Because if you did what I suggested with the list, the time to look up the key would be linear in the length of the list. You'd have to look at each element until you found the key.

字典是用一种很神奇的，叫做散列法的算法,来实现的，后面我们将,会学到一点关于。

麻省理工公开课 - 计算机科学及编程导论课程节选

And they got in this habit of, every time they had a bright, happy text, they'd set this in one kind of mode or key-- a major mode--and every time they had a sad one, they'd set it in minor.

于是人们养成了这样的习惯,每当人们书写欢乐,愉悦的歌词时,他们就设定一个调式或调性,即大调,每当人们书写忧伤的歌词时,他们就定为小调

耶鲁公开课 - 聆听音乐课程节选