我不是XML专家。XML使用 . 解析文件时遇到问题rentrez。我试图将每个 pmid（PubMed 数据库中的文章 ID）的作者和隶属关系作为输出。我的代码运行良好，除非作者有多个从属关系。当作者有多个从属关系时，列的长度first_names、、 last_names和affiliation变得不同，它会返回错误。我真的没有 xml 解析方面的专业知识来处理这个问题。我严格期待如下结果：
pmid first_names last_names affiliation
27869504 Luca Villa Division of Experimental Oncology/Unit of Urology, URI , IRCCS Ospedale San Raffaele, Milan, Italy
27869504 Luca Villa Department of Urology, Tenon Hospital, Pierre and Marie Curie University , Paris, France
27869504 Tarik Emre Şener Department of Urology, Tenon Hospital, Pierre and Marie Curie University , Paris, France
27869504 Tarik Emre Şener Department of Urology, Marmara University School of Medicine, Istanbul, Turkey
entrez_fetch 返回的示例 XML 文件的结构如下：
<?xml version="1.0"?>
<!DOCTYPE PubmedArticleSet PUBLIC "-//NLM//DTD PubMedArticle, 1st January 2017//EN" "https://dtd.nlm.nih.gov/ncbi/pubmed/out/pubmed_170101.dtd">
<PubmedArticleSet>
<PubmedArticle>
<MedlineCitation Status="In-Data-Review" Owner="NLM">
<PMID Version="1">27869504</PMID>
<DateCreated>
<Year>2016</Year>
<Month>11</Month>
<Day>21</Day>
</DateCreated>
<DateRevised>
<Year>2017</Year>
<Month>01</Month>
<Day>06</Day>
</DateRevised>
<Article PubModel="Print-Electronic">
<Journal>
<ISSN IssnType="Electronic">1557-900X</ISSN>
<JournalIssue CitedMedium="Internet">
<Volume>31</Volume>
<Issue>1</Issue>
<PubDate>
<Year>2017</Year>
<Month>Jan</Month>
</PubDate>
</JournalIssue>
<Title>Journal of endourology</Title>
<ISOAbbreviation>J. Endourol.</ISOAbbreviation>
</Journal>
<ArticleTitle>Initial Content Validation Results of a New Simulation Model for Flexible Ureteroscopy: The Key-Box.</ArticleTitle>
<Pagination>
<MedlinePgn>72-77</MedlinePgn>
</Pagination>
<ELocationID EIdType="doi" ValidYN="Y">10.1089/end.2016.0677</ELocationID>
<Abstract>
<AbstractText Label="PURPOSE" NlmCategory="OBJECTIVE">We sought to test the content validity of a new training model for flexible ureteroscopy: the Key-Box.</AbstractText>
<AbstractText Label="MATERIAL AND METHODS" NlmCategory="METHODS">Sixteen medical students were randomized to undergo a 10-day training consisting of performing 10 different exercises aimed at learning specific movements with the flexible ureteroscope, and how to catch and release stones with a nitinol basket using the Key-Box (n = 8 students in the training group, n = 8 students in the nontraining control group). Subsequently, an expert endourologist (O.T.) blindly assessed skills acquired by the whole cohort of students through two exercises on ureteroscope manipulation and one exercise on stone capture selected among those used for the training. A performance scale (1-5) assessing different steps of the procedure was used to evaluate each student. Time to complete the exercises was measured. Mann-Whitney Rank Sum test was used for comparisons between the two groups.</AbstractText>
