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«标准差»

💻 Python

你是一家研究实验室的数据分析师。你被要求编写一个程序，根据一系列测量值计算标准差。测量值是一串自然数。为了方便数据输入，规定序列的结束由数字0表示，该数字不参与计算。

样本的标准差称为 & NBSP; & Sigma; (Sigma)，其计算公式如下。你可以在这里看到))) & nbsp; https: //habr.com/en/companies/raft/articles/857064/

输入格式

自然数序列，每个数字占一行。输入以数字 0 结束。

输出格式

一个材料编号 & mdash; 标准偏差，以尽可能高的精度计算得出，必须四舍五入到小数点后两位。

示例

输入

600
470
170
430
300
0

输出

164.71

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

# Считываем последовательность чисел до тех пор, пока не встретим 0
numbers = []
while True:
    num = int(input())
    if num == 0:
        break
    numbers.append(num)

# Проверяем различные случаи
n = len(numbers)

# Если последовательность пустая или содержит только один элемент
if n == 0:
    print("0.00")
elif n == 1:
    # Для одного элемента стандартное отклонение не определено (деление на 0 в формуле)
    print("0.00")
else:
    # Вычисляем среднее арифметическое
    mean = sum(numbers) / n
    
    # Вычисляем сумму квадратов отклонений от среднего
    sum_of_squares = 0
    for num in numbers:
        deviation = num - mean
        sum_of_squares += deviation ** 2
    
    # Вычисляем стандартное отклонение для выборки
    # Формула: σ = √(Σ(xᵢ - μ)² / (n - 1))
    # n-1 = 0 при n=1, но мы уже обработали этот случай выше
    variance = sum_of_squares / (n - 1)
    std_deviation = variance ** 0.5
    
    # Округляем до 2 знаков после запятой и выводим
    print(f"{std_deviation:.2f}")