crf - 如何制作CRF++的模板文件？

Question

我是CRF++的新手。我在教自己看它的手册:
http://crfpp.googlecode.com/svn/trunk/doc/index.html?source=navbar#templ
我不明白这是什么意思:

This is a template to describe unigram features. When you give a

template "U01:%x[0,1]", CRF++ automatically generates a set of feature

functions (func1 ... funcN) like:

func1 = if (output = B-NP and feature="U01:DT") return 1 else return 0

func2 = if (output = I-NP and feature="U01:DT") return 1 else return 0

func3 = if (output = O and feature="U01:DT") return 1 else return 0

.... funcXX = if (output = B-NP and feature="U01:NN") return 1 else return 0

funcXY = if (output = O and feature="U01:NN") return 1 else return 0. The number of feature functions generated by a template

amounts to (L * N), where L is the number of output

为什么Unigram功能有很多行，它们是什么意思？

Answer 1

在看了the documentation足够长的时间之后，我想我已经明白了。

以文档中的示例为例，输入数据为:

He        PRP  B-NP
reckons   VBZ  B-VP
the       DT   B-NP 
current   JJ   I-NP 
account   NN   I-NP

而相关的功能模板(格式为 %x[row, col]，其中 row相对于您当前位置)为 %x[0,1]
扩展 %x[0,1]时，根据当前 token 的不同，它可以扫描设置的 [PRP, VBZ, DT, JJ, NN]内部的字符串之一(即第1列中唯一的字符串之一，其中最左边的列是第0列)。对于这些字符串中的每一个，它都会创建一组形式如下的特征函数(查看输入数据的第三行):

func1 = if (output = B-NP and feature="U01:DT") return 1 else return 0
func2 = if (output = I-NP and feature="U01:DT") return 1 else return 0
func3 = if (output = O    and feature="U01:DT") return 1 else return 0
...

将该特定字符串(以上代码中的 DT)与每个单个输出类进行比较。

因此，如果输出类是 [B-NP, I-NP, O]，则扩展为要素函数的要素模板将如下所示:

# row 1 (He, PRP, B-NP)
func1 = if (output = B-NP and feature="U01:PRP") return 1 else return 0
func2 = if (output = I-NP and feature="U01:PRP") return 1 else return 0
func3 = if (output = O    and feature="U01:PRP") return 1 else return 0

# row 2 (Reckons, VBZ, B-VP)
func4 = if (output = B-NP and feature="U01:VBZ") return 1 else return 0
func5 = if (output = I-NP and feature="U01:VBZ") return 1 else return 0
func6 = if (output = O    and feature="U01:VBZ") return 1 else return 0

# Row 3 (the, DT, B-NP)
func7 = if (output = B-NP and feature="U01:DT") return 1 else return 0
func8 = if (output = I-NP and feature="U01:DT") return 1 else return 0
func9 = if (output = O    and feature="U01:DT") return 1 else return 0

# Row 4 (current, JJ, I-NP)
func10 = if (output = B-NP and feature="U01:JJ") return 1 else return 0
func11 = if (output = I-NP and feature="U01:JJ") return 1 else return 0
func12 = if (output = O    and feature="U01:JJ") return 1 else return 0

# Row 5 (account, NN, I-NP)
func13 = if (output = B-NP and feature="U01:NN") return 1 else return 0
func14 = if (output = I-NP and feature="U01:NN") return 1 else return 0
func15 = if (output = O    and feature="U01:NN") return 1 else return 0

关于文档中提到的内容:

The number of feature functions generated by a template amounts to (L * N), where L is the number of output classes and N is the number of unique strings expanded from the given template.

在这种情况下，L为3，N为5。