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Leçon 3. FOR cycle
Difficulté :
Exercice«Éléments voisins»
💻 Python
Un nouveau scanner de colis est installé à l'usine. Il vérifie le lot un par un. Vous — le programmeur chargé d'écrire un programme pour lui.
Le programme reçoit d'abord le nombre de colis dans le lot (n). Ensuite, il commence à recevoir les codes entiers de chaque colis un par un.
Le scanner fonctionne selon les règles suivantes :
- Si le code du colis — un nombre négatif, cela est considéré comme une erreur de scan. Ce colis doit être ignoré, le message est "Code incorrect, passez." Et passez immédiatement au colis suivant.
- Si le code du colis est divisible par 13 sans reste (par exemple, 0, 13, 26, ...), alors le colis est considéré comme dangereux. Il faut immédiatement arrêter toute la vérification, afficher le message "Un colis dangereux avec le code a été trouvé :
[code] ! La vérification est arrêtée." et terminer le programme. - Si le code du colis — un nombre normal, non négatif qui n'est pas divisible par 13, alors sa valeur est ajoutée à la valeur totale de tous les colis vérifiés.
- Si tout le lot a été vérifié et qu'aucun colis dangereux n'a été trouvé, le programme doit afficher le message "La vérification est terminée. La valeur totale des colis :
[somme]."
Format d entrée
La première ligne contient un nombre entier & nbsp; n & nbsp; (1 & le; n & le; 100) & mdash; Le nombre de colis dans le lot.
Les & nbsp; n lignes suivantes contiennent un nombre entier & mdash; Le code du colis suivant.
Format de sortie
le programme affiche un ou plusieurs messages en fonction des codes des colis :
- avec un code négatif :
code incorrect, passez. - lorsqu'un colis dangereux est détecté :
un colis dangereux avec le code a été trouvé : [code] ! L'audit est arrêté.(où
[code] — est le code lui-même). - Si la vérification s'est terminée avec succès :
La vérification est terminée. La valeur totale des colis : [sum].(où
[sum] — est le montant total de la valeur).
Exemple
Entrée
5
10
-5
20
15
2
Sortie
Code incorrect, passez.
La vérification est terminée. Valeur totale des colis : 47.
Indice
Basics of the forcycle
The for loop in Python is your main tool for iterating through elements in any sequence. Imagine that you have a box with different items. The for loop allows you to take one item at a time, do something with it, and move on to the next until the items in the box run out.
It is used to iterate over sequences such as lists, strings, tuples, sets, or even keys in a dictionary. The same block of code is executed for each element of the sequence.
Syntax:
for element in sequence:
# a block of code that will be executed for each element
# IMPORTANT: this block must be indented!
Here, the element is a temporary variable that is assigned a different value from the sequence at each iteration. You can name this variable any way you want, but it's better to choose a meaningful name (for example, fruit for a list of fruits or letter for a string).
Iterating through the list
A list is an ordered collection of items. The for loop will run through it from the first element to the last.
fruits = ['apple', 'banana', 'cherry']
for fruit in fruit:
print(fruit)
Conclusion:
apple
banana
cherry
Iterating over the line
A string is, in fact, a sequence of characters. Therefore, the for loop can iterate over it just as easily.
word = "Python"
for a letter in a word:
print(letter)
Conclusion:
P
y
t
h
o
n
Cycle control: break and continue
Sometimes you don't have to go through the whole cycle to the end. There are special operators for this.
The break operator
The break operator is used to immediately and completely terminate the loop. As soon as the program encounters a break, it immediately exits the loop and continues execution from the next line of code after it.
A useful tip: break is indispensable when you are looking for something. Once you've found what you're looking for, there's no point in continuing the search.
Example: We have a list of numbers, and we want to find the first even number and stop the loop as soon as it is found.
numbers = [1, 3, 5, 6, 7, 9, 10]
for num in numbers:
if num % 2 == 0: # Check for parity
print(f"The first even number is found: {num}")
break # Immediately exit the loop
print("The loop is completed.")
