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Lektion 5. While cycle
Schwierigkeit:
Aufgabe«Countful-Countdown»
💻 Python
In einer Welt, in der die Zeit in die entgegengesetzte Richtung fließt, führt die berühmte Forscherin Dr. Elara Wence ein Experiment mit ihrer Zeitmaschine durch. Sie startet das Fahrzeug von den anfänglichen „Zeitkoordinaten“ n und die Maschine beginnt den Countdown bis 1 , dem Bezugspunkt (unserer Gegenwart).
Der Bordcomputer des Fahrzeugs muss jede Zeitkoordinate aufzeichnen, durch die das Fahrzeug fährt. Für wichtige Reisepunkte soll er jedoch spezielle Notizen machen:
- am Startpunkt ( n ) muss der Computer die Meldung ausgeben:
": der Beginn der Rückwärtsreise!" - genau in der Mitte der Strecke (ganzzahlige Division von n durch 2) soll der Computer notieren:
": die Hälfte der Strecke ist geschafft!" - ganz am Ende, bei 1 , wird die Meldung ausgegeben:
": Ankunft am Bezugspunkt!"
Ihre Aufgabe — Schreiben Sie ein Programm, das die Arbeit des Bordjournals der Zeitmaschine von Dr. Vance emuliert. Das Programm erhält eine anfängliche Koordinate n und gibt ein vollständiges Reisejournal aus.
Klarstellung: Wenn die Mitte der Strecke mit dem Endpunkt zusammenfällt (z. B. für n = 2 oder n = 3), hat die Meldung über die Ankunft am Bezugspunkt Vorrang.
Eingabeformat
10
Ausgabeformat
10: Start der Rückfahrt!
9
8
7
6
5: Die Hälfte der Strecke ist zurückgelegt!
4
3
2
1: Ankunft am Referenzpunkt!
Beispiel
Eingabe
Ausgabe
Hinweis
All about thewhile loop in Python
The while cycle (translated as "for now") allows you to repeat the execution of a block of code as long as a certain logical condition remains true (True). This is an indispensable tool when you don't know in advance exactly how many times you need to perform an action. The main difference from the for loop is that for is usually used to iterate over a known sequence (list, string, range), and while when the number of iterations depends on the fulfillment of some condition in during the program operation.
Loop syntax while
Here is the basic syntax of the while loop in Python:
while condition:
code block_
- Condition: this is any expression that returns
TrueorFalse. This can be not only a comparison of variables, but also, for example, checking the presence of items in the list or the value returned by the function. If the condition is true (True), theblock_codeis executed. - Code block: one or more indented statements that are executed each time the loop passes (each iteration).
Example of a simple loop while
Let's look at an example where we output numbers from 1 to 5:
count = 1
while count <= 5:
print(count)
count += 1 # The most important line: increase the value of the count variable by 1
Output:
1
2
3
4
5
Here we start with the value count = 1. Before each iteration, Python checks the condition count<=5. As long as it is true, a block of code is executed: the current value of count is printed, and then it increases by 1. As soon as count becomes 6, the condition 6 <= 5 becomes false (False), and the loop ends its work.
A useful tip: the variable that controls the loop (in our case count) is often called a counter or a loop control variable. It is extremely important to ensure that it changes within the loop in such a way that the condition sooner or later becomes false.
Potential problems: endless loops
An infinite loop is a cycle that never ends. This happens when his condition always remains true.
count = 1
while count <= 5:
print(count)
# Error: we forgot to change the count value, so the condition will always be True.
To stop the execution of such a cycle, you need to interrupt the program manually (for example, by pressing Ctrl+C in the terminal).
However, it is worth noting that infinite loops are not always a mistake. Sometimes they are used intentionally, for example, in programs that need to run all the time: servers listening to connections, or game loops. In such cases, the while True construction is often used.:, and the exit from the loop is organized using a break for some special event.
