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Leçon 5. While cycle
Difficulté :
Exercice«Doublement de la séquence»
💻 Python
Vous — Jeune apprenti dans le laboratoire du grand alchimiste. Votre mentor a découvert un moyen de doubler la puissance des essences magiques, qui sont représentées par des valeurs numériques. Cependant, sa méthode ne fonctionne qu'avec des essences « pures » ayant une puissance positive. Une essence avec une puissance nulle ou négative est considérée comme instable et ne peut pas être transformée.
L'alchimiste vous dicte les valeurs des essences une par une. Votre tâche — Créer un programme qui prendra ces valeurs jusqu'à ce que l'alchimiste dise « stop ». Le programme doit traiter toutes les essences reçues : doubler la force de chaque essence « pure » (positive) et afficher le résultat à l'écran. Les essences instables doivent simplement être ignorées.
Tâche :
Écrivez un programme qui :
- lit la séquence d'entiers saisis par l'utilisateur un par un sur une ligne.
- arrête la lecture lorsque l'utilisateur saisit le mot « stop »
- pour chaque nombre positif saisi, calcule sa valeur multipliée par 2.
- affiche les résultats obtenus, chacun sur une nouvelle ligne. Les nombres qui ne sont pas positifs (zéro et négatifs) sont ignorés, et rien ne doit être affiché pour eux.
Format d entrée
La séquence de données où chaque ligne contient soit un entier (int), soit le mot "stop". La saisie se termine après la réception de la ligne "Stop".
Format de sortie
La séquence d'entiers (int), chacun sur une nouvelle ligne. Ce sont les valeurs doubles de tous les nombres positifs de la séquence d'entrée, dérivées dans le même ordre.
Exemple
Entrée
10
-5
25
0
8
Stop
Sortie
20
50
16
Indice
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
Solution du développeur
# Создаем пустой список, куда будем сохранять числовые значения эссенций.
essences = []
# Запускаем бесконечный цикл для ввода данных.
# Он будет прерван, когда пользователь введет "стоп".
while True:
# Считываем очередную строку, которую ввел пользователь.
user_input = input()
# Проверяем, не является ли введенная строка командой для остановки.
if user_input == "стоп":
# Если да, то выходим из цикла ввода.
break
# Если это не команда "стоп", значит, это число.
# Преобразуем строку в целое число и добавляем в наш список.
essences.append(int(user_input))
# Теперь проходим по всему списку собранных эссенций.
# Цикл for идеально подходит для перебора элементов списка.
for value in essences:
# Применяем правило алхимика: работаем только с чистыми (положительными) эссенциями.
if value > 0:
# Если значение положительное, умножаем его на 2 и выводим результат.
print(value * 2)
