完美偶数计数

Sure! Let’s break down the problem and the provided solution step by step.

Problem Statement

We need to count how many even numbers exist in a given list that fall within a specified range [l, r]. The function solution takes four parameters:

• n: the number of elements in the list a.
• l: the lower bound of the range.
• r: the upper bound of the range.
• a: a list of integers.

Breakdown of the Code

The function works as follows:

1. Initialize Count: Start by initializing a counter variable count to zero. This variable will store the number of even integers found within the specified range.

2. Iterate through the List: Use a for loop to iterate over each integer x in the list a.

3. Check Conditions:

   • Determine if x is even using the condition x % 2 == 0.
   • Check if x falls within the specified range using l <= x <= r.
   • If both conditions are satisfied, increment the count by one.

4. Return the Count: After processing all elements in the list, return the final count.

Example Walkthrough

Let’s go through an example to illustrate how the function works:

Example 1:

Input: n = 5, l = 3, r = 8, a = [1, 2, 6, 8, 7]

• Only even numbers are 2, 6, 8.
• Within the range [3, 8], the even numbers are 6 and 8.
• Count = 2.

Example 2:

Input: n = 4, l = 10, r = 20, a = [12, 15, 18, 9]

• The even numbers here are 12, 18.
• Both 12 and 18 fall within the range [10, 20].
• Count = 2.

Example 3:

Input: n = 3, l = 1, r = 10, a = [2, 4, 6]

• All numbers are even and fall within the range [1, 10].
• Count = 3.

Overall Explanation

The function efficiently counts even numbers in a specific range using a simple loop and conditional checks. It has a time complexity of O(n), where n is the length of the list, as it makes a single pass through the list.

Conclusion

The provided solution effectively meets the problem's requirements. Here’s the complete solution code again for clarity:

def solution(n: int, l: int, r: int, a: list) -> int:
    count = 0
    for x in a:
        if x % 2 == 0 and l <= x <= r:
            count += 1
    return count
 
if __name__ == '__main__':
    print(solution(5, 3, 8, [1, 2, 6, 8, 7]) == 2)  # Expected output: 2
    print(solution(4, 10, 20, [12, 15, 18, 9]) == 2) # Expected output: 2
    print(solution(3, 1, 10, [2, 4, 6]) == 3)        # Expected output: 3

This effectively illustrates how the function processes the data to yield the correct count of even numbers within the specified bounds.