DE
RU
EN
ES
ZH
FR
PT
AR
HI
🎉 Glückwunsch! 🎉
Du hast die Aufgabe hervorragend gelöst! Es war schwierig, aber du hast die Lösung gefunden. Du bist dem Meister einen Schritt näher! Mach weiter so.
Lektion 3. FOR cycle
Schwierigkeit:
Aufgabe«Cybersicherheit»
💻 Python
Ihr Freund hat einen Defekt, und er hat Sie um Hilfe gebeten. Nach einer schnellen Diagnose haben Sie festgestellt, dass das System mit Viren infiziert ist. Um den Behandlungsprozess zu verfolgen, haben Sie beschlossen, ein kleines Programm zu schreiben.
rnDas Programm muss zuerst abfragen, wie viele Viren gefunden wurden.
rnDann muss es die Funktionsweise des Antivirenprogramms nachahmen: nacheinander einen Virus "entfernen" und jedes Mal melden, wie viele noch übrig sind. Wenn alle Viren gelöscht sind, muss das Programm anzeigen, dass der Computer jetzt sicher ist.
rnWenn der Benutzer anfangs eine 0 oder eine negative Zahl eingibt, muss das Programm melden, dass der Computer nicht infiziert ist und keine Behandlung erforderlich ist.
Eingabeformat
Die anfängliche Anzahl der Viren auf dem Computer (ganze Zahl).
Ausgabeformat
Meldungen über den Entfernungsprozess und das Endergebnis (Zeile).
Beispiel
Eingabe
4
Ausgabe
Entferne den Virus. Übrig: 3
Entferne den Virus. Übrig: 2
Entferne den Virus. Übrig: 1
Entferne den Virus. Übrig: 0
Alle Viren wurden entfernt. Sicherheitssystem!
Hinweis
Diving into cycles: for and range()
The for loop is one of the most powerful and frequently used tools in Python. It allows you to repeat (or, as programmers say, iterate) a certain block of code a set number of times. His main idea is to sequentially iterate over elements from a collection (for example, a list of numbers, characters in a string, etc.) and perform the same action for each element.
The basic syntax looks like this:
for element in sequence:
# a block of code that will be repeated
Here, the sequence can be not only numbers, but also, for example, a list of strings or just a string.
# Example of iterating through a list
for fruit in ["apple", "banana", "cherry"]:
print(fruit)
# Example of line iteration
for letter in "Python":
print(letter)
Generating sequences with range()
We often need to execute the code a certain number of times. The range() function is ideal for this, which creates a sequence of numbers. For example, range(5) creates a sequence from 0 to 4 (i.e. 0, 1, 2, 3, 4).
Combining for and range(), we get a classic construction for repetitions.:
for i in range(number):
# code block
Let's analyze this construction:
foris the keyword that tells Python that we are starting a cycle.iis a counter variable. At each step of the cycle, it will take the next value from the sequence. A useful tip: the variable name can be anything (i,j,k- these are generally accepted conventions for simple counters), but if you are going over something specific, it is better to give a meaningful name. for examplefor student in students:.inis the keyword that binds the counter variable and the sequence.range(number)calling a function that creates a sequence of integers from0tonumber - 1. It is important to remember that the upper bound is not included.- The code block (indented) is the commands that you want to execute for each number in the sequence.
Practical examples
Output of numbers from 0 to 4
for i in range(5):
print(i)
# output: 0
# output: 1
# output: 2
# output: 3
# output: 4
range(5)creates an invisible sequence for us: 0, 1, 2, 3, 4.- The
forloop assigns each of these values to theivariable in turn. print(i)prints the current value ofiat each step.
Re-executing the code 5 times
for i in range(5):
print("Hello world!")
# output: Hello world!
# conclusion: Hello, world!
# conclusion: Hello, world!
# conclusion: Hello, world!
# conclusion: Hello, world!
range(5)creates the sequence again 0, 1, 2, 3, 4.- The
forloop goes through each of these numbers, but in this case we don't use the value of theivariable inside the loop. We just use the loop as a counter to runprint("Hello, world!")exactly 5 times.
