所以，在程序开始遍历这些文件的时候，假设为了确定目录的总大小，就必须逐一重新查询文件系统。
So when the program starts to loop through the files, perhaps to determine the directories overall size, it has to requery the file system one by one.
假设一个不同的场景，系统要遍历相同的集合，但是要执行不同的操作。
Given a different scenario, the system would traverse the same collection but perform a different action.
例如，假设您有一个应用程序，它必须遍历数据库中大量行。
For example, imagine that you have an application that must iterate through a very large number of rows in a database.
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