Python – Group contiguous strings in List

Given a mixed list, the task is to write a Python program to group all the contiguous strings. 

Input : test_list = [5, 6, 'g', 'f', 'g', 6, 5, 'i', 's', 8, 'be', 'st', 9]
Output : [5, 6, ['g', 'f', 'g'], 6, 5, ['i', 's'], 8, ['be', 'st'], 9]
Explanation : Strings are grouped to form result.

Input : test_list = [5, 6, 6, 5, 'i', 's', 8, 'be', 'st', 9]
Output : [5, 6, 6, 5, ['i', 's'], 8, ['be', 'st'], 9]
Explanation : Strings are grouped to form result.

Method 1: Using isinstance() + generator + groupby()

In this, we perform the task of identifying strings using isinstance() and str, groupby() is used to group all the strings found by key using isinstance(). The generator way of building a list helps to convert the intermediate results to a list.

Python3

# Python3 code to demonstrate working of
# Group contiguous strings in List
# Using isinstance() + generator + groupby()

from itertools import groupby

# checking string instance


def str_check(ele):
    return isinstance(ele, str)

# Group contiguous strings
def group_strs(test_list):

    # grouping list by cont. strings
    for key, grp in groupby(test_list, key=str_check):

        if key:
            yield list(grp)
        else:
            yield from grp


# Initializing list
test_list = [5, 6, 'g', 'f', 'g', 6, 5,
             'i', 's', 8, 'be', 'st', 9]

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

# Calling recursive method
res = [*group_strs(test_list)]

# Printing result
print("List after grouping : " + str(res))

Output

The original list is : [5, 6, 'g', 'f', 'g', 6, 5, 'i', 's', 8, 'be', 'st', 9]
List after grouping : [5, 6, ['g', 'f', 'g'], 6, 5, ['i', 's'], 8, ['be', 'st'], 9]

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

Method 2: Using type() and simple iteration

1. Define a function named group_strs that takes a list test_list as input.
2. Initialize an empty list called res.
3. Initialize an empty list called temp.
4. Use a for loop to iterate over each element ele in test_list.
   • Check if the type of ele is str using the type() function.
   • If the type of ele is str, append ele to temp.
   • If the type of ele is not str, check if the length of temp is greater than 0 using the len() function.
   • If the length of temp is greater than 0, append temp to res and reset temp to an empty list.
   • Append ele to res.
   • If the length of temp is greater than 0 after the loop has finished, append temp to res.
   • Return the res list.
5. Initialize a list called test_list with a mix of integers and strings.
6. Print the original list test_list.
7. Call the group_strs() function with test_list as the argument and assign the returned value to res.
8. Print the new list res after grouping the strings together.

Example:

Python3

# Using loop
def group_strs(test_list):

    res = []
    temp = []

  for ele in test_list:

  if type(ele)==str:
            temp.append(ele)
        else:

  if len(temp) > 0:
                res.append(temp)
                temp = []
            res.append(ele)

  if len(temp) > 0:
        res.append(temp)
    return res

# initializing list
test_list = [5, 6, 'g', 'f', 'g', 6, 5,
             'i', 's', 8, 'be', 'st', 9]

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

# calling recursive function
res = group_strs(test_list)

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

Output

The original list is : [5, 6, 'g', 'f', 'g', 6, 5, 'i', 's', 8, 'be', 'st', 9]
List after grouping : [5, 6, ['g', 'f', 'g'], 6, 5, ['i', 's'], 8, ['be', 'st'], 9]

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

Method 3: Using itertools.groupby() function

Python3

# Python3 code to demonstrate working of
# Group contiguous strings in List

import itertools

# Method to group contiguous strings


def group_strs(test_list):

    res = []

    for k, g in itertools.groupby(test_list, key=lambda x: isinstance(x, str)):

      if k:
            res.append(''.join(list(g)))
        else:
            res.extend(list(g))

    return res


# Initializing list
test_list = [5, 6, 'g', 'f', 'g', 6, 5,
             'i', 's', 8, 'be', 'st', 9]

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

# Calling the function
res = group_strs(test_list)

# Printing list after grouping as result
print("List after grouping : " + str(res))

Output

The original list is : [5, 6, 'g', 'f', 'g', 6, 5, 'i', 's', 8, 'be', 'st', 9]
List after grouping : [5, 6, 'gfg', 6, 5, 'is', 8, 'best', 9]

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 4: Using a for loop and conditional statements. 

Step-by-step approach:

1. Create an empty list called “res” to store the grouped strings.
2. Create an empty string called “temp” to store the current group of strings.
3. Loop through each element in the “test_list”.
4. Check if the element is a string by using the “isinstance()” function.
5. If the element is a string, append it to the “temp” string.
6. If the element is not a string, append the “temp” string to the “res” list if it is not empty.
7. Append the current element to the “res” list.
8. If the loop is finished and there are still strings in the “temp” string, append it to the “res” list.
9. Return the “res” list.

Python3

def group_strs(test_list):
    res = []
    temp = ''
    for ele in test_list:
        if isinstance(ele, str):
            temp += ele
        else:
            if temp:
                res.append(temp)
                temp = ''
            res.append(ele)
    if temp:
        res.append(temp)
    return res

# initializing list
test_list = [5, 6, 'g', 'f', 'g', 6, 5, 'i', 's', 8, 'be', 'st', 9]

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

# calling group_strs function
res = group_strs(test_list)

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

Output

The original list is : [5, 6, 'g', 'f', 'g', 6, 5, 'i', 's', 8, 'be', 'st', 9]
List after grouping : [5, 6, 'gfg', 6, 5, 'is', 8, 'best', 9]

Time complexity: O(n), where n is the length of the “test_list”.
Auxiliary space: O(1), because we are using a constant amount of extra space for the “res” and “temp” variables.