Python – Group Consecutive elements by Sign

Given a list group of consecutive elements on the basis of signs.

Input : test_list = [5, -3, 2, 4, 6, -2, -1, -7, -9, 2, 3] 
Output : [[5], [-3], [2, 4, 6], [-2, -1, -7, -9], [2, 3]] 
Explanation : Elements inserted into new list on sign change.

Input : test_list = [-2,3,4,5,6,-3]
Output : [[-2], [3, 4, 5, 6], [-3]]
Explanation : Elements inserted into new list on sign change.  

Method #1: Using the loop

In this, whenever a sign(positive or negative) change occurs, a new list is initiated otherwise the elements are appended to a similar list as initialized.

Python3

# Python3 code to demonstrate working of
# Group Consecutive elements by Sign
# Using loop

# initializing list
test_list = [5, -3, 2, 4, 6, -2, -1, -7,
             -9, 2, 3, 10, -3, -5, 3]

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

res = [[]]
for (idx, ele) in enumerate(test_list):

    # checking for similar signs by XOR
    if ele ^ test_list[idx - 1] < 0:
        res.append([ele])
    else:
        res[-1].append(ele)

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

Output

The original list is : [5, -3, 2, 4, 6, -2, -1, -7, -9, 2, 3, 10, -3, -5, 3]
Elements after sign grouping : [[5], [-3], [2, 4, 6], [-2, -1, -7, -9], [2, 3, 10], [-3, -5], [3]]

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

Method #2: Using groupby() + list comprehension

In this, we perform the task of grouping using groupby(), and list comprehension is used to perform the task of iterating through the list. The condition for the sign is injected using the lambda function.

Python3

# Python3 code to demonstrate working of
# Group Consecutive elements by Sign
# Using groupby() + list comprehension

import itertools

# initializing list
test_list = [-2, 3, 4, 5, 6, -3]

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

# grouped using groupby()
res = [list(ele) for idx, ele in itertools.groupby(test_list, lambda a: a > 0)]

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

Output

The original list is : [-2, 3, 4, 5, 6, -3]
Elements after sign grouping : [[-2], [3, 4, 5, 6], [-3]]

Time Complexity: O(n), where n is the length of the input list. This is because we’re using the built-in groupby() + list comprehension which has a time complexity of O(n) in the worst case.
Auxiliary Space: O(n), where n is the length of the input list as we’re using additional space other than the input list itself. 

Method 3: using the numpy library

Steps:

1. Import the numpy library.
2. Initialize the input list test_list.
3. Convert the input list to a numpy array using the numpy.array() method.
4. Create a boolean array by checking if each element in the numpy array is greater than zero using the numpy.greater() method.
5. Use the numpy.split() method to split the numpy array into sub-arrays based on the boolean array created in step 4.
6. Convert each sub-array to a list using the tolist() method.
7. Print the result.

Python3

import numpy as np

# initializing list
test_list = [-2, 3, 4, 5, 6, -3]

# convert to numpy array
arr = np.array(test_list)

# create boolean array
bool_arr = np.greater(arr, 0)

# split array based on boolean array
res_arr = np.split(arr, np.where(bool_arr[:-1] != bool_arr[1:])[0]+1)

# convert each sub-array to a list
res = [i.tolist() for i in res_arr]

# print result
print("Elements after sign grouping : " + str(res))

OUTPUT : 
Elements after sign grouping : [[-2], [3, 4, 5, 6], [-3]]

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