说明：  PCA降维实例，使用数据集，数据集衡量美国329个城市生活质量的9个指标：气候、住房、健康、犯罪率、交通、教育、艺术、娱乐和经济。对于各指标，越高表示越好，如高的犯罪指标表示低的犯罪率。
(PCA dimensionality reduction example uses data sets and data sets to measure 9 indicators of quality of life in 329 cities: climate, housing, health, crime rate, transportation, education, art, entertainment and economy. For each index, the higher the better, as the high crime indicator indicates a low rate of crime.)