 0, 'Gfg', 0, 'is', 0, 'Best', 0, 0, 0, 0, 0, 0]

Time Complexity : O(N)
Auxiliary Space : O(N)