3. 编写一个程序，接受一个字符串输入，并输出该字符串中包含的单词数量。


"Hello world, how are you doing today?"
这是一个计算字符串中单词数量的示例Python程序：

```python
str = "Hello world, how are you doing today?"
words = str.split()
print("该字符串中包含", len(words), "个单词。")
```

输出：

```
该字符串中包含 7 个单词。
```