6.1 for循环

---

文章目录

• 前言
• 一、遍历序列类型
• • [1. 遍历字符串](#1. 遍历字符串)
  • [2. 遍历列表](#2. 遍历列表)
  • [3. 遍历元组](#3. 遍历元组)
• 二、遍历字典
• • [1. 基本遍历方式](#1. 基本遍历方式)
  • [2. 字典遍历的实用技巧](#2. 字典遍历的实用技巧)
• 三、range()函数的使用
• • [1. range()的基本用法](#1. range()的基本用法)
  • [2. range()的实际应用](#2. range()的实际应用)
  • [3. range()的进阶用法](#3. range()的进阶用法)
• 四、循环控制语句
• • [1. break语句](#1. break语句)
  • [2. continue语句](#2. continue语句)
  • [3. break和continue结合使用](#3. break和continue结合使用)

---

前言

本文主要介绍遍历序列类型、遍历字典、range（）函数的使用和循环控制语句等知识点。

---

一、遍历序列类型

1. 遍历字符串

python
# 遍历字符串中的每个字符
text = "Python"

# 基本遍历
for char in text:
    print(char)
# 输出：P y t h o n（每个字符占一行）

# 同时获取索引和字符
for index, char in enumerate(text):
    print(f"索引 {index}: 字符 '{char}'")
# 输出：
# 索引 0: 字符 'P'
# 索引 1: 字符 'y'
# 索引 2: 字符 't'
# ...

# 统计字符串中的元音字母
text = "Hello World"
vowels = "aeiouAEIOU"
vowel_count = 0

for char in text:
    if char in vowels:
        vowel_count += 1
        print(f"找到元音: {char}")

print(f"总共找到 {vowel_count} 个元音字母")

2. 遍历列表

python
# 基本遍历
fruits = ["apple", "banana", "orange", "grape"]

print("水果列表：")
for fruit in fruits:
    print(f"- {fruit}")

# 使用enumerate获取索引和值
print("\n带索引的水果列表：")
for i, fruit in enumerate(fruits):
    print(f"{i+1}. {fruit}")

# 修改列表元素
numbers = [1, 2, 3, 4, 5]
print("\n原列表：", numbers)

for i in range(len(numbers)):
    numbers[i] = numbers[i] * 2  # 每个元素乘以2

print("修改后：", numbers)  # [2, 4, 6, 8, 10]

# 处理嵌套列表
matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
print("\n矩阵遍历：")
for row in matrix:
    for element in row:
        print(element, end=" ")
    print()  # 换行
# 输出：
# 1 2 3
# 4 5 6
# 7 8 9

3. 遍历元组

python
# 元组遍历与列表类似
coordinates = ((0, 0), (1, 2), (3, 4), (5, 6))

print("坐标点：")
for point in coordinates:
    print(f"({point[0]}, {point[1]})")

# 元组解包遍历
print("\n解包遍历：")
for x, y in coordinates:
    print(f"x={x}, y={y}")

# 多个序列同时遍历（使用zip）
names = ("Alice", "Bob", "Charlie")
ages = (25, 30, 35)

print("\n姓名和年龄：")
for name, age in zip(names, ages):
    print(f"{name}: {age}岁")

二、遍历字典

1. 基本遍历方式

python
student = {
    "name": "张三",
    "age": 18,
    "grade": "高三",
    "scores": {"数学": 90, "语文": 85, "英语": 92}
}

# 1. 遍历键（默认方式）
print("键的遍历：")
for key in student:
    print(key)
# 等价于：for key in student.keys():

# 2. 遍历值
print("\n值的遍历：")
for value in student.values():
    print(value)

# 3. 遍历键值对（最常用）
print("\n键值对遍历：")
for key, value in student.items():
    print(f"{key}: {value}")

# 4. 使用enumerate获取序号
print("\n带序号的遍历：")
for i, (key, value) in enumerate(student.items()):
    print(f"{i+1}. {key} = {value}")

