理论与实践

数据输入输出

任务

条件语句

任务

for循环

任务

字符串

任务

while循环

任务

列表

任务

二维数组

任务

字典

任务

集合

任务

函数与递归

任务

课程 5. While cycle

难度：

任务«数字之夏（带通行证）»

💻 Python

学校会计部门正在为体育馆的维修筹集捐款。为了简化核算，决定忽略一些小额及特定金额。总会计师决定，在最终结算时，需要忽略所有能被3或5整除的捐款金额。其余所有金额需进行累加。

你需要编写一个程序，该程序将依次询问用户输入的捐款金额。用户输入整数。输入过程在用户输入单词后结束。之后，程序应根据会计规则显示已计入的捐款总金额。

输入格式

用户输入数据行。每行包含一个整数（类型为 number，按 line 读取），或者单词 stop（类型为 line），表示输入结束的信号。

输出格式

程序输出一个数字（类型为 number）——满足条件的捐款总额。

示例

输入

100
50
27
11
7
停止

输出

提示

While in python

In the Python programming language, the while loop construction can have an optional else block. This block is executed only when the loop completes in its natural way, that is, when its condition becomes false, and not as a result of a forced interruption using the break operator.

Syntax:

while condition:
    # the body of the loop
    # ...the code that runs on each iteration...
else:
    # else block
    # ...the code that is executed if the loop ended without a break...

You can think of the else block as a "plan B": it is triggered if the main goal within the loop (which is usually completed through a break) has not been achieved.

Usage example: Searching for an element

Consider a classic problem where we search for a number in a list. If a number is found, we immediately break the loop using break. If we have checked all the elements and still haven't found what we are looking for, the else block is executed.

numbers = [1, 2, 3, 4, 5]
target = 8 # Let's try to find a number that is not in the list.

index = 0
while index < len(numbers):
    if numbers[index] == target:
        print(f"Success! {target} found at {index}.")
break # Goal reached, exit the loop
    index += 1
else:
    # This block will be executed because break was not called
    print(f"The search is completed. {target} not found in the list.")

How it works:

The whileloop is executed as long as the index < len(numbers) condition is true.
Inside the loop, we check whether the current element is equal to the desired one (target).
If we find a value, a success message is displayed and the loop is immediately interrupted by the break statement. The else block is ignored in this case.
If the loop reaches the end (checked all the elements), it means that break has never worked. The index < len(numbers) condition becomes false, and then the else block is executed, reporting a failed search.

A useful tip: Using while...else helps to avoid "flagged" variables. Without this block, the code would look like this:

# Alternative version with "flag"
numbers = [1, 2, 3, 4, 5]
target = 8
found = False # Flag variable

index = 0
while index < len(numbers):
    if numbers[index] == target:
        found = True
        break
    index += 1

if found:
    print(f"Success! {target} found.")
else:
print(f"Search completed. {target} not found in the list.")

As you can see, the while...else construction makes the code cleaner, more concise, and more "Pythonic", eliminating the need to manually control the flag.

If `break` is missing

If there is no break operator in the body of the while loop, then the else block will always be executed after the loop ends.

count = 0
while count < 3:
print(f"Iteration number {count}")
count += 1
else:
print("The loop ended naturally, so the else block is executed.")

Here, the while loop runs until count is less than 3. As soon as count becomes 3, the loop condition becomes false, and control is transferred to the else block.

Important nuances

What about continue? The continue operator does not interrupt the loop, but only ends the current iteration ahead of schedule. It does not affect the execution of the else block in any way.
If the loop was never executed? If the while condition is initially false, the loop body will not be executed even once. However, the else block will still run, since the loop ended "normally" (it was not interrupted via break).

value = 10
while value < 5: # The condition is immediately false
    print("This text will never be printed")
else:
print("The cycle didn't even start, but it ended normally, so else worked.")

This also works for for loops! The same logic applies to for...else loops. The else block after for is executed if the cycle has passed through all the elements and has not been interrupted using break.

Output

The while...else construction is a powerful and elegant Python tool. It is ideal for situations where you need to perform an action only if the loop has completed its work completely without being interrupted. This is especially useful in search algorithms, condition checking, or when working with data streams, where break signals a special event, and else indicates a regular termination without any.

main.py

测试 1

测试 2

测试 3

测试 4

测试 5

测试 6

测试 7

测试 8

测试 9

测试 10

开发者解答

# Инициализируем переменную для хранения общей суммы пожертвований.
# Начинаем с нуля, так как еще ничего не посчитано.
total_sum = 0

# Создаем бесконечный цикл для непрерывного ввода данных.
# Цикл прервется только специальной командой.
while True:
    # Запрашиваем у пользователя ввод (это может быть число или слово "стоп").
    # Введенные данные всегда считываются как строка.
    donation_input = input()

    # Проверяем, ввел ли пользователь слово "стоп" для завершения работы.
    # Это наше условие выхода из цикла while.
    if donation_input == 'стоп':
        # Если пользователь ввел "стоп", выходим из бесконечного цикла.
        break

    # Если введено не "стоп", то это должна быть сумма.
    # Преобразуем введенную строку в целое число для математических операций.
    donation = int(donation_input)

    # Это главное условие задачи. Проверяем, что сумма не делится на 3 И не делится на 5.
    # Оператор % (остаток от деления) помогает это выяснить.
    # Если остаток от деления на 3 не равен 0 И остаток от деления на 5 не равен 0, то...
    if donation % 3 != 0 and donation % 5 != 0:
        # ...добавляем эту сумму к нашей общей сумме.
        total_sum = total_sum + donation

# После завершения цикла (когда было введено "стоп") выводим итоговую посчитанную сумму.
print(total_sum)