<AbstractText Label="RESULTS" NlmCategory="RESULTS">Mean scores obtained by trained students were significantly higher compared with those obtained by nontrained students (all p < 0.001). All trained students were able to complete the two exercises on ureteroscope manipulation within 3 minutes, whereas two students (25%) were not able to finish the exercise on stone capture. Conversely, four (50%) and six (75%) nontrained students were not able to finish one out of the two exercises on ureteroscope manipulation and the exercise on stone capture, respectively. The mean time to complete the three exercises was 76.3, 69.9, and 107 and 172.5, 137.9, and 168 seconds in the trained and nontrained groups, respectively (all p < 0.001).</AbstractText>
<AbstractText Label="CONCLUSIONS" NlmCategory="CONCLUSIONS">The K-Box(®) seems to be a valid easy-to-use training model for initiating novel endoscopists to flexible ureteroscopy.</AbstractText>
</Abstract>
<AuthorList CompleteYN="Y">
<Author ValidYN="Y">
<LastName>Villa</LastName>
<ForeName>Luca</ForeName>
<Initials>L</Initials>
<AffiliationInfo>
<Affiliation>1 Division of Experimental Oncology/Unit of Urology, URI , IRCCS Ospedale San Raffaele, Milan, Italy .</Affiliation>
</AffiliationInfo>
<AffiliationInfo>
<Affiliation>2 Department of Urology, Tenon Hospital, Pierre and Marie Curie University , Paris, France .</Affiliation>
</AffiliationInfo>
</Author>
<Author ValidYN="Y">
<LastName>Şener</LastName>
<ForeName>Tarik Emre</ForeName>
<Initials>TE</Initials>
<AffiliationInfo>
<Affiliation>2 Department of Urology, Tenon Hospital, Pierre and Marie Curie University , Paris, France .</Affiliation>
</AffiliationInfo>
<AffiliationInfo>
<Affiliation>3 Department of Urology, Marmara University School of Medicine , Istanbul, Turkey .</Affiliation>
</AffiliationInfo>
</Author>
<Author ValidYN="Y">
<LastName>Somani</LastName>
<ForeName>Bhaskar K</ForeName>
<Initials>BK</Initials>
<AffiliationInfo>
<Affiliation>4 Department of Urology, University Hospital Southampton NHS Trust , Southampton, United Kingdom .</Affiliation>
</AffiliationInfo>
</Author>
<Author ValidYN="Y">
<LastName>Cloutier</LastName>
<ForeName>Jonathan</ForeName>
<Initials>J</Initials>
<AffiliationInfo>
<Affiliation>2 Department of Urology, Tenon Hospital, Pierre and Marie Curie University , Paris, France .</Affiliation>
</AffiliationInfo>
<AffiliationInfo>
<Affiliation>5 Department of Urology, University Hospital Centre of Quebec City , Quebec, Canada .</Affiliation>
</AffiliationInfo>
</Author>
<Author ValidYN="Y">
<LastName>Butticè</LastName>
<ForeName>Salvatore</ForeName>
<Initials>S</Initials>
<AffiliationInfo>
<Affiliation>2 Department of Urology, Tenon Hospital, Pierre and Marie Curie University , Paris, France .</Affiliation>
</AffiliationInfo>
<AffiliationInfo>
<Affiliation>6 Department of Urology, University of Messina , Messina, Italy .</Affiliation>
</AffiliationInfo>
</Author>
<Author ValidYN="Y">
<LastName>Marson</LastName>
<ForeName>Francesco</ForeName>
<Initials>F</Initials>
<AffiliationInfo>
<Affiliation>2 Department of Urology, Tenon Hospital, Pierre and Marie Curie University , Paris, France .</Affiliation>
</AffiliationInfo>
<AffiliationInfo>
<Affiliation>7 Department of Urology, Città della Salute e della Scienza, Turin, Italy .</Affiliation>
</AffiliationInfo>
</Author>
<Author ValidYN="Y">
<LastName>Doizi</LastName>
<ForeName>Steeve</ForeName>
<Initials>S</Initials>
<AffiliationInfo>
