# 第3章 条件判断与循环

## 3.1 条件判断

```python
age = 18
if age < 18:
    print("未成年")
elif age < 60:
    print("成年人")
else:
    print("老年人")
```

### 比较运算符

| 运算符 | 含义 |
|--------|------|
| `==` | 等于 |
| `!=` | 不等于 |
| `>` | 大于 |
| `<` | 小于 |

### 逻辑运算符

```python
if age >= 18 and age <= 60:
    print("劳动年龄")

if age < 12 or age > 65:
    print("优惠票")
```

---

## 3.2 for 循环

```python
fruits = ["苹果", "香蕉", "橙子"]
for fruit in fruits:
    print(f"我喜欢吃{fruit}")
```

### range() 函数

```python
for i in range(5):
    print(i)  # 0 1 2 3 4

for i in range(1, 6):
    print(i)  # 1 2 3 4 5
```

---

## 3.3 while 循环

```python
total = 0
i = 1
while i <= 100:
    total += i
    i += 1
print(total)  # 5050
```

---

## 3.4 break 和 continue

```python
for i in range(10):
    if i == 5:
        break
    print(i)  # 0 1 2 3 4

for i in range(10):
    if i % 2 == 0:
        continue
    print(i)  # 1 3 5 7 9
```

---

## 本章练习

1. 编写程序，判断一个年份是否为闰年
2. 打印100以内的所有质数
3. 编写程序，计算斐波那契数列的前20项