Summation of numbers from 0 to 4
amount = 0 # 1. Creating a "piggy bank" for the amount
for i in range(5):
#2. At each step, we add the current number to the piggy bank
sum += i
print(sum) # 3. We show what has accumulated in the end
# output: 10
amount = 0initialize the variableamount. This is a critical step.range(5)creates a sequence 0, 1, 2, 3, 4.- The
forloop runs through each number. In the first step, thesumbecomes0 + 0 = 0. On the second one,0 + 1 = 1. On the third one,1 + 2 = 3. On the fourth floor,3 + 3 = 6. On the fifth -6 + 4 = 10. sum +=iis a shortened and more "pythonic" notation forsum = sum + i. It increases the value ofsumby the current value ofi.print(amount)outputs the total amount after the cycle is completely completed.
Flexibility range(): parameters start, stop, step
The range() function can take up to three parameters: start (start), stop (end) and step (step).
range(stop): creates a sequence from0tostop - 1.range(start, stop): creates a sequence fromstarttostop - 1.range(start, stop, step): creates a sequence fromstarttostop - 1instepincrements.
A useful tip: The step may be negative. This allows you to generate sequences in reverse order. In this case, the start value should be greater than stop.
range(5)-> 0, 1, 2, 3, 4range(2, 6)-> 2, 3, 4, 5range(1, 10, 2)-> 1, 3, 5, 7, 9range(5, 0, -1)-> 5, 4, 3, 2, 1
What are increment and decrement?
An increment is simply an increase in the value of a variable, usually by 1. A decrement is a decrease in the value of a variable, usually by 1.
Some programming languages have special operators ++ and --, but Python does not have them. Instead, more explicit and flexible assignment operators with the operation are used.
Increment in Python: operator +=
We can easily increase the value of a variable using +=.
number = 5
# Increasing the value of the variable by 1
number += 1
print(number) # will output 6
the number = 5sets the initial value.number += 1is an elegant shorthand fornumber = number + 1. She says, "take the current value of thenumber, add 1 to it, and write the result back to thenumber."
Decrement in Python: operator -=
Similar to increment, the -= operator is used for decrement.
number = 5
# Reducing the value of a variable by 1
number -= 1
print(number) # will output 4
the number = 5sets the initial value.number -= 1is an abbreviation fornumber = number - 1.
Other useful operators
This principle works with all basic mathematical operations in Python. Instead of the number on the right, there may be another variable (a +=b).
a = 10
a += 5 # Equivalent to a = a + 5 (a will become 15)
a -= 3 # Equivalent to a = a - 3 (a will become 12)
a *= 2 # Equivalent to a = a * 2 (a will become 24)
a /= 4 # Is equivalent to a = a / 4 (a will become 6.0)
a %= 5 # Is equivalent to a = a %5 (a will become 1.0, the remainder of dividing 6 by 5)
a //= 3 # Is equivalent to a = a // 3 (a will become 0.0, integer division of 1 by 3)
Key ideas and useful tips
- The
forloop is designed to iterate through elements in any sequence (lists, strings,range, etc.). - The
range()function is your main tool for creating sequences of numbers for loops. Remember that it does not include an upper bound! - Increment (
+=) and decrement (-=) are the standard and preferred way to change numeric variables in Python. - Meaningful names. Give the loop variables understandable names (
for book in books:), it makes the code much more readable. - Practice is the key to success! Try writing loops to solve different problems: find the sum of even numbers, print the multiplication table, and print a ladder of symbols.
main.py
Test 1
Test 2
Test 3
Test 4
Test 5
Test 6
Test 7
Test 8
Test 9
Test 10
Entwicklerlösung
# Получаем от пользователя начальное количество вирусов и преобразуем в целое число.
total_viruses = int(input())
# Проверяем, есть ли вообще вирусы.
if total_viruses <= 0:
# Если вирусов нет (или введено отрицательное число), выводим сообщение, что все в порядке.
print("Вирусов не обнаружено. Компьютер чист!")
else:
# Если вирусы есть, запускаем процесс удаления.
# Используем цикл for с функцией range(), чтобы пройти от начального количества вирусов до 1.
# range(total_viruses, 0, -1) создает последовательность чисел от total_viruses до 1 с шагом -1.
for i in range(total_viruses, 0, -1):
# Внутри цикла выводим сообщение о том, сколько вирусов осталось после "удаления" текущего.
# i-1, потому что i - это текущее количество, а после удаления останется на один меньше.
print(f"Удален вирус. Осталось: {i - 1}")
# После того как цикл завершится (все вирусы удалены), выводим финальное сообщение.
print("Все вирусы удалены. Система в безопасности!")
