Like filter , map evaluates the string passed as its second argument, passing each list element in turn, via v: val.
和filter一样，map计算作为第二个参数传递的字符串，并且通过v： val传递每个列表元素。
The call to map walks through each value, aliases it to v: val, evaluates the expression in the string, and replaces the value with the result of that expression.
调用map可以遍历各个值，将其作为别名赋给v： val，在字符串中计算表达式，并用表达式结果替换值。
The filter function works much the same way as for lists, except that in addition to testing each entry's value using v: val, you can also test its key using v: key. For example.
filter函数的工作方法和列表中的相同，除了用v： val来检测各条目的值，您还可以用v： key来检测它的键。
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