Using break to exit the loop early
We can use the break operator to immediately and completely terminate the loop, even if its main condition is still true.
count = 1
while count <= 10: # The condition allows you to go up to 10
if count == 3:
print("We have met the number 3, we exit the loop.")
break # Exit the loop when count is 3
print(count)
count += 1
Output:
1
2
We have met the number 3, we exit the cycle.
A useful tip: if you have nested loops (one while inside the other), the break statement will interrupt execution of only the innermost loop in which it is located.
Using continue to skip an iteration
The continue operator skips the rest of the code in the current iteration and immediately proceeds to check the condition for the next iteration.
count = 0
while count < 5:
count += 1
if count == 3:
print("Skipping the number 3...")
continue # Skip the remaining code and move on to the next iteration
print(f"Processing the number: {count}")
Output:
Processing the number: 1
Processing the number: 2
Skip the number 3...
Process the number: 4
Processing the number: 5
It is important not to confuse continue with break. break stops the loop completely, and continue only completes the current step ahead of schedule.
Loop while with user input
while loops are ideal for processing user input when data needs to be requested until it meets the required criteria.
import random
secret_number = random.randint(1, 10)
guess = None # Initialize the variable before the loop
# The loop will run until guess is equal to secret_number
while guess != secret_number:
guess_text = input("Guess a number from 1 to 10:")
# Check if the user has entered a number
if not guess_text.isdigit():
print("Please enter a number.")
continue # Skip the rest of the logic and request input again.
guess = int(guess_text)
if guess < secret_number:
print("Your number is less than the number you have guessed.")
elif guess> secret_number:
print("Your number is more than the number you have guessed.")
print(f"Congratulations! You guessed the number {secret_number}!")
Loop while with block else
Loops in Python have an interesting and not the most well-known feature - the else block. The code in the else block is executed only if the loop ended naturally (that is, its condition became False), and was not interrupted by the break operator.
# Example where else will execute
count = 1
while count <= 3:
print(f"Iteration {count}")
count += 1
else:
print("The loop ended naturally.")
# Output:
# Iteration 1
# Iteration 2
# Iteration 3
# The cycle ended naturally.
# Example where else fails due to break
count = 1
while count <= 3:
if count == 2:
print("The loop is interrupted by the break statement.")
break
print(f"Iterate {count}")
count += 1
else:
print("This block will not be executed.")
# Output:
# Iteration 1
# The loop is interrupted by the break statement.
Conclusion
The while loop is a powerful tool in Python. Here are the main points to remember:
- Initializing variables: Make sure that the variables used in the condition exist before the loop starts.
- Condition change: Something must happen inside the loop that will eventually make the condition false in order to avoid an infinite loop.
breakandcontinue: Usebreakto exit the loop completely andcontinueto skip the current iteration.- The
while Truepattern: This is a convenient way to organize a cycle that must be interrupted by a complex internal condition usingbreak. - The
elseblock: Keep in mind that you can use theelseblock for code that should be executed only at the normal end of the loop.
main.py
Test 1
Test 2
Test 3
Test 4
Test 5
Test 6
Test 7
Test 8
Test 9
Test 10
Entwicklerlösung
# Получаем начальную временную координату от пользователя
n = int(input())
# Рассчитываем координату середины пути с помощью целочисленного деления
midpoint = n // 2
# Запускаем цикл, который будет идти от n до 1 включительно, с шагом -1
for i in range(n, 0, -1):
# Проверяем, является ли текущая координата начальной точкой
if i == n:
print(f"{i}: Начало обратного путешествия!")
# Если нет, проверяем, является ли она конечной точкой
elif i == 1:
print(f"{i}: Прибытие в точку отсчета!")
# Если нет, проверяем, является ли она серединой пути
elif i == midpoint:
print(f"{i}: Половина пути пройдена!")
# Если это ни одна из ключевых точек, просто выводим координату
else:
print(i)
