条件语句（if-else）
让程序做决策 —— 根据条件执行不同代码
什么是条件语句？
条件语句让程序根据不同情况执行不同代码，类似于：
- Stata:
if条件筛选 - R:
if-else语句 - 日常生活: "如果下雨就带伞，否则不带"
基本语法：if 语句
1. 简单 if
python
age = 20
if age >= 18:
print("你是成年人")
print("可以投票")
# 输出:
# 你是成年人
# 可以投票语法要点：
if后面跟条件，以冒号:结尾- 缩进（4个空格或1个Tab）表示代码块
- 缩进的代码只在条件为真时执行
对比 Stata/R：
stata
* Stata (没有真正的 if 语句，只有条件筛选)
gen adult = "是成年人" if age >= 18r
# R
if (age >= 18) {
print("你是成年人")
print("可以投票")
}2. if-else（二选一）
python
age = 16
if age >= 18:
print("你是成年人")
else:
print("你是未成年人")
# 输出: 你是未成年人实战示例：收入分类
python
income = 75000
if income >= 100000:
category = "高收入"
else:
category = "中低收入"
print(f"收入分类: {category}")
# 输出: 收入分类: 中低收入3. if-elif-else（多条件）
python
score = 85
if score >= 90:
grade = "A"
elif score >= 80:
grade = "B"
elif score >= 70:
grade = "C"
elif score >= 60:
grade = "D"
else:
grade = "F"
print(f"成绩等级: {grade}")
# 输出: 成绩等级: B注意：
elif是 "else if" 的缩写- 条件从上到下检查，一旦满足就停止
else是可选的（兜底情况）
对比：Stata vs R vs Python
Stata 方式（生成分类变量）
stata
* Stata
gen income_group = ""
replace income_group = "低收入" if income < 30000
replace income_group = "中收入" if income >= 30000 & income < 80000
replace income_group = "高收入" if income >= 80000R 方式
r
# R
if (income < 30000) {
income_group <- "低收入"
} else if (income < 80000) {
income_group <- "中收入"
} else {
income_group <- "高收入"
}Python 方式
python
# Python
if income < 30000:
income_group = "低收入"
elif income < 80000:
income_group = "中收入"
else:
income_group = "高收入"实战案例
案例 1：问卷数据验证
python
# 受访者数据
age = 25
income = 50000
education_years = 16
# 数据验证
print("=== 数据质量检查 ===")
# 检查年龄
if age < 0 or age > 120:
print(" 年龄数据异常")
elif age < 18:
print("️ 未成年受访者")
else:
print(" 年龄数据正常")
# 检查收入
if income < 0:
print(" 收入数据异常（负数）")
elif income == 0:
print("️ 收入为零，可能失业")
elif income > 1000000:
print("️ 收入过高，需核实")
else:
print(" 收入数据正常")
# 检查教育年限
if education_years < 0 or education_years > 30:
print(" 教育年限异常")
else:
print(" 教育年限正常")案例 2：BMI 健康评估
python
# 计算 BMI
height_m = 1.75
weight_kg = 70
bmi = weight_kg / (height_m ** 2)
# BMI 分类与建议
print(f"你的 BMI: {bmi:.2f}")
if bmi < 18.5:
category = "偏瘦"
advice = "建议增加营养摄入"
elif bmi < 25:
category = "正常"
advice = "保持当前状态"
elif bmi < 30:
category = "超重"
advice = "建议适量运动和控制饮食"
else:
category = "肥胖"
advice = "建议咨询医生，制定减重计划"
print(f"分类: {category}")
print(f"建议: {advice}")输出：
你的 BMI: 22.86
分类: 正常
保持当前状态案例 3：学术表现评估
python
# 学生数据
gpa = 3.7
attendance_rate = 0.95
assignments_completed = 18
total_assignments = 20
# 综合评估
print("=== 学术表现评估 ===")
# GPA 评估
if gpa >= 3.5:
gpa_level = "优秀"
elif gpa >= 3.0:
gpa_level = "良好"
elif gpa >= 2.5:
gpa_level = "中等"
else:
gpa_level = "需要改进"
