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Lektion 3. FOR cycle
Schwierigkeit:
Aufgabe«Festliche Einkäufe»
💻 Python
Sie planen ein festliches Abendessen und haben ein streng begrenztes Budget. Ihre Aufgabe — Schreiben Sie ein Assistentenprogramm, das Ihre Kosten in Echtzeit verfolgt.
Das Programm muss zuerst Ihr Gesamtbudget und die Anzahl der Waren abfragen, die Sie kaufen möchten. Dann soll es für jedes Produkt dessen Namen und Preis abfragen.
Nach dem Hinzufügen jedes Produkts muss das Programm ausgeben, welches Produkt hinzugefügt wurde und wie viel Geld Ihnen noch übrig ist. Falls Ihre Ausgaben irgendwann das Budget überschreiten, sollte das Programm sofort eine Warnung dazu anzeigen, aber dennoch weiterhin Waren zur Liste hinzufügen.
Nachdem Sie die Informationen zu allen Waren eingegeben haben, muss das Programm den Gesamtbetrag des ausgegebenen Geldes und den endgültigen Kontostand (dieser kann negativ sein) anzeigen.
Eingabeformat
Erste Zeile: das Budget (ganze Zahl).
Zweite Zeile: die Anzahl der Waren (ganze Zahl).
Anschließend folgen für jedes Produkt Paare von Zeilen:
- der Name der Ware (Zeile).
- der Preis der Ware (ganze Zahl).
Ausgabeformat
für jedes eingegebene Produkt:
rn- rn
- die Ware und der Saldo des Budgets (Zeile). rn
- wenn das Budget überschritten wird, wird zusätzlich eine Warnung (Zeile) angezeigt. rn
nach der Eingabe aller Produkte:
rn- rn
- der Abschlussbericht, der den Gesamtbetrag der Ausgaben und den Endsaldo enthält (mehrere Zeilen). rn
Beispiel
Eingabe
1500
3
Napoleon cake
800
champagne
500
Candles
150
Ausgabe
Hinzugefügte Waren: Napoleon-Kuchen. Budgetrest: 700
Hinzugefügte Waren: Champagner. Budgetrest: 200
Hinzugefügte Waren: Kerzen. Budgetrest: 50
Die Einkäufe sind abgeschlossen.
Gesamtausgaben: 1450
Endgültiger Saldo: 50
Hinweis
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
Entwicklerlösung
# Запрашиваем и считываем общий бюджет, преобразуя его в целое число
budget = int(input())
# Запрашиваем и считываем количество товаров, преобразуя его в целое число
num_items = int(input())
# Создаем переменную для хранения общей суммы потраченных средств, изначально она равна 0
total_spent = 0
remaining_budget = budget
# Запускаем цикл, который повторится столько раз, сколько товаров мы планируем купить
for i in range(num_items):
# Внутри цикла запрашиваем название очередного товара
item_name = input()
# Затем запрашиваем его цену и преобразуем в целое число
item_price = int(input())
# Добавляем цену текущего товара к общей сумме трат
total_spent = total_spent + item_price
# Вычисляем, сколько денег осталось от первоначального бюджета
remaining_budget = budget - total_spent
# Выводим сообщение о том, какой товар добавлен и какой остаток бюджета
print(f"Добавлен товар: {item_name}. Остаток бюджета: {remaining_budget}")
# Проверяем условие: если остаток бюджета стал меньше нуля...
if remaining_budget < 0:
# ...то выводим предупреждение с указанием суммы превышения
# Мы используем -remaining_budget, чтобы показать положительное число долга
print(f"Внимание! Бюджет превышен на {-remaining_budget}.")
# После завершения цикла выводим пустую строку для лучшей читаемости
print("\nПокупки завершены.")
# Выводим итоговую сумму всех трат
print(f"Всего потрачено: {total_spent}")
# Выводим финальный остаток, который может быть положительным или отрицательным
print(f"Итоговый остаток: {remaining_budget}")
