Python循环语句：从基础到进阶的实用指南

# 无循环的冗余写法（仅展示3名学生）
score1 = 85
score2 = 90
score3 = 78
total = score1 + score2 + score3
average = total / 3

当学生数量增加到100人时，这种写法显然不现实。循环的作用就是让计算机自动完成重复操作，你只需告诉它"重复做什么"和"重复多少次"。

for 变量 in 序列:
    # 循环体

scores = [85, 90, 78, 92, 88]
total = 0
for score in scores:
    total += score  # 等价于 total = total + score
print("平均分:", total / len(scores))

运行结果：

平均分: 86.6

这里 for 循环依次取出  scores 中的每个分数，累加到 total 变量中，最后计算平均值。

range(start, stop, step)
# start: 起始值（默认0）
# stop: 结束值（不包含）
# step: 步长（默认1）

for i in range(1, 11, 2):  # 从1开始，到10结束（不含），步长2
    print(i, end=" ")

输出：

1 3 5 7 9

word_counts = {"apple": 3, "banana": 2, "orange": 5}
for word, count in word_counts.items():
    print(f"{word}出现了{count}次")

输出：

apple出现了3次
banana出现了2次
orange出现了5次

for i in range(1, 10):      # 外层循环控制行
    for j in range(1, i+1): # 内层循环控制列
        print(f"{j}x{i}={i*j}", end="\t")
    print()  # 每行结束后换行

输出（前3行）：

1x1=1	
1x2=2	2x2=4	
1x3=3	2x3=6	3x3=9

while 条件:
    # 循环体

import random
target = random.randint(1, 100)
guess = None

while guess != target:
    guess = int(input("猜一个1-100的数字: "))
    if guess < target:
        print("太小了！")
    elif guess > target:
        print("太大了！")
print("恭喜猜中！")

这个游戏中，while 循环会持续询问用户输入，直到猜中正确数字。

while True:  # 缺少退出条件
    print("无限循环中...")

count = 0
while True:
    print("当前计数:", count)
    count += 1
    if count >= 5:  # 当计数达到5时退出
        break

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9]
for num in numbers:
    if num % 2 != 0:  # 如果是奇数
        continue      # 跳过本次循环
    print(num, end=" ")

输出：

2 4 6 8

num = int(input("输入一个数字检查是否为质数: "))
i = 2
is_prime = True
while i * i <= num:  # 只需检查到√num
    if num % i == 0:
        is_prime = False
        break
    i += 1
print("是质数" if is_prime else "不是质数")

[表达式 for 变量 in 序列]

squares = [x**2 for x in range(1, 6)]
print(squares)  # 输出: [1, 4, 9, 16, 25]

even_squares = [x**2 for x in range(1, 11) if x % 2 == 0]
print(even_squares)  # 输出: [4, 16, 36, 64, 100]

fruits = ["apple", "banana", "orange"]
for index, fruit in enumerate(fruits):
    print(f"索引{index}: {fruit}")

索引0: apple
索引1: banana
索引2: orange

names = ["Alice", "Bob", "Charlie"]
scores = [85, 90, 78]
for name, score in zip(names, scores):
    print(f"{name}的成绩是{score}")

输出：

Alice的成绩是85
Bob的成绩是90
Charlie的成绩是78

for i in range(1000):
    result = i * (len(some_list) + 1)  # len()在每次循环都计算

list_length = len(some_list) + 1
for i in range(1000):
    result = i * list_length  # 只需计算一次

list1 = [1, 2, 3, 4, 5]
list2 = [4, 5, 6, 7, 8]
common = []
for x in list1:
    for y in list2:
        if x == y:
            common.append(x)

set1 = set(list1)
set2 = set(list2)
common = list(set1 & set2)  # 交集操作

def read_large_file(file_path):
    with open(file_path, 'r') as file:
        for line in file:  # 逐行读取，不占用大量内存
            yield line.strip()

for line in read_large_file("huge_file.txt"):
    process(line)  # 处理每一行

import requests
from bs4 import BeautifulSoup

def extract_links(url):
    response = requests.get(url)
    soup = BeautifulSoup(response.text, 'html.parser')
    links = []
    for link in soup.find_all('a'):  # 查找所有<a>标签
        href = link.get('href')
        if href and href.startswith('http'):
            links.append(href)
    return links

website_links = extract_links("https://example.com")
for link in website_links[:5]:  # 打印前5个链接
    print(link)

这个案例展示了如何用 for 循环遍历HTML标签，提取网页中的所有外部链接。

import pandas as pd

# 模拟数据
data = {
    'Name': ['Alice', 'Bob', 'Charlie', 'David'],
    'Math': [85, 90, 78, 92],
    'English': [88, 92, 85, 89]
}
df = pd.DataFrame(data)

# 计算每门课的平均分
for subject in ['Math', 'English']:
    avg_score = df[subject].mean()
    print(f"{subject}平均分: {avg_score:.2f}")

# 找出所有科目都高于85分的学生
high_achievers = []
for _, row in df.iterrows():  # 遍历DataFrame的每一行
    if all(row[1:] > 85):  # 检查Math和English列
        high_achievers.append(row['Name'])
print("优秀学生:", high_achievers)

输出：

Math平均分: 86.25
English平均分: 88.50
优秀学生: ['Bob', 'David']