# 出勤率评估
if attendance_rate >= 0.9:
attendance_level = "优秀"
elif attendance_rate >= 0.75:
attendance_level = "良好"
else:
attendance_level = "需要改进"
# 作业完成率
completion_rate = assignments_completed / total_assignments
if completion_rate >= 0.9:
assignment_level = "优秀"
elif completion_rate >= 0.75:
assignment_level = "良好"
else:
assignment_level = "需要改进"
# 总体评价
if gpa_level == "优秀" and attendance_level == "优秀" and assignment_level == "优秀":
overall = " 优秀学生"
elif gpa_level == "需要改进" or attendance_level == "需要改进":
overall = "️ 需要辅导"
else:
overall = " 合格学生"
print(f"GPA: {gpa} ({gpa_level})")
print(f"出勤率: {attendance_rate*100:.1f}% ({attendance_level})")
print(f"作业完成率: {completion_rate*100:.1f}% ({assignment_level})")
print(f"总体评价: {overall}")嵌套条件语句
条件语句可以嵌套（在 if 内部再有 if）。
python
age = 25
income = 75000
has_degree = True
# 嵌套条件
if age >= 18:
if has_degree:
if income >= 50000:
eligibility = "完全符合"
else:
eligibility = "符合但收入偏低"
else:
eligibility = "符合但需要学历"
else:
eligibility = "年龄不符"
print(f"贷款资格: {eligibility}")
# 输出: 贷款资格: 完全符合更好的写法（避免过度嵌套）：
python
age = 25
income = 75000
has_degree = True
# 用逻辑运算符简化
if age >= 18 and has_degree and income >= 50000:
eligibility = "完全符合"
elif age < 18:
eligibility = "年龄不符"
elif not has_degree:
eligibility = "需要学历"
elif income < 50000:
eligibility = "收入不符"
else:
eligibility = "未知情况"
print(f"贷款资格: {eligibility}")条件表达式（三元运算符）
对于简单的 if-else，可以用一行代码：
python
# 传统写法
age = 20
if age >= 18:
status = "成年人"
else:
status = "未成年人"
# 条件表达式（一行）
age = 20
status = "成年人" if age >= 18 else "未成年人"
print(status) # 成年人语法：
python
value_if_true if condition else value_if_false实用示例：
python
# 收入分类
income = 120000
category = "高收入" if income >= 100000 else "中低收入"
# 及格判断
score = 85
result = "及格" if score >= 60 else "不及格"
# 性别编码
gender = "Male"
gender_code = 1 if gender == "Male" else 0
print(category, result, gender_code)
# 输出: 高收入 及格 1多条件判断技巧
1. 使用 in 简化多个 or
python
# 繁琐写法
major = "Economics"
if major == "Economics" or major == "Finance" or major == "Business":
print("商科专业")
# 简洁写法
major = "Economics"
if major in ["Economics", "Finance", "Business"]:
print("商科专业")2. 使用范围判断
python
# 检查年龄是否在合理范围
age = 25
# Python 支持链式比较
if 18 <= age <= 65:
print("工作年龄人口")
# 等价于（但不推荐）
if age >= 18 and age <= 65:
print("工作年龄人口")3. 提前返回（在函数中）
python
def check_eligibility(age, income, has_job):
# 提前返回不符合的情况
if age < 18:
return "年龄不符"
if income < 30000:
return "收入不符"
if not has_job:
return "需要有工作"
# 所有条件都满足
return "符合条件"
result = check_eligibility(25, 50000, True)
print(result) # 符合条件常见错误
错误 1：忘记冒号
python
if age >= 18 # SyntaxError: 缺少冒号
print("成年人")
if age >= 18: #
print("成年人")错误 2：缩进错误
