sync from local backup

media/learn/courses/12/chapters/48.md

@@ -0,0 +1,89 @@
+# 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