Python | Indices of Kth element value

Sometimes, while working with records, we might have a problem in which we need to find all the indices of elements for a particular value at a particular Kth position of tuple. This seems to be a peculiar problem but while working with many keys in records, we encounter this problem. Let’s discuss certain ways in which this problem can be solved. 

Method #1 : Using loop This is the brute force method by which this problem can be solved. In this, we keep a counter for indices and append to list if we find the specific record at Kth position in tuple. 

Python3

# Python3 code to demonstrate working of
# Indices of Kth element value
# Using loop

# initialize list
test_list = [(3, 1, 5), (1, 3, 6), (2, 5, 7),
             (5, 2, 8), (6, 3, 0)]

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

# initialize ele
ele = 3

# initialize K
K = 1

# Indices of Kth element value
# Using loop
# using y for K = 1
res = []
count = 0
for x, y, z in test_list:
    if y == ele:
        res.append(count)
    count = count + 1

# printing result
print("The indices of element at Kth position : " + str(res))

Output : 

The original list is : [(3, 1, 5), (1, 3, 6), (2, 5, 7), (5, 2, 8), (6, 3, 0)] The indices of element at Kth position : [1, 4]

Time Complexity: O(n), where n is the length of the list test_list 
Auxiliary Space: O(n) additional space of size n is created where n is the number of elements in the res list 

Method #2: Using enumerate() + list comprehension

The combination of the above functions can be used to solve this problem. In this, we enumerate for the indices using enumerate(), rest is performed as in the above method but in a compact way. 

Python3

# Python3 code to demonstrate working of
# Indices of Kth element value
# Using enumerate() + list comprehension

# initialize list
test_list = [(3, 1, 5), (1, 3, 6), (2, 5, 7),
             (5, 2, 8), (6, 3, 0)]

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

# initialize ele
ele = 3

# initialize K
K = 1

# Indices of Kth element value
# Using enumerate() + list comprehension
res = [a for a, b in enumerate(test_list) if b[K] == ele]

# printing result
print("The indices of element at Kth position : " + str(res))

Output : 

The original list is : [(3, 1, 5), (1, 3, 6), (2, 5, 7), (5, 2, 8), (6, 3, 0)] The indices of element at Kth position : [1, 4]

Method 3: Using sorted() function

Approach:

This function takes in an input list and an integer k, and returns a list of indices where the kth smallest element appears in the original input list.

Python3

def indices_of_kth_element_sorted(input_list, k):
    sorted_list = sorted(input_list)
    indices = [i for i, x in enumerate(input_list) if sorted_list[k-1] == x]
    return indices


input_list = [(3, 1, 5), (1, 3, 6), (2, 5, 7), (5, 2, 8), (6, 3, 0)]
k = 2
indices = indices_of_kth_element_sorted(input_list, k)
print(f"The original list is : {input_list}")
print(f"The indices of element at Kth position : {indices}")

Output

The original list is : [(3, 1, 5), (1, 3, 6), (2, 5, 7), (5, 2, 8), (6, 3, 0)]
The indices of element at Kth position : [2]

Time complexity: Sorting the list takes O(n log n) time in the average and worst cases, where n is the length of the input list. The list comprehension to find the indices takes O(n) time in the worst case. Therefore, the overall time complexity is O(n log n).
Auxiliary Space: Sorting the list takes O(n) extra space, where n is the length of the input list. The list comprehension to find the indices takes O(n) extra space in the worst case. Therefore, the overall space complexity is O(n).