了解地图语法 [英] Understanding map syntax

问题描述

我在理解 map 应该如何使用时遇到了一些问题.

I have some problems understanding how the map should be used.

按照这个教程我创建了一个包含以下文字:

Following this tutorial I created a file containing the following text:

[open#apache]
[apache#hadoop]

，我能够毫无错误地加载该文件:

The, I was able to load that file without errors:

a = load 'data/file_name.txt' as (M:map [])

现在，我如何获取所有值"的列表?即

Now, how can I take the list of all the "values"? I.e.

(apache)
(hadoop) 

此外，我刚刚开始学习 Pig，因此每个提示都会非常有帮助.

Furthermore, I have just started to learn Pig, therefore every hints is going to be very helpful.

推荐答案

与地图交互的方式只有一种，那就是使用 # 运算符.为了让它有更多的功能，你必须定义一些UDFs.因此，地图可以真正用于纯猪的唯一方法是:

There is only one way to interact with a map, and that is to use the # operator. In order for it to have more functionality, you'll have to define some UDFs. Therefore the only way a map can really be used in pure pig is like:

B = FOREACH A GENERATE M#'open' ;

将其作为输出生成:

(apache)
()

请注意，# 后面的值是一个带引号的字符串，它不能更改，必须在运行作业之前设置.

Note that the value after the # is a quoted string, it cannot change and must be set before the you run the job.

另外，请注意，它为第二行创建了一个 NULL，因为该映射不包含值为 'open' 的键.这与在两个字符数组键和值的模式上使用 FILTER 略有不同:

Also, notice that is creates a NULL for the second line, because that map does not contain a key with the vaule 'open'. This is slightly different then using FILTER on a schema of two chararrays key and value:

B = FILTER A BY key=='open' ;

产生输出:

(open,apache)

如果只需要值，那么可以简单地通过:

If only the value is desired, then it can be done simply by:

B = FOREACH (FILTER A BY key=='open') GENERATE value ;

产生:

(apache)

如果保留 NULL 很重要，它们也可以通过使用 bincond:

If keeping the NULLs is important, they can also be generated by using a bincond:

B = FOREACH A GENERATE (key=='open'?value:NULL) ;

产生与 M#'open' 相同的输出.

Which produces the same output as M#'open'.

根据我的经验，地图不是很有用，因为它们的限制很大.

From my experience maps are not very useful because of how restrictive they are.

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