Python | Get indices of True values in a binary list

Boolean lists are often used by the developers to check for True values during hashing. Boolean list is also used in certain dynamic programming paradigms in dynamic programming. Let’s discuss certain ways to get indices of true values in list in Python.

Method #1 : Using enumerate() and list comprehension

enumerate() can do the task of hashing index with its value and coupled with list comprehension can let us check for the true values.

# Python3 code to demonstrate
# to return true value indices.
# using enumerate() + list comprehension

# initializing list
test_list = [True, False, True, False, True, True, False]

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

# using enumerate() + list comprehension
# to return true indices.
res = [i for i, val in enumerate(test_list) if val]

# printing result
print ("The list indices having True values are : " +  str(res))

Output:

The original list is : [True, False, True, False, True, True, False]
The list indices having True values are : [0, 2, 4, 5]

Method #2 : Using lambda + filter() + range()

filter() function coupled with lambda can perform this task with help of range function. range() function is used to traverse the entire list and filter checks for true values.

# Python3 code to demonstrate
# to return true value indices.
# using lambda + filter() + range()

# initializing list
test_list = [True, False, True, False, True, True, False]

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

# using lambda + filter() + range()
# to return true indices.
res = list(filter(lambda i: test_list[i], range(len(test_list))))

# printing result
print ("The list indices having True values are : " +  str(res))

Output:

The original list is : [True, False, True, False, True, True, False]
The list indices having True values are :  [0, 2, 4, 5]

Method #3 : Using itertools.compress()

compress function checks for all the elements in list and returns the list of indices with True values. This is most Pythonic and elegant way to perform this particular task.

# Python3 code to demonstrate
# to return true value indices.
# using itertools.compress()
from itertools import compress

# initializing list
test_list = [True, False, True, False, True, True, False]

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

# using itertools.compress()
# to return true indices.
res = list(compress(range(len(test_list)), test_list))

# printing result
print ("The list indices having True values are : " +  str(res))

Output:

The original list is : [True, False, True, False, True, True, False]
The list indices having True values are : [0, 2, 4, 5]