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exercise4.变量和命名

1.给每一行代码加上注释，给自己解释一下这一行的作用。 2.倒着读你的.py 文件。 3.朗读你的.py 文件，将每个字符朗读出来。

cars = 100
space_in_a_car = 4.0
drivers = 30
passengers = 90
cars_not_driven = cars - drivers
cars_driven = drivers
carpool_capacity = cars_driven * space_in_a_car
average_passengers_per_car = passengers / cars_driven

print "There are", cars, "cars available."
print "There are only", drivers, "drivers available."
print "There will be", cars_not_driven, "empty cars today."
print "We can transport", carpool_capacity, "people today."
print "We have", passengers, "to carpool today."
print "We need to put about", average_passengers_per_car, "in each car."

NOTE: space_in_a_car 中的_是下划线。你要自己学会怎样打出这个字符来。这个符号在变量里通常被用作假想的空格，用来隔开单词。

你应该看到的结果

\$ python ex4.py
There are 100 cars available.
There are only 30 drivers available.
There will be 70 empty cars today.
We can transport 120.0 people today.
We have 90 to carpool today.
We need to put about 3 in each car.

附加题

Traceback (most recent call last):
File "ex4.py", line 8, in <module>
average_passengers_per_car = car_pool_capacity / passenger
NameError: name 'car_pool_capacity' is not defined

1.我在程序里用了 4.0 作为space_in_a_car的值，这样做有必要吗？如果只用 4 会有什么问题? 2.记住 4.0 是一个浮点数，自己研究一下这是什么意思。浮点数是带有小数点的数字。 3.在每一个变量赋值的上一行加上一行注释。 4.记住=的名字是等于(equal)，它的作用是为东西取名。 5.记住_是下划线字符(underscore)。 6.将 python 作为计算器运行起来，就跟以前一样，不过这一次在计算过程中使用变量名来做计算，常见的变量名有 i, x, j 等等。

常见问题

Q: = (单等号)和==(双等号)之间的区别?

= (单等号) 用来赋值，==(双等号)用来判断等号两边的值是否相等。你会在 27 节习题里学到这些。