THEORY AND PRACTICE

Input and Output Data

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For Loop

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Functions and Recursion

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Занятие 3. FOR cycle

Difficulty level:

Task«Cybersecurity»

your friend has broken, and he asked you for help. After quick diagnosis, you found that the system is infected with viruses. To track the treatment process, you decided to write a small program.

the program must first request how many viruses were found.

rn Then it must imitate the operation of the antivirus: to sequentially "remove" one virus, each time reporting how much it remains. When all the viruses are deleted, the program must display that the computer is now safe.

Input format

The initial number of viruses on the computer (integer).

Output format

Example

Input

Output

Remove the virus. It remains: 3
Remove the virus. It remains: 2
Remove the virus. It remains: 1
Remove the virus. It remains: 0
All viruses are removed. Safety system!

Hint

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:

for is the keyword that tells Python that we are starting a cycle.
i is 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 example for student in students:.
in is the keyword that binds the counter variable and the sequence.
range(number) calling a function that creates a sequence of integers from 0 to number - 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 for loop assigns each of these values to the i variable in turn.
print(i) prints the current value of i at 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 for loop goes through each of these numbers, but in this case we don't use the value of the i variable inside the loop. We just use the loop as a counter to run print("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 = 0 initialize the variable amount. This is a critical step.
range(5) creates a sequence 0, 1, 2, 3, 4.
The for loop runs through each number. In the first step, the sum becomes 0 + 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 +=i is a shortened and more "pythonic" notation for sum = sum + i. It increases the value of sum by the current value of i.
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 from 0 to stop - 1.
range(start, stop): creates a sequence from start to stop - 1.
range(start, stop, step): creates a sequence from start to stop - 1 in step increments.

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, 4
range(2, 6) -> 2, 3, 4, 5
range(1, 10, 2) -> 1, 3, 5, 7, 9
range(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 = 5 sets the initial value.
number += 1 is an elegant shorthand for number = number + 1. She says, "take the current value of the number, add 1 to it, and write the result back to the number."

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 = 5 sets the initial value.
number -= 1 is an abbreviation for number = 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 for loop 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

Developer’s solution

# 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!")

🎉 Congratulations! 🎉

You did an excellent job with the task! It was a challenging problem, but you found the correct solution. You are one step closer to mastering programming! Keep up the good work, because every stage you pass makes you even stronger.