如何从XMLDocumentContent对象提取$children$html内容

提前道歉，我相信这很简单，但我不知道我做错了什么

除其他事项外，此代码

study.name <- 'NLSY79'
library(XML)
library(httr)
sub.study <- paste0( "https://www.nlsinfo.org/investigator/servlet1?get=SUBSTUDIES&study=" , study.name )
study.html <- GET( sub.study )
content( study.html )
study.block <- htmlParse( study.html , asText = TRUE )

---

study.name您可以使用
xpathsaply
提取所需的元素

xpathSApply(study.block, "//option")
# [[1]]
# <option value="-1" selected="selected">(Choose One)</option> 
# [[2]]
# <option value="343.06">NLSY79 (1979-2010)</option> 

您可以使用
xpathsaply
提取所需的元素

xpathSApply(study.block, "//option")
# [[1]]
# <option value="-1" selected="selected">(Choose One)</option> 
# [[2]]
# <option value="343.06">NLSY79 (1979-2010)</option> 

您还可以使用
xmlGetAtrr

xpathSApply(study.block, "//option", xmlGetAttr, "value")
[1] "-1"     "343.06"

或

您还可以使用
xmlGetAtrr

xpathSApply(study.block, "//option", xmlGetAttr, "value")
[1] "-1"     "343.06"

或

xpathSApply(study.block, "//option[not(@selected)]", xmlGetAttr, "value")
[1] "343.06"