2. 字典遍历的实用技巧

python
# 示例：统计单词频率
text = "apple banana apple orange banana apple mango"
words = text.split()

word_count = {}
for word in words:
    # 如果单词已存在，计数+1；否则初始化为1
    word_count[word] = word_count.get(word, 0) + 1

print("单词频率统计：")
for word, count in word_count.items():
    print(f"{word}: {count}次")

# 示例：筛选字典内容
inventory = {
    "apple": {"price": 5.0, "quantity": 100},
    "banana": {"price": 3.0, "quantity": 50},
    "orange": {"price": 4.0, "quantity": 0},
    "grape": {"price": 8.0, "quantity": 20}
}

print("\n库存充足的商品：")
for item, details in inventory.items():
    if details["quantity"] > 0:
        print(f"{item}: {details['quantity']}个，单价{details['price']}元")

# 示例：字典推导式与遍历结合
scores = {"Math": 90, "English": 85, "Science": 88, "History": 78}

# 找出分数大于85的科目
high_scores = {subject: score for subject, score in scores.items() if score > 85}
print("\n高分科目：", high_scores)

三、range()函数的使用

range()函数生成一个整数序列，常用于控制循环次数。

1. range()的基本用法

python
# 1. range(stop) - 从0到stop-1
print("range(5):")
for i in range(5):
    print(i, end=" ")  # 0 1 2 3 4

# 2. range(start, stop) - 从start到stop-1
print("\n\nrange(2, 7):")
for i in range(2, 7):
    print(i, end=" ")  # 2 3 4 5 6

# 3. range(start, stop, step) - 从start到stop-1，步长为step
print("\n\nrange(1, 10, 2):")
for i in range(1, 10, 2):
    print(i, end=" ")  # 1 3 5 7 9

print("\n\nrange(10, 0, -1):")
for i in range(10, 0, -1):
    print(i, end=" ")  # 10 9 8 7 6 5 4 3 2 1

2. range()的实际应用

python
# 1. 生成序列并转换为列表
numbers = list(range(10))  # [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
even_numbers = list(range(0, 20, 2))  # [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

print("偶数列表：", even_numbers)

# 2. 计算累加和
total = 0
for i in range(1, 101):  # 1到100
    total += i
print(f"1到100的和：{total}")  # 5050

# 3. 生成乘法表
print("\n九九乘法表：")
for i in range(1, 10):  # 1-9
    for j in range(1, i+1):  # 1-i
        print(f"{j}×{i}={i*j:2}", end="  ")
    print()  # 换行

# 4. 列表索引遍历（常用模式）
fruits = ["apple", "banana", "orange", "grape", "mango"]

print("\n列表索引遍历：")
for i in range(len(fruits)):
    print(f"索引 {i}: {fruits[i]}")

# 更Pythonic的方式（使用enumerate）
print("\n使用enumerate：")
for i, fruit in enumerate(fruits):
    print(f"索引 {i}: {fruit}")

3. range()的进阶用法

python
# 1. 生成特定范围的浮点数
# range()不支持浮点数，但可以模拟
start = 0.0
stop = 1.0
step = 0.1

float_range = []
current = start
while current < stop:
    float_range.append(round(current, 1))
    current += step

print("浮点数范围：", float_range)

# 或者使用numpy的arange（需要安装numpy）
# import numpy as np
# float_range = np.arange(0.0, 1.0, 0.1)

# 2. 生成日期范围（使用datetime）
from datetime import datetime, timedelta

start_date = datetime(2023, 1, 1)
end_date = datetime(2023, 1, 10)

print("\n日期范围：")
for i in range((end_date - start_date).days + 1):
    current_date = start_date + timedelta(days=i)
    print(current_date.strftime("%Y-%m-%d"))

# 3. 倒序遍历列表
numbers = [10, 20, 30, 40, 50]

print("\n倒序遍历列表：")
for i in range(len(numbers)-1, -1, -1):
    print(f"索引 {i}: {numbers[i]}")

# 更简单的方式：使用reversed()
print("\n使用reversed()：")
for num in reversed(numbers):
    print(num)

四、循环控制语句

1. break语句

break用于完全终止循环，跳出循环体。

python
# 示例1：找到第一个满足条件的元素
numbers = [23, 45, 12, 67, 89, 34, 56]
target = 67

print("查找数字 67：")
for num in numbers:
    print(f"检查: {num}")
    if num == target:
        print(f"找到了！")
        break  # 找到后立即停止循环
else:
    print("未找到")  # 如果循环正常结束（没被break中断）则执行