python
# 缩进不一致
if age >= 18:
print("成年人")
print("可以投票") # IndentationError
# 缩进一致
if age >= 18:
print("成年人")
print("可以投票")错误 3：使用赋值而不是比较
python
age = 18
if age = 18: # SyntaxError: 应该用 ==
print("刚好18岁")
if age == 18: #
print("刚好18岁")错误 4：空 if 块
python
if age >= 18:
# SyntaxError: 不能为空
if age >= 18:
pass # 用 pass 占位完整实战：问卷数据处理
python
# === 问卷数据 ===
respondent_id = 1001
age = 35
gender = "Female"
education = "Master's Degree"
employment_status = "Employed"
annual_income = 85000
marital_status = "Married"
num_children = 2
# === 数据验证与分类 ===
print(f"=== 受访者 {respondent_id} 数据报告 ===\n")
# 1. 年龄组
if age < 25:
age_group = "青年（<25）"
elif age < 45:
age_group = "中青年（25-44）"
elif age < 65:
age_group = "中老年（45-64）"
else:
age_group = "老年（65+）"
print(f"年龄组: {age_group}")
# 2. 教育水平编码
education_levels = {
"High School": 1,
"Associate Degree": 2,
"Bachelor's Degree": 3,
"Master's Degree": 4,
"Doctoral Degree": 5
}
if education in education_levels:
education_code = education_levels[education]
if education_code >= 4:
education_category = "高学历"
elif education_code >= 3:
education_category = "本科"
else:
education_category = "专科及以下"
else:
education_code = 0
education_category = "未知"
print(f"教育水平: {education} ({education_category})")
# 3. 收入分组
if annual_income < 30000:
income_quartile = "Q1（低收入）"
elif annual_income < 60000:
income_quartile = "Q2（中低收入）"
elif annual_income < 100000:
income_quartile = "Q3（中高收入）"
else:
income_quartile = "Q4（高收入）"
print(f"收入分组: ${annual_income:,} ({income_quartile})")
# 4. 家庭结构
if marital_status == "Married" and num_children > 0:
family_type = "已婚有子女"
elif marital_status == "Married":
family_type = "已婚无子女"
elif num_children > 0:
family_type = "单亲家庭"
else:
family_type = "单身"
print(f"家庭结构: {family_type}")
# 5. 目标人群判断（示例：高学历高收入中青年）
is_target_demographic = (
(age_group == "中青年（25-44）") and
(education_category == "高学历") and
(income_quartile in ["Q3（中高收入）", "Q4（高收入）"])
)
if is_target_demographic:
print("\n 符合目标人群特征")
else:
print("\n 不符合目标人群特征")练习题
练习 1：税率计算
python
# 根据收入计算税率
income = 75000
# 税率表：
# 0-50000: 10%
# 50001-100000: 20%
# 100001+: 30%
# 计算应缴税额并输出练习 2：学生奖学金评定
python
gpa = 3.8
volunteer_hours = 50
research_papers = 2
# 奖学金规则：
# - 一等奖学金：GPA >= 3.8 且 (志愿时长 >= 40 或 论文 >= 2)
# - 二等奖学金：GPA >= 3.5
# - 三等奖学金：GPA >= 3.0
# - 无奖学金：其他情况
# 判断奖学金等级练习 3：健康风险评估
python
age = 55
bmi = 28
smoking = True
exercise_days_per_week = 1
family_history = True # 家族病史
# 风险评分规则：
# - 年龄 > 50: +2 分
# - BMI >= 25: +2 分
# - 吸烟: +3 分
# - 运动天数 < 3: +1 分
# - 有家族病史: +2 分
# 计算总分并给出风险等级：
# 0-2: 低风险
# 3-5: 中风险
# 6+: 高风险下一步
在下一节中，我们将学习 循环（for/while），让程序重复执行任务。
继续加油！