<Affiliation>2 Department of Urology, Tenon Hospital, Pierre and Marie Curie University , Paris, France .</Affiliation>
</AffiliationInfo>
</Author>
<Author ValidYN="Y">
<LastName>Proietti</LastName>
<ForeName>Silvia</ForeName>
<Initials>S</Initials>
<AffiliationInfo>
<Affiliation>2 Department of Urology, Tenon Hospital, Pierre and Marie Curie University , Paris, France .</Affiliation>
</AffiliationInfo>
<AffiliationInfo>
<Affiliation>8 Department of Urology, IRCCS San Raffaele Scientific Institute , Ville Turro Division, Milan, Italy .</Affiliation>
</AffiliationInfo>
</Author>
<Author ValidYN="Y">
<LastName>Traxer</LastName>
<ForeName>Olivier</ForeName>
<Initials>O</Initials>
<AffiliationInfo>
<Affiliation>2 Department of Urology, Tenon Hospital, Pierre and Marie Curie University , Paris, France .</Affiliation>
</AffiliationInfo>
</Author>
</AuthorList>
<Language>eng</Language>
<PublicationTypeList>
<PublicationType UI="D016428">Journal Article</PublicationType>
</PublicationTypeList>
<ArticleDate DateType="Electronic">
<Year>2016</Year>
<Month>12</Month>
<Day>16</Day>
</ArticleDate>
</Article>
<MedlineJournalInfo>
<Country>United States</Country>
<MedlineTA>J Endourol</MedlineTA>
<NlmUniqueID>8807503</NlmUniqueID>
<ISSNLinking>0892-7790</ISSNLinking>
</MedlineJournalInfo>
<KeywordList Owner="NOTNLM">
<Keyword MajorTopicYN="N">flexible ureteroscopy</Keyword>
<Keyword MajorTopicYN="N">learning curve</Keyword>
<Keyword MajorTopicYN="N">training model</Keyword>
<Keyword MajorTopicYN="N">ureteroscopy curriculum</Keyword>
</KeywordList>
</MedlineCitation>
<PubmedData>
<History>
<PubMedPubDate PubStatus="pubmed">
<Year>2016</Year>
<Month>11</Month>
<Day>22</Day>
<Hour>6</Hour>
<Minute>0</Minute>
</PubMedPubDate>
<PubMedPubDate PubStatus="medline">
<Year>2016</Year>
<Month>11</Month>
<Day>22</Day>
<Hour>6</Hour>
<Minute>0</Minute>
</PubMedPubDate>
<PubMedPubDate PubStatus="entrez">
<Year>2016</Year>
<Month>11</Month>
<Day>22</Day>
<Hour>6</Hour>
<Minute>0</Minute>
</PubMedPubDate>
</History>
<PublicationStatus>ppublish</PublicationStatus>
<ArticleIdList>
<ArticleId IdType="pubmed">27869504</ArticleId>
<ArticleId IdType="doi">10.1089/end.2016.0677</ArticleId>
</ArticleIdList>
</PubmedData>
</PubmedArticle>
</PubmedArticleSet>
以下是我正在使用的代码，它运行良好，除非 PubMed 数据库中一篇文章的作者有多个隶属关系：
library(rentrez)
library(XML)
pubmedSearch <- entrez_search("pubmed", term = "flexible ureteroscope Simulation Model",
retmax = 10)
SearchResults <- entrez_fetch(db="pubmed", pubmedSearch$ids, rettype="xml",
parsed=TRUE)
xmlGetValue <- function(x, node){
a <- xpathSApply(x, node, xmlValue)
if(length(a) == 0) {a <- NA} else {a}
}
parse_paper <- function(paper){
pmid <- xmlGetValue(paper, ".//ArticleId[@IdType='pubmed']")
first_names <- xmlGetValue(paper, ".//Author/ForeName")
last_names <- xmlGetValue(paper, ".//Author/LastName")
affiliation <- xmlGetValue(paper, ".//AffiliationInfo/Affiliation")
data.frame(pmid=pmid, first_names=first_names, last_names=last_names,
affiliation=affiliation)
}
parse_multiple_papers <- function(papers){
res <- xpathApply(papers, "/PubmedArticleSet/*", parse_paper)
do.call(rbind.data.frame, res)
}
test_df <- parse_multiple_papers(SearchResults)
非常感谢任何帮助和支持。