5 Best Ways to Print Elements on Even Positions in a Python Array

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πŸ’‘ Problem Formulation: This article demonstrates how to print array elements that are located at even positions using Python. For an array [3, 1, 4, 1, 5, 9, 2, 6], we aim to extract and print the elements at the 2nd, 4th, 6th, and so on positions, resulting in the desired output: [1, 1, 9, 6].

Method 1: Using a For Loop

This method iterates over the array using a for loop with a step of two, starting from index 1 which represents the second position in a zero-indexed array. The function is straightforward and is excellent for beginners to understand how indexing and loops work.

Here’s an example:

arr = [3, 1, 4, 1, 5, 9, 2, 6]
for i in range(1, len(arr), 2):
    print(arr[i])

Output:

1
1
9
6

This code snippet creates a loop that starts at index 1 (second position), ends at the length of the array minus one (last position), and increments by two, thus only printing the elements at the even positions.

Method 2: List Slicing

List slicing is a clean and efficient way to access specific portions of a list in Python. By using slicing, we can easily grab the even-positioned elements of an array without explicitly iterating over them.

Here’s an example:

arr = [3, 1, 4, 1, 5, 9, 2, 6]
print(arr[1::2])

Output:

[1, 1, 9, 6]

This snippet utilizes the syntax of list slicing. arr[1::2] starts the slice at the second element (index 1) of the array and proceeds to the end, selecting every second element.

Method 3: Using List Comprehension

Python’s list comprehensions are a concise way to create lists. They can be used to derive a new list by evaluating an expression over an iterable, selectively including elements.

Here’s an example:

arr = [3, 1, 4, 1, 5, 9, 2, 6]
even_pos_elements = [arr[i] for i in range(1, len(arr), 2)]
print(even_pos_elements)

Output:

[1, 1, 9, 6]

This code utilizes list comprehension to iterate over the array and create a new list that contains the array’s elements at the even positions (or odd indexes).

Method 4: Using enumerate and Filtering

The enumerate function in Python adds a counter to an iterable. In conjunction with filtering, this can be used to identify and print elements at even positions.

Here’s an example:

arr = [3, 1, 4, 1, 5, 9, 2, 6]
for index, value in enumerate(arr):
    if index % 2 != 0:
        print(value)

Output:

1
1
9
6

By combining enumerate with a simple conditional within a for loop, this code checks if the index is not divisible by two (meaning it’s an odd index which corresponds to an even position) and prints the corresponding element.

Bonus One-Liner Method 5: Using filter and lambda

Python’s filter function allows you to process an iterable and extract elements that meet a condition. Combined with a lambda function, this makes for a terse one-liner that gets the job done.

Here’s an example:

arr = [3, 1, 4, 1, 5, 9, 2, 6]
print(list(filter(lambda x: arr.index(x) % 2 != 0, arr)))

Output:

[1, 1, 9, 6]

This snippet uses filter along with a lambda function to iterate over the array and select elements whose index value is odd. Note that using arr.index(x) is not very efficient for large arrays as it searches the array every time it’s called.

Summary/Discussion

  • Method 1: For Loop. Suitable for teaching purposes. The iteration approach might be slower for very large arrays.
  • Method 2: List Slicing. Elegant and Pythonic. Offers the best performance for most cases but may be less readable for beginners.
  • Method 3: List Comprehension. Flexible and Pythonic. Balances readability with efficiency. Good for intermediate Python users.
  • Method 4: Enumerate and Filtering. Explicit and easy to understand what each iteration is doing. Might be a bit slower due to the conditional check every loop.
  • Method 5: filter and lambda. Very concise. However, it’s not recommended for performance-sensitive tasks due to arr.index inherent inefficiency.