5 Best Ways to Print Elements at Odd Positions in a Python Array

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💡 Problem Formulation: In Python, arrays are often represented by lists. The challenge is to iterate through an array and print the elements that are situated at the odd indices, not odd numerical values. For instance, if the input array is [2, 5, 3, 8, 6, 7], the output should be 5, 8, 7 because these values are at positions 1, 3, and 5 respectively (considering 0 as the first position).

Method 1: Using a For Loop With Range Step

The typical way to access elements at odd positions is by using a for loop with a range that starts at 1 and increases in steps of 2. This way, it skips over the even indices and iterates only over the odd ones.

Here’s an example:

my_array = [2, 5, 3, 8, 6, 7]
for i in range(1, len(my_array), 2):
    print(my_array[i])

Output:

5
8
7

This code snippet begins the loop from index 1 and jumps every two elements, effectively printing the second, fourth, and any other element at an odd position within the array.

Method 2: List Comprehension

List comprehensions provide a concise way to create lists or extract elements based on conditions. You can use this to generate a sub-list of elements at odd positions and then print them.

Here’s an example:

my_array = [2, 5, 3, 8, 6, 7]
odd_position_elements = [my_array[i] for i in range(1, len(my_array), 2)]
print(odd_position_elements)

Output:

[5, 8, 7]

This snippet uses list comprehension to create a new list that contains only the items at the odd indices of the original array. It’s then printed as a list.

Method 3: Enumerate Function

The enumerate function adds a counter to an iterable and returns it (the enumerate object). In the provided example, we use enumerate in a list comprehension to filter elements where the index is odd.

Here’s an example:

my_array = [2, 5, 3, 8, 6, 7]
odd_position_elements = [value for index, value in enumerate(my_array) if index % 2 != 0]
print(odd_position_elements)

Output:

[5, 8, 7]

This code block uses the enumerate function within a list comprehension to check if the index is odd, and if so, it includes the element in the resulting list, which is then printed.

Method 4: While Loop

Another approach is to use a while loop that increments the index by 2 until it reaches the end of the array, printing each element at these odd positions.

Here’s an example:

my_array = [2, 5, 3, 8, 6, 7]
i = 1  # Start at the first odd index
while i < len(my_array):
    print(my_array[i])
    i += 2

Output:

5
8
7

The snippet demonstrates a while loop that starts at index 1 and continues until it exceeds the array length, printing elements at odd indices each iteration by increasing the index by 2.

Bonus One-Liner Method 5: Using Slicing

Python’s slicing feature allows you to slice the array and extract elements starting from the second element (index 1) to the end of the list with a step of 2 to get elements at odd positions.

Here’s an example:

my_array = [2, 5, 3, 8, 6, 7]
print(my_array[1::2])

Output:

[5, 8, 7]

This one-liner utilizes slicing to extract and print a sub-list of elements located at the odd indices of the original array. It’s simple, elegant, and efficient.

Summary/Discussion

  • Method 1: For Loop With Range Step. Strengths: Straightforward and robust. Weaknesses: More verbose than some other methods.
  • Method 2: List Comprehension. Strengths: Concise and easy to read. Weaknesses: Creates an intermediate list that could consume memory if the array is large.
  • Method 3: Enumerate Function. Strengths: Very Pythonic and clear intent. Weaknesses: Also creates an intermediate list similarly to method 2.
  • Method 4: While Loop. Strengths: Offers fine control over loop execution. Weaknesses: More error-prone due to manual index management.
  • Method 5: Using Slicing. Strengths: Extremely concise and the most performant for large lists. Weaknesses: Might not be as readable to beginners.