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什么是MapReduce? Google的分布运算开发工具MapReduce，用于大规模数据集（大于1TB）的并行运算! 可以定义一些函数，对批量数据进行操作。比如可以定义使每个数据乘以二的函数，这样，可以并行执行，而且可以分发到不同服务器进行分布式运算。而且，在某些时候，即使是单机运算也有不小的意义，比如求平均数时很容易就可以实现，相邻数相加的运算，可以使参与运算的项快速减少一半。如果采用普通算法逐个运算。首先，如果一直加下去，很容易溢出，如果采用边加边计算，逐项求平均，很容易造成精确度丢失，…
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