过滤器是一个条件表达式，用于选择行。
A filter is a conditional expression that is used to select rows.
条件中指定的变量必须在设置了断点的行中，否则表达式就没有什么意义！
The variables specified in the condition must be in scope at whatever line you set the breakpoint on, otherwise the expression wouldn't make sense!
在Ruby中，如果条件结构的体只有一行，那么可以编写清单11中所示的表达式。
In Ruby, if the body of a conditional is a single line, you can write expressions like the one shown in Listing 11. Also note that the same conditional could also be written as definition.word.
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