python - 无法为元组列表中的数据返回无

Question

我的函数应该获取一个元组列表，并返回车辆达到或超过 n 所需的天数，从列表中的第一天开始，仅使用 while 循环和一个 return 语句。

def days_to_reach_n_vehicles(vehicle_records, n):
    """Returns number of days taken to reach or exceed a total of n vehicles"""
    cumulative_total = 0
    num_days = 0
    index = 0

    while index < len(vehicle_records):
        cumulative_total += vehicle_records[index][1]
        index += 1
        num_days += 1

        if cumulative_total >= n:
            break

    return num_days

它为我提供了以下测试代码的正确输出 2:

some_records = [('2010-01-01',1),
                ('2010-01-02',2),
                ('2010-01-03',3)]
days = days_to_reach_n_vehicles(some_records, 3)
print(days)

但是，如果在 vehicle_records 中的最后一天没有到达 n 辆车，则需要返回 None。我无法让它为下一个测试数据返回 None，有人可以告诉我我需要修复什么吗？

some_records = [('2010-01-01',1),
                ('2010-01-02',2),
                ('2010-01-03',3)]
days = days_to_reach_n_vehicles(some_records, 40)
print(days)

Answer 1

也许您需要返回 num_days if cumulative_total >= n else None。

def days_to_reach_n_vehicles(vehicle_records, n):
    """Returns number of days taken to reach or exceed a total of n vehicles"""
    cumulative_total = 0
    num_days = 0
    index = 0

    while index < len(vehicle_records):
        cumulative_total += vehicle_records[index][1]
        index += 1
        num_days += 1

        if cumulative_total >= n:
            break

    return num_days if cumulative_total >= n else None