我无法从 xml 中提取节点。 xml_find_all 未按预期工作答案

【问题标题】：I'm having trouble to extract a node from xml. xml_find_all not working as expected我无法从 xml 中提取节点。 xml_find_all 未按预期工作
【发布时间】：2020-03-05 23:34:37
【问题描述】：

我的问题可能相当简单，但我在使用 xml 时遇到了问题。我有一个代谢物列表和一个数据库，我可以在其中找到有关它们的 xml 格式的信息。我正在尝试创建一个同义词表，以便将我必须的代谢物名称翻译成更适合下游分析的名称。这是一个简单的代码，我正在尝试访问同义词节点，但由于某种原因无法正常工作。我成功地尝试了另一个 xml 文件。此外，任何有关如何构建此表的提示都将不胜感激。

library(xml2)

metabolites <- read_xml('<?xml version="1.0" encoding="UTF-8"?>
                    <hmdb xmlns="http://www.hmdb.ca">
                    <metabolite>
                    <version>4.0</version>
                    <creation_date>2005-11-16 15:48:42 UTC</creation_date>
                    <update_date>2019-01-11 19:13:56 UTC</update_date>
                    <accession>HMDB0000001</accession>
                    <status>quantified</status>
                    <secondary_accessions>
                    <accession>HMDB00001</accession>
                    <accession>HMDB0004935</accession>
                    </secondary_accessions>
                    <name>1-Methylhistidine</name>
                    <cs_description>1-Methylhistidine, also known as 1-mhis...</cs_description>
                    <description>One-methylhistidine (1-MHis) is derived ...</description>
                    <synonyms>
                    <synonym>(2S)-2-amino-3-(1-Methyl-1H-imidazol-4-yl)propanoic acid</synonym>
                    <synonym>1-Methylhistidine</synonym>
                    <synonym>Pi-methylhistidine</synonym>
                    <synonym>(2S)-2-amino-3-(1-Methyl-1H-imidazol-4-yl)propanoate</synonym>
                    <synonym>1 Methylhistidine</synonym>
                    </synonyms>
                    <chemical_formula>C7H11N3O2</chemical_formula>
                    <average_molecular_weight>169.1811</average_molecular_weight>
                    </metabolite>
                    </hmdb>')


syn <- xml_find_all(metabolites, "//synonyms")

谢谢！

【问题讨论】：

标签： r xml-parsing xml2

【解决方案1】：

它与命名空间声明有关。请参阅此处的讨论：https://github.com/r-lib/xml2/issues/222

library(xml2)

metabolites <- read_xml('<hmdb xmlns="http://www.hmdb.ca">
<metabolite>
<version>4.0</version>
<creation_date>2005-11-16 15:48:42 UTC</creation_date>
<update_date>2019-01-11 19:13:56 UTC</update_date>
<accession>HMDB0000001</accession>
<status>quantified</status>
<secondary_accessions>
<accession>HMDB00001</accession>
<accession>HMDB0004935</accession>
</secondary_accessions>
<name>1-Methylhistidine</name>
<cs_description>1-Methylhistidine, also known as 1-mhis...</cs_description>
<description>One-methylhistidine (1-MHis) is derived ...</description>
<synonyms>
<synonym>(2S)-2-amino-3-(1-Methyl-1H-imidazol-4-yl)propanoic acid</synonym>
<synonym>1-Methylhistidine</synonym>
<synonym>Pi-methylhistidine</synonym>
<synonym>(2S)-2-amino-3-(1-Methyl-1H-imidazol-4-yl)propanoate</synonym>
<synonym>1 Methylhistidine</synonym>
</synonyms>
<chemical_formula>C7H11N3O2</chemical_formula>
<average_molecular_weight>169.1811</average_molecular_weight>
</metabolite>
</hmdb>')

# namespace d1
xml_ns(metabolites)
#> d1 <-> http://www.hmdb.ca
#doesn't work
xml_find_all(metabolites, "//synonyms")
#> {xml_nodeset (0)}
#works
xml_find_all(metabolites, "//d1:synonyms")
#> {xml_nodeset (1)}
#> [1] <synonyms>\n  <synonym>(2S)-2-amino-3-(1-Methyl-1H-imidazol-4-yl)pro ...

^{由reprex package (v0.3.0) 于 2019 年 11 月 9 日创建}

【讨论】：