Python - Indices of Atmost K Elements in List

Finding indices of elements that are at most K means locating positions of elements that are less than or equal to a given value K. Python provides several approaches using built-in functions like enumerate(), filter(), and NumPy functions.

Problem Statement

Given a list [10, 8, 13, 29, 7, 40, 91] and K value of 13, we need to find indices of elements ? 13. The elements [10, 8, 13, 7] are at positions [0, 1, 2, 4].

Using List Comprehension

List comprehension with enumerate() provides a clean solution ?

k = 10
numbers = [10, 4, 11, 12, 14]
indices = [i for i, num in enumerate(numbers) if num <= k]
print("Indices of elements ?", k, ":", indices)
print("Elements:", [numbers[i] for i in indices])

Indices of elements ? 10 : [0, 1]
Elements: [10, 4]

Using Loop and Append

Traditional approach using a for loop ?

k = 40
numbers = [10, 20, 50, 60, 30, 80, 90]
indices = []

for i in range(len(numbers)):
    if numbers[i] <= k:
        indices.append(i)

print("Indices of elements ?", k, ":", indices)
print("Elements:", [numbers[i] for i in indices])

Indices of elements ? 40 : [0, 1, 4]
Elements: [10, 20, 30]

Using NumPy

NumPy's where() function efficiently handles array operations ?

import numpy as np

k = 12
numbers = [10, 4, 11, 12, 14]
arr = np.array(numbers)
indices = np.where(arr <= k)[0]

print("Indices of elements ?", k, ":", indices)
print("Elements:", arr[indices])

Indices of elements ? 12 : [0 1 2 3]
Elements: [10  4 11 12]

Using enumerate() and filter()

Functional programming approach using filter() and lambda ?

k = 8
numbers = [10, 4, 11, 12, 14, 1, 2, 89, 0]
indices = list(map(lambda x: x[0], filter(lambda x: x[1] <= k, enumerate(numbers))))

print("Indices of elements ?", k, ":", indices)
print("Elements:", [numbers[i] for i in indices])

Indices of elements ? 8 : [1, 5, 6, 8]
Elements: [4, 1, 2, 0]

Comparison

Conclusion

Use list comprehension for readable and efficient code. For large datasets, NumPy provides the best performance. The traditional loop approach is most beginner-friendly and easy to debug.