Python – Concatenate Strings in the Given Order

Given a String List and order list, perform string concatenation in a specific order.

Input : test_list = [“best”, “Gfg”, “for”, “is”], sort_order = [1, 3, 0, 2] 
Output : Gfgisbestfor 
Explanation : Combined as per order of indices.

Input : test_list = [“best”, “Gfg”], sort_order = [1, 0] 
Output : Gfgbest 
Explanation : Combined as per order of indices. 

Method #1: Using loop

In this, we iterate order elements in the loop and perform concatenation of strings of the similar index in similar order.

Python3

# Python3 code to demonstrate working of
# Concatenate Strings in Order
# Using loop

# initializing list
test_list = ["best", "Gfg", "for", "is", "geeks"]

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

# initializing join order
sort_order = [1, 3, 0, 2, 4]

res = ''
for order in sort_order:

    # concatenating by order
    res += test_list[order]

# printing result
print("Ordered concatenation : " + str(res))

Output

The original list is : ['best', 'Gfg', 'for', 'is', 'geeks']
Ordered concatenation : Gfgisbestforgeeks

Time Complexity: O(n) where n is the number of elements in the list “test_list”. 
Auxiliary Space: O(1) additional space is not needed

Method #2: Using join() + list comprehension

In this, we perform the task of concatenation using join(), list comprehension is used for iteration of order.

Python3

# Python3 code to demonstrate working of
# Concatenate Strings in Order
# Using join() + list comprehension

# initializing list
test_list = ["best", "Gfg", "for", "is", "geeks"]

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

# initializing join order
sort_order = [1, 3, 0, 2, 4]

# join() performs concatenation
res = ''.join([test_list[order] for order in sort_order])

# printing result
print("Ordered concatenation : " + str(res))

Output

The original list is : ['best', 'Gfg', 'for', 'is', 'geeks']
Ordered concatenation : Gfgisbestforgeeks

The Time and Space Complexity for all the methods are the same:

Time Complexity: O(n)
Space Complexity: O(n)

Method #3: Using map()

Here’s a different approach, which leverages the python map() function:

Python3

# Python3 code to demonstrate working of
# Concatenate Strings in Order
# Using map()

# initializing list
test_list = ["best", "Gfg", "for", "is", "geeks"]

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

# initializing join order
sort_order = [1, 3, 0, 2, 4]

# using map() to get the concatenated string
res = ''.join(map(lambda x: test_list[x], sort_order))

# printing result
print("Ordered concatenation : " + str(res))

Output

The original list is : ['best', 'Gfg', 'for', 'is', 'geeks']
Ordered concatenation : Gfgisbestforgeeks

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

The code leverages the map() function to iterate over the sort_order list, and using a lambda function, it retrieves the corresponding string from the test_list. The join() function is then used to concatenate the retrieved strings into a single string.