Conclusion:
The first even number was found: 6
The cycle is completed.
Please note that the numbers 7, 9, and 10 were not checked, as the cycle was interrupted by the number 6.
The continue operator
The continue operator is used to skip the current iteration and immediately move on to the next one. All the code that is in the loop after continue will not be executed for the current element.
A useful tip: continue is very convenient for "filtering" data inside a loop. If the current item doesn't suit you by some criterion, you just skip it and move on to the next one.
Example: We have a list of numbers, and we want to output all the numbers except the even ones.
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
for num in numbers:
if num % 2 == 0: # Check for parity
continue # If the number is even, skip print() and go to the next number.
print(num)
Conclusion:
1
3
5
7
9
When num was equal to 2, 4, 6, etc., the if condition was met, continue skipped the print(num) command, and the loop immediately started a new iteration.
The difference between break and continue is in a nutshell
break: "Stop, car!". Stops the cycle completely.continue: "This one doesn't fit, let's do the next one!". Skips only the current step.
The else block in the for
loop
The for loop may have an optional else block. This block is executed only if the loop has completed all its iterations naturally, that is, it has not been interrupted by the break operator.
A useful tip: It is a very powerful tool for search tasks. The else block makes it easy to handle a situation where the search has ended but nothing has been found.
Example 1: The loop ended normally
for number in range(3):
print(number)
else:
print("The cycle is completed without interruptions.")
Conclusion:
0
1
2
The cycle is completed without interruptions.
Example 2: The loop is interrupted by the break
for number in range(3):
if number == 1:
print("We found 1, we break the loop.")
break
print(number)
else:
# This block will not be executed because there was a break
print("The cycle is completed without interruptions.")
Conclusion:
0
We found 1, we interrupt the cycle.
Practical examples
The sum of all the numbers in the list
This is a classic task for understanding how to work with a cycle and accumulate results.
numbers = [1, 2, 3, 4, 5]
amount = 0 # Initialize the variable to store the amount BEFORE the loop
for a number in numbers:
sum += number # At each iteration, we add the current number to the total
print(f' Sum: {sum}') # Output the final result AFTER the loop
Conclusion: Amount: 15
A useful tip: Although this method perfectly demonstrates how the loop works, in real code it is better to use the built-in sum() function to find the sum of the list elements: print(sum(numbers)). It's shorter and more efficient.
Finding the maximum number in the list
Here we assume that the first element is the maximum, and then in the loop we compare it with all the others.
numbers = [1, 7, 3, 9, 5]
maximum = numbers[0] # We take the first element as the temporary maximum
for a number in numbers:
if the number is > maximum: # If we find a number greater than our maximum...
maximum = number # ...then we update the maximum
print(f' Maximum: {maximum}')
Conclusion: Maximum: 9
A useful tip: As with the sum, Python has a built-in function max() for this task: print(max(numbers)). Knowing these functions saves time, but understanding how they work using the for loop is a key skill for a programmer.
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
# Сначала считываем количество посылок в партии
num_packages = int(input())
# Инициализируем переменную для хранения общей ценности
total_value = 0
# Запускаем цикл for, который будет выполняться num_packages раз
for _ in range(num_packages):
# Считываем код очередной посылки
package_code = int(input())
# 1. Проверяем на ошибку сканирования (отрицательный код)
if package_code < 0:
print("Неверный код, пропуск.")
# Пропускаем остаток кода и переходим к следующей итерации
continue
# 2. Проверяем, не является ли посылка опасной
if package_code % 13 == 0:
print(f"Обнаружена опасная посылка с кодом: {package_code}! Проверка остановлена.")
# Немедленно выходим из цикла
break
# 3. Если посылка обычная, добавляем ее ценность к общей сумме
total_value = total_value + package_code
# 4. Этот блок выполнится, только если цикл for завершился без break
else:
print(f"Проверка завершена. Общая ценность посылок: {total_value}.")
