Python – String Repetition and spacing in List

Sometimes while working with Python, we can have a problem in which we need to perform the repetition of each string in list and also attach a deliminator to each occurrence. This kind of problem can occur in day-day programming. Lets discuss certain ways in which this task can be performed.

Method #1 : Using loop
This task can be performed in brute force way using loop. In this, we iterate the list and perform string addition and multiplication while iteration using suitable operators.

Python3

# Python3 code to demonstrate working of
# String Repetition and spacing in List
# Using loop

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

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

# initializing delim
delim = '-'

# initializing K
K = 3

# String Repetition and spacing in List
# Using loop
res = []
for sub in test_list:
    res.append((sub + delim) * K)

# printing result
print("List after performing operations : " + str(res))

Output : 

The original list is : ['gfg', 'is', 'best']
List after performing operations : ['gfg-gfg-gfg-', 'is-is-is-', 'best-best-best-']

Time complexity: O(n), where n is the length of the input list, because we loop through each element of the list once.
Auxiliary space: O(n), because we create a new list of length n to store the results.

Method #2 : Using join() + list comprehension
The combination of above functionalities can also be used to perform this task. In this, we perform the task of attaching delim using join() and list comprehension performs the task of repetition. Avoids trailing delim.

Python3

# Python3 code to demonstrate working of
# String Repetition and spacing in List
# Using join() + list comprehension

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

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

# initializing delim
delim = '-'

# initializing K
K = 3

# String Repetition and spacing in List
# Using join() + list comprehension
res = []
for sub in test_list:
    res.append(delim.join([sub for _ in range(K)]))

# printing result
print("List after performing operations : " + str(res))

Output : 

The original list is : ['gfg', 'is', 'best']
List after performing operations : ['gfg-gfg-gfg', 'is-is-is', 'best-best-best']

Time complexity: O(n*K), where n is the length of the input list and K is the repetition factor.
Auxiliary space: O(n*K), as we are creating a new list with K repetitions of each element in the input list.

Method #3: Using itertools.repeat() 

Here’s an example of using itertools.repeat() to repeat each string in the list test_list and attach a delimiter to each occurrence:

Step-by-step approach:

• Initializes an empty list res.
• Iterates over each element in the test_list using a for loop.
• For each element, it creates a list with K number of repetitions of the element using the itertools.repeat() function and then joins them with the delim separator using the join() method of strings. The resulting string is then appended to the res list.
• Prints the resulting list res using the print() function and string concatenation.

Below is the implementation of the above approach:

Python3

import itertools

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

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

# initializing delim
delim = '-'

# initializing K
K = 3

# String Repetition and spacing in List
# Using itertools.repeat()
res = []
for sub in test_list:
    res.append(delim.join(list(itertools.repeat(sub, K))))

# printing result
print("List after performing operations : " + str(res))

#This code is contributed by Edula Vinay Kumar Reddy

Output

The original list is : ['gfg', 'is', 'best']
List after performing operations : ['gfg-gfg-gfg', 'is-is-is', 'best-best-best']

Time complexity: O(n * K) where n is the length of the list test_list and K is the number of repetitions.
Auxiliary Space: O(n * K) as we are creating a list of length n * K to store the results.

Method 4: Using map() and lambda functions.

Step-by-step approach:

• Initialize the list of strings to be repeated.
• Initialize the delimiter to be used for the repetition of strings.
• Initialize the value of K, the number of times a string should be repeated.
• Create a lambda function that takes a string as input and returns the string repeated K times, separated by the delimiter.
• Use map() function to apply the lambda function to each string in the input list.
• Convert the output of map() function to a list.
• Print the final list after performing the operations.

Below is the implementation of the above approach:

Python3

# Python3 code to demonstrate working of
# String Repetition and spacing in List
# Using map() and lambda functions

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

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

# initializing delim
delim = '-'

# initializing K
K = 3

# String Repetition and spacing in List
# Using map() and lambda functions
res = list(map(lambda x: delim.join([x]*K), test_list))

# printing result
print("List after performing operations : " + str(res))

Output

The original list is : ['gfg', 'is', 'best']
List after performing operations : ['gfg-gfg-gfg', 'is-is-is', 'best-best-best']

Time complexity: O(n), where n is the length of the input list.
Auxiliary space: O(n) as we are storing the output in a list.

Method 5: Using numpy.tile() and numpy.ravel()

Step-by-step approach:

1. Import the numpy module.
2. Initialize the list test_list.
3. Print the original list.
4. Initialize the delimiter delim and the number of repetitions K.
5. Use a list comprehension to create a new list res with the repeated and spaced strings. The expression inside the brackets uses numpy.tile() to repeat each string in test_list K times vertically and 1 time horizontally, and numpy.ravel() to flatten the resulting 2D array into a 1D array. The join() method is then used to concatenate the resulting array of strings with the delimiter delim.
6. Print the resulting list.

Python3

import numpy as np

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

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

# initializing delim
delim = '-'

# initializing K
K = 3

# String Repetition and spacing in List
# Using numpy.tile() and numpy.ravel()
res = [delim.join(np.ravel(np.tile([x], (K, 1)))) for x in test_list]

# printing result
print("List after performing operations : " + str(res))

OUTPUT:

The original list is : ['gfg', 'is', 'best']
List after performing operations : ['gfg-gfg-gfg', 'is-is-is', 'best-best-best']

Time complexity: The time complexity of this method is O(nk), where n is the length of the input list test_list and k is the number of repetitions K, since the operation of repeating each string in test_list K times has a time complexity of O(k|s|), where |s| is the length of the string.

Auxiliary space: The auxiliary space used by this method is O(nk), since a new list of size n*k is created.

Method 6: Using list multiplication and string concatenation

Step-by-step approach:

• Define the delimiter string delim and the integer K representing the number of times each string should be repeated.
• Create a new list res using a list comprehension that multiplies each string in test_list by K, concatenates K-1 instances of the delimiter delim between them using the string multiplication operator *, and finally appends the resulting string to res.
• Print the result using the print() function and passing the string representation of res.

Below is the implementation of the above approach:

Python3

# Method 6: Using list multiplication and string concatenation

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

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

# initializing delim
delim = '-'

# initializing K
K = 3

# String Repetition and spacing in List
# Using list multiplication and string concatenation
res = [delim.join([s] * K) for s in test_list]

# printing result
print("List after performing operations : " + str(res))

Output

The original list is : ['gfg', 'is', 'best']
List after performing operations : ['gfg-gfg-gfg', 'is-is-is', 'best-best-best']

 Time complexity: O(N * K), where N is the length of test_list.
Auxiliary space: O(N * K), where N is the length of test_list. This is because a new list of length N * K is created to store the result.