重读《Programming Pearls》之七：The Back of The Envelope

封底计算

涉及的是日常生活中的估计问题，比较常用。

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Basic Skills：

1、Two answers are better than one.

2、Quick checks. 根据波利亚的《How To Solve It》，量纲检测法、舍九法等

3、Rules of Thumb（经验法则）.

"Rule of 72":Assume that you invest a sum of money for y years at an interest rate of r percent per year. The financial version of the rule says that if r*y=72, then your money will roughly double.

72法则：假如你投入一笔钱，时间是y年，每年的利率是r%。则当r*y=72时，钱将翻倍。

4、Practice.这个基本上市不变的真理了（虽然说只有改变时不会变的）

Performance Estimates.性能估计

Safty Factors.安全系数

Little‘s Law:The average number of things in the system is the product of the average rate at which things leave the system and the average time each one spends in the system.

队列中的平均物体数量是进入量和平均滞留时间的乘积。

Principles

Everything should be made as simple as possible, but no simpler.

相关：

有一本书叫《How To Lie With Statistics》

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