05. 练习：变量和赋值运算符

练习：赋值和修改变量

现在该你来使用变量了。这道练习的注释（以 # 开头的行）提供了创建和修改变量的说明。请在每条注释后面根据说明写一行代码。

注意，这段代码使用了科学记数法来定义很大的数字。4.445e8 等于 4.445 * 10 ** 8，也就是 444500000.0。

#代码注释 - 中英对照

单词表：water reservoir 水库；volume：体积；rainfall 降雨；cubic metre 立方米

The current volume of a water reservoir (in cubic metres) 目前水库的蓄水量（单位：立方米）
reservoir_volume = 4.445e8

The amount of rainfall from a storm (in cubic metres) 一场暴雨的降雨量（单位：立方米）
rainfall = 5e6

decrease the rainfall variable by 10% to account for runoff 考虑到流失的水量，将降雨量这个变量降低10%

add the rainfall variable to the reservoir_volume variable 将降雨量与蓄水量相加

increase reservoir_volume by 5% to account for stormwater that flows into the reservoir in the days following the storm 暴雨后的一段时间内，雨水会不断流入水库，考虑这部分水量，需要将水库的蓄水量变量增加 5%

decrease reservoir_volume by 5% to account for evaporation 考虑到雨水蒸发情况，将蓄水量变量减小5%
subtract 2.5e5 cubic metres from reservoir_volume to account for water that's piped to arid regions. 水库里的水会用来浇灌干涸地区，所以需要将蓄水量减少 2.5e5 立方米

print the new value of the reservoir_volume variable 打印蓄水量变量的最新值

Start Quiz:

# The current volume of a water reservoir (in cubic metres)
reservoir_volume = 4.445e8
# The amount of rainfall from a storm (in cubic metres)
rainfall = 5e6

# decrease the rainfall variable by 10% to account for runoff

# add the rainfall variable to the reservoir_volume variable

# increase reservoir_volume by 5% to account for stormwater that flows
# into the reservoir in the days following the storm

# decrease reservoir_volume by 5% to account for evaporation

# subtract 2.5e5 cubic metres from reservoir_volume to account for water
# that's piped to arid regions.

# print the new value of the reservoir_volume variable

更改变量

更改变量会如何影响到用该变量定义的另一个变量？我们来看一个示例。

这是关于山景城人口和人口密度的原始数据。

>>> mv_population = 74728
>>> mv_area = 11.995
>>> mv_density = mv_population/mv_area

现在我们重新定义 mv_population 变量：

（注意：后续代码跟在上面三行代码后面，而非重新开始）

>>> mv_population = 75000