习题 3: 数字和数学计算
每一种编程语言都包含处理数字和进行数学计算的方法。不必担心，程序员经常撒谎说他们是多么牛的\ 数学天才，其实他们根本不是。如果他们真是数学天才，他们早就去从事数学相关的行业了，而不是\ 写写广告程序和社交网络游戏，从人们身上偷赚点小钱而已。
这章练习里有很多的数学运算符号。我们来看一遍它们都叫什么名字。你要一边写一边念出它们的\ 名字来，直到你念烦了为止。名字如下：
+plus 加号-minus 减号/slash 斜杠*asterisk 星号%percent 百分号<less-than 小于号>greater-than 大于号<=less-than-equal 小于等于号>=greater-than-equal 大于等于号
有没有注意到以上只是些符号，没有运算操作呢？写完下面的练习代码后，再回到上面的列表，写出每\
个符号的作用。例如 + 是用来做加法运算的。
print "I will now count my chickens:"
print "Hens", 25 + 30 / 6
print "Roosters", 100 - 25 * 3 % 4
print "Now I will count the eggs:"
print 3 + 2 + 1 - 5 + 4 % 2 - 1 / 4 + 6
print "Is it true that 3 + 2 < 5 - 7?"
print 3 + 2 < 5 - 7
print "What is 3 + 2?", 3 + 2
print "What is 5 - 7?", 5 - 7
print "Oh, that's why it's False."
print "How about some more."
print "Is it greater?", 5 > -2
print "Is it greater or equal?", 5 >= -2
print "Is it less or equal?", 5 <= -2
在你运行之前完全确保你键入的字符同上。把你敲的每行代码都同我的做一下对比。
What You Should See
$ python ex3.py
I will now count my chickens:
Hens 30
Roosters 97
Now I will count the eggs:
7
Is it true that 3 + 2 < 5 - 7?
False
What is 3 + 2? 5
What is 5 - 7? -2
Oh, that's why it's False.
How about some more.
Is it greater? True
Is it greater or equal? True
Is it less or equal? False
Study Drills
- 使用
#在代码每一行的前一行为自己写一个注解，说明一下这一行的作用。 - 记得开始时的 <练习 0> 吧？用里边的方法把 Python 运行起来，然后使用刚才学到的运算符号，把Python当做计算器玩玩。
- 自己找个想要计算的东西，写一个
.py文件把它计算出来。 - 有没有发现计算结果是"错"的呢？计算结果只有整数，没有小数部分。研究一下这是为什么，搜索一下“浮点数(floating point number)”是什么东西。
- 使用浮点数重写一遍
ex3.py，让它的计算结果更准确(提示: 20.0 是一个浮点数)。
Common Student Questions
Why is the % character a "modulus" and not a "percent"? |
|---|
| Mostly that's just how the designers chose to use that symbol. In normal writing you are correct to read it as a "percent." In programming this calculation is typically done with simple division and the / operator. The % modulus is a different operation that just happens to use the % symbol. |
How does % work? |
Another way to say it is, "X divided by Y with J remaining." For example, "100 divided by 16 with 4 remaining." The result of % is the J part, or the remaining part. |
| What is the order of operations? |
| In the United States we use an acronym called PEMDAS which stands for Parentheses Exponents Multiplication Division Addition Subtraction. That's the order Python follows as well. |
Why does / (divide) round down? |
It's not really rounding down; it's just dropping the fractional part after the decimal. Try doing 7.0 / 4.0 and compare it to 7 / 4 and you'll see the difference. |
Copyright (C) 2010 by
Author: Zed Shaw
Translator：Zander Wong