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Leçon 3. FOR cycle
Difficulté :
Exercice«Distributeur»
💻 Python
Imaginez que vous configurez un programme pour un distributeur automatique de billets. Pour des raisons de sécurité, lors de la saisie du compte personnel, le distributeur demande à l'utilisateur de répéter une phrase spécifique un nombre de fois déterminé. Cependant, afin de ne pas ennuyer le client, le système de sécurité a fixé une limite : vous ne pouvez pas demander de répéter la phrase plus de 10 fois.
Votre tâche — Écrire un programme qui :
- lit la phrase à répéter.
- lit le nombre de répétitions requises.
- vérifie si le nombre de répétitions dépasse 10.
- si le nombre de répétitions est de 10 ou moins, le programme affiche la phrase donnée le nombre de fois indiqué. Chaque répétition doit être affichée sur une nouvelle ligne.
- si le nombre de répétitions est supérieur à 10, le programme doit afficher un message d'erreur : "Error: the number of repetitions cannot exceed 10".
Format d entrée
Première ligne : la phrase définie par le distributeur automatique (type : ligne).
Deuxième ligne : le nombre de fois que vous devez répéter la phrase (type : nombre).
Format de sortie
Si le nombre de répétitions ne dépasse pas 10 : une phrase donnée, répétée le bon nombre de fois. Chaque phrase sur une nouvelle ligne (type : ligne).
Si le nombre de répétitions dépasse 10 : message d'erreur (type : ligne).
Exemple
Entrée
Je confirme
3
Sortie
Je confirme
Je confirme
Je confirme
Indice
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
Solution du développeur
# 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)
