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课程 3. FOR cycle
难度：
任务«统计评论数量»
💻 Python
你——热门移动应用“智能脚踝”的开发者。在最近一次更新后，你想通过分析用户在应用商店中的评分来快速评估他们的反应。评分采用5星制。你决定将评分为4星或5星的评论视为正面评价，而评分为1星、2星或3星的评论视为负面评价。
编写一个程序，首先请求输入待分析的评论总数。然后程序应在循环中请求输入每条评论的评分（一个从1到5的整数）。输入所有评分后，程序应计算并显示正面和负面评论的总数。
输入格式
第一行：用于分析的总评论数（整数）。
后续行：每条评论的评分（1到5之间的整数），每行一个。
输出格式
第一行：最终好评数量（文本和整数）。
第二行：最终差评数量（文本和整数）。
示例
输入
5
5
1
4
2
3
输出
正面评价：2
负面评价：3
提示
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
测试 1
测试 2
测试 3
测试 4
测试 5
测试 6
测试 7
测试 8
测试 9
测试 10
开发者解答
# Спрашиваем у пользователя, сколько всего отзывов нужно обработать.
# Функция input() считывает строку, а int() превращает её в целое число.
total_reviews = int(input())
# Создаем переменные-счетчики для положительных и отрицательных отзывов.
# В начале их значения равны нулю.
positive_reviews_count = 0
negative_reviews_count = 0
# Запускаем цикл for, который повторится столько раз, сколько отзывов указал пользователь.
# range(total_reviews) создает последовательность чисел от 0 до total_reviews-1.
for _ in range(total_reviews):
# Внутри цикла на каждой итерации запрашиваем оценку очередного отзыва.
rating = int(input())
# Используем условие if/else для проверки значения оценки.
# Если оценка 4 или 5, то отзыв считается положительным.
if rating >= 4:
# Увеличиваем счетчик положительных отзывов на единицу.
positive_reviews_count += 1
else:
# В противном случае (оценка 1, 2 или 3), отзыв считается отрицательным.
# Увеличиваем счетчик отрицательных отзывов на единицу.
negative_reviews_count += 1
# После того как цикл завершится, выводим итоговые результаты.
# f-строки используются для удобного вывода текста вместе со значениями переменных.
print(f"Положительных отзывов: {positive_reviews_count}")
print(f"Отрицательных отзывов: {negative_reviews_count}")
