for循环与while循环¶
问题69：Python中有哪些循环语句？¶
Python中有两种主要的循环语句：
1. for循环¶
# 基本for循环
fruits = ["苹果", "香蕉", "橙子"]
for fruit in fruits:
print(fruit)
# 使用range()函数
for i in range(5):
print(i) # 0, 1, 2, 3, 4
# 指定范围
for i in range(1, 6):
print(i) # 1, 2, 3, 4, 5
# 指定步长
for i in range(0, 10, 2):
print(i) # 0, 2, 4, 6, 8
2. while循环¶
# 基本while循环
count = 0
while count < 5:
print(count)
count += 1
# 条件循环
password = ""
while password != "123456":
password = input("请输入密码: ")
print("密码正确！")
问题70：for循环的常见用法有哪些？¶
# 1. 遍历列表
numbers = [1, 2, 3, 4, 5]
for num in numbers:
print(num * 2)
# 2. 遍历字符串
text = "Python"
for char in text:
print(char)
# 3. 遍历字典
person = {"name": "张三", "age": 25, "city": "北京"}
for key, value in person.items():
print(f"{key}: {value}")
# 4. 使用enumerate()获取索引
fruits = ["苹果", "香蕉", "橙子"]
for index, fruit in enumerate(fruits):
print(f"{index}: {fruit}")
# 5. 使用zip()遍历多个序列
names = ["张三", "李四", "王五"]
ages = [25, 30, 28]
for name, age in zip(names, ages):
print(f"{name}: {age}岁")
问题71：while循环的常见用法有哪些？¶
# 1. 计数器循环
count = 0
while count < 3:
print(f"第{count + 1}次循环")
count += 1
# 2. 条件循环
import random
target = random.randint(1, 10)
guess = 0
while guess != target:
guess = int(input("猜一个1-10的数字: "))
if guess < target:
print("太小了")
elif guess > target:
print("太大了")
print("猜对了！")
# 3. 无限循环（需要break退出）
while True:
user_input = input("输入'quit'退出: ")
if user_input.lower() == 'quit':
break
print(f"你输入了: {user_input}")
# 4. 文件读取
# 模拟文件读取
lines = ["第一行", "第二行", "第三行"]
index = 0
while index < len(lines):
print(lines[index])
index += 1
问题72：循环中的break和continue如何使用？¶
# break - 跳出整个循环
for i in range(10):
if i == 5:
break
print(i) # 0, 1, 2, 3, 4
# continue - 跳过当前迭代
for i in range(5):
if i == 2:
continue
print(i) # 0, 1, 3, 4
# 实际应用示例
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
for num in numbers:
if num % 2 == 0:
continue # 跳过偶数
if num > 7:
break # 遇到大于7的数就停止
print(num) # 1, 3, 5, 7
问题73：嵌套循环如何使用？¶
# 基本嵌套循环
for i in range(3):
for j in range(3):
print(f"({i}, {j})")
# 打印乘法表
for i in range(1, 10):
for j in range(1, i + 1):
print(f"{j} × {i} = {i * j}", end="\t")
print() # 换行
# 二维列表遍历
matrix = [
[1, 2, 3],
[4, 5, 6],
[7, 8, 9]
]
for row in matrix:
for element in row:
print(element, end=" ")
print()
问题74：循环的else子句如何使用？¶
# for循环的else子句
for i in range(5):
print(i)
else:
print("循环正常结束")
# 如果循环被break中断，else不会执行
for i in range(5):
if i == 3:
break
print(i)
else:
print("这行不会执行")
# while循环的else子句
count = 0
while count < 3:
print(count)
count += 1
else:
print("while循环正常结束")
# 实际应用：查找元素
def find_element(lst, target):
for i, element in enumerate(lst):
if element == target:
print(f"找到元素 {target}，位置: {i}")
break
else:
print(f"未找到元素 {target}")
find_element([1, 2, 3, 4, 5], 3) # 找到元素 3，位置: 2
find_element([1, 2, 3, 4, 5], 6) # 未找到元素 6
问题75：循环的性能优化技巧有哪些？¶
# 1. 使用列表推导式替代简单循环
# 传统方式
squares = []
for i in range(10):
squares.append(i**2)
# 列表推导式
squares = [i**2 for i in range(10)]
# 2. 使用生成器表达式处理大数据
# 传统方式（占用大量内存）
large_list = [i**2 for i in range(1000000)]
# 生成器表达式（节省内存）
large_generator = (i**2 for i in range(1000000))
# 3. 使用enumerate()替代range(len())
# 不推荐
fruits = ["苹果", "香蕉", "橙子"]
for i in range(len(fruits)):
print(f"{i}: {fruits[i]}")
# 推荐
for i, fruit in enumerate(fruits):