# 示例2：用户登录尝试限制
max_attempts = 3
correct_password = "123456"

for attempt in range(1, max_attempts + 1):
    password = input(f"第{attempt}次尝试，请输入密码：")
    
    if password == correct_password:
        print("登录成功！")
        break  # 密码正确，跳出循环
    
    print(f"密码错误，还剩{max_attempts - attempt}次机会")
else:
    # 循环正常结束（所有尝试都失败）
    print("密码错误次数过多，账户已锁定！")

# 示例3：嵌套循环中的break
print("\n搜索二维列表：")
matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]
target = 5
found = False

for i, row in enumerate(matrix):
    for j, value in enumerate(row):
        print(f"检查 matrix[{i}][{j}] = {value}")
        if value == target:
            print(f"找到 {target} 在位置 ({i}, {j})")
            found = True
            break  # 只跳出内层循环
    
    if found:
        break  # 跳出外层循环

2. continue语句

continue用于跳过当前循环的剩余语句，直接进入下一次循环。

python
# 示例1：跳过特定元素
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

print("1-10中的奇数：")
for num in numbers:
    if num % 2 == 0:  # 如果是偶数
        continue  # 跳过偶数，不执行下面的print
    print(num, end=" ")  # 只打印奇数
# 输出：1 3 5 7 9

# 示例2：处理有效数据
user_inputs = ["123", "abc", "45.6", "78", "xyz", "100"]

print("\n处理数字输入：")
total = 0
for value in user_inputs:
    if not value.isdigit():  # 如果不是纯数字
        print(f"跳过非数字: '{value}'")
        continue
    
    # 只有数字才执行到这里
    num = int(value)
    total += num
    print(f"添加数字: {num}")

print(f"数字总和: {total}")

# 示例3：复杂条件跳过
students = [
    {"name": "Alice", "score": 85, "attendance": 0.9},
    {"name": "Bob", "score": 45, "attendance": 0.8},
    {"name": "Charlie", "score": 92, "attendance": 0.6},  # 出勤率低
    {"name": "David", "score": 78, "attendance": 0.95},
    {"name": "Eve", "score": 88, "attendance": 0.7}      # 出勤率低
]

print("\n合格学生名单：")
for student in students:
    # 跳过不及格的学生
    if student["score"] < 60:
        continue
    
    # 跳过出勤率低于70%的学生
    if student["attendance"] < 0.7:
        continue
    
    # 只处理合格的学生
    print(f"- {student['name']}: 分数{student['score']}, 出勤率{student['attendance']*100}%")

3. break和continue结合使用

python
# 示例：学生成绩处理系统
students = [
    {"name": "Alice", "scores": [85, 90, 78]},
    {"name": "Bob", "scores": [60, 55, 65]},      # 有不及格
    {"name": "Charlie", "scores": [92, 95, 90]},
    {"name": "David", "scores": [45, 50, 40]},    # 多门不及格
    {"name": "Eve", "scores": [88, 85, 90]}
]

print("优秀学生检查：")
for student in students:
    print(f"\n检查 {student['name']} 的成绩: {student['scores']}")
    
    # 检查是否有不及格科目
    has_failed = False
    for score in student["scores"]:
        if score < 60:
            print(f"  发现不及格科目: {score}")
            has_failed = True
            break  # 发现一个不及格就跳出内层循环
    
    # 如果有不及格，跳过该学生
    if has_failed:
        print(f"  {student['name']} 有不及格科目，跳过")
        continue
    
    # 计算平均分
    avg_score = sum(student["scores"]) / len(student["scores"])
    
    # 检查是否优秀（平均分>=85）
    if avg_score >= 85:
        print(f"  {student['name']} 是优秀学生，平均分: {avg_score:.1f}")
    else:
        print(f"  {student['name']} 成绩良好，平均分: {avg_score:.1f}")

---