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Lektion 3. FOR cycle
Schwierigkeit:
Aufgabe«Geldautomat»
💻 Python
Stellen Sie sich vor, Sie richten ein Programm für einen Geldautomaten ein. Aus Sicherheitsgründen fordert der Geldautomat den Benutzer bei der Eingabe des persönlichen Kontos auf, einen bestimmten Satz eine bestimmte Anzahl von Malen zu wiederholen. Um den Kunden jedoch nicht zu langweilen, hat das Sicherheitssystem eine Grenze festgelegt: Sie können nicht verlangen, den Satz mehr als 10 Mal zu wiederholen.
rnIhre Aufgabe — Schreiben Sie ein Programm, das:
rn- rn
- den Satz liest, der wiederholt werden soll. rn
- die Anzahl der erforderlichen Wiederholungen liest. rn
- überprüft, ob die Anzahl der Wiederholungen 10 oder weniger beträgt. rn
- wenn die Anzahl der Wiederholungen 10 oder weniger beträgt, gibt das Programm den angegebenen Satz die angegebene Anzahl von Malen aus. Jede Wiederholung soll in einer neuen Zeile angezeigt werden. rn
- wenn die Anzahl der Wiederholungen mehr als 10 beträgt, muss das Programm eine Fehlermeldung ausgeben: "Error: the number of repetitions cannot exceed 10". rn
Eingabeformat
Erste Zeile: der vom Geldautomaten festgelegte Satz (Typ: Zeile).
Zweite Zeile: die Anzahl der Wiederholungen des Satzes (Typ: Ganzzahl).
Ausgabeformat
Wenn die Anzahl der Wiederholungen 10 nicht überschreitet: eine gegebene Phrase, die die richtige Anzahl von Malen wiederholt wird. Jede Phrase in einer neuen Zeile (Typ: Zeile).
Wenn die Anzahl der Wiederholungen 10 überschreitet: Fehlermeldung (Typ: Zeile).
Beispiel
Eingabe
Ich bestätige
3
Ausgabe
Ich bestätige
Ich bestätige
Ich bestätige
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
Entwicklerlösung
# Запрашиваем у пользователя фразу, которую нужно будет повторить.
# Функция input() считывает одну строку текста.
secret_phrase = input()
# Запрашиваем количество повторений.
# Функция input() считывает строку, а int() превращает её в целое число.
repeat_count = int(input())
# Это условие. Проверяем, больше ли введенное число, чем 10.
if repeat_count > 10:
# Если условие истинно (число больше 10), выводим сообщение об ошибке.
print("Ошибка: количество повторений не может превышать 10")
else:
# Если условие ложно (число 10 или меньше), выполняется эта часть кода.
# Запускаем цикл for. Он будет работать столько раз, сколько указано в repeat_count.
# range(repeat_count) создает последовательность чисел от 0 до repeat_count-1.
for i in range(repeat_count):
# На каждом шаге цикла (итерации) выводим секретную фразу.
print(secret_phrase)