print(f"{i}: {fruit}")
# 4. 使用zip()处理多个序列
# 不推荐
names = ["张三", "李四", "王五"]
ages = [25, 30, 28]
for i in range(len(names)):
print(f"{names[i]}: {ages[i]}岁")
# 推荐
for name, age in zip(names, ages):
print(f"{name}: {age}岁")
问题76：循环的实际应用场景有哪些？¶
# 1. 数据处理
def process_scores(scores):
total = 0
count = 0
for score in scores:
if score >= 0: # 过滤无效分数
total += score
count += 1
return total / count if count > 0 else 0
# 2. 文件处理
def count_lines(filename):
try:
with open(filename, 'r', encoding='utf-8') as file:
count = 0
for line in file:
count += 1
return count
except FileNotFoundError:
return 0
# 3. 用户输入验证
def get_valid_input():
while True:
try:
number = int(input("请输入一个正整数: "))
if number > 0:
return number
else:
print("请输入正整数")
except ValueError:
print("请输入有效数字")
# 4. 游戏循环
def number_guessing_game():
import random
target = random.randint(1, 100)
attempts = 0
while attempts < 7: # 最多7次机会
guess = int(input("猜一个1-100的数字: "))
attempts += 1
if guess == target:
print(f"恭喜！你用了{attempts}次猜对了！")
return
elif guess < target:
print("太小了")
else:
print("太大了")
print(f"游戏结束！答案是{target}")
# 5. 数据统计
def analyze_data(data):
stats = {
'count': 0,
'sum': 0,
'max': float('-inf'),
'min': float('inf')
}
for value in data:
stats['count'] += 1
stats['sum'] += value
stats['max'] = max(stats['max'], value)
stats['min'] = min(stats['min'], value)
stats['average'] = stats['sum'] / stats['count']
return stats
问题77：如何避免常见的循环错误？¶
# 1. 避免在循环中修改列表
# 错误做法
numbers = [1, 2, 3, 4, 5]
for i in range(len(numbers)):
if numbers[i] % 2 == 0:
numbers.remove(numbers[i]) # 这会导致索引错误
# 正确做法1：倒序遍历
numbers = [1, 2, 3, 4, 5]
for i in range(len(numbers) - 1, -1, -1):
if numbers[i] % 2 == 0:
numbers.pop(i)
# 正确做法2：创建新列表
numbers = [1, 2, 3, 4, 5]
numbers = [x for x in numbers if x % 2 != 0]
# 2. 避免无限循环
# 错误做法
# count = 0
# while count < 5:
# print(count) # 忘记增加count，导致无限循环
# 正确做法
count = 0
while count < 5:
print(count)
count += 1
# 3. 避免在循环中重复计算
# 错误做法
data = [1, 2, 3, 4, 5]
for i in range(len(data)): # 每次都计算len(data)
print(data[i])
# 正确做法
data = [1, 2, 3, 4, 5]
length = len(data) # 只计算一次
for i in range(length):
print(data[i])
# 或者直接遍历
for item in data:
print(item)
问题78：循环的最佳实践¶
# 1. 使用描述性的变量名
def calculate_total_price(items):
total = 0
for item in items:
total += item['price'] * item['quantity']
return total
# 2. 使用函数封装复杂循环逻辑
def find_duplicates(lst):
seen = set()
duplicates = set()
for item in lst:
if item in seen:
duplicates.add(item)
else:
seen.add(item)
return list(duplicates)
# 3. 使用生成器处理大数据
def read_large_file(filename):
with open(filename, 'r') as file:
for line in file:
yield line.strip()
# 4. 使用适当的循环类型
# 当知道循环次数时使用for循环
for i in range(10):
print(i)
# 当条件不确定时使用while循环
user_input = ""
while user_input.lower() != 'quit':
user_input = input("输入'quit'退出: ")
# 5. 使用循环的else子句
def search_item(items, target):
for i, item in enumerate(items):
if item == target:
return i
else:
return -1
# 6. 合理使用break和continue
def process_numbers(numbers):
result = []
for num in numbers:
if num < 0:
continue # 跳过负数
if num > 100:
break # 遇到大于100的数就停止
result.append(num)
return result
总结¶
循环是编程中的基础概念，要点包括： - for循环：适合已知循环次数或遍历序列 - while循环：适合条件不确定的循环 - break和continue：控制循环流程 - 嵌套循环：处理多维数据 - else子句：循环正常结束时的处理 - 性能优化：使用推导式、生成器等 - 最佳实践：避免常见错误，使用合适的循环类型