r - 如果for循环中发生错误，如何更改值？

Question

我有一个循环，可从〜440个网页中读取HTML表数据。每页上的代码并不完全相同，因此有时我需要表节点1，有时需要表节点2。现在，我只是在列表中手动设置节点号并将其输入循环。我的问题是页面节点已开始更改和更新节点＃列表变得很麻烦。

如果循环遇到错误的节点号(即:1而不是2，或者取反)，它将给出错误并关闭。如果遇到错误，是否有办法让循环将错误的节点号替换为正确的节点号，然后继续运行循环，就好像什么都没发生一样？

这是我循环中代码的readHTML部分，并带有示例url:

url <- "http://espn.go.com/nba/player/gamelog/_/id/2991280/year/2013/"

html.page <- htmlParse(url)

tableNodes <- getNodeSet(html.page, "//table")

x <- as.numeric(Players$Nodes[s])

tbl = readHTMLTable(tableNodes[[x]], colClasses = c("character"),stringsAsFactors = FALSE)

这是节点号错误时出现的错误:

"Error in readHTMLTable(tableNodes[[x]], colClasses = c("character"), stringsAsFactors = FALSE) : error in evaluating the argument 'doc' in selecting a method for function 'readHTMLTable': Error in tableNodes[[x]] : subscript out of bounds"

示例代码:

A <- c("dog", "cat")

Nodes <- as.data.frame(1:1) 

#)Nodes <- as.data.frame(1:2) <-- This works without errors

colnames(Nodes)[1] <- "Col1"

Nodes2 <- 2

url <-c("http://espn.go.com/nba/player/gamelog/_/id/6639/year/2013/","http://espn.go.com/nba/player/gamelog/_/id/6630/year/2013/")

for (i in 1:length(A))  
{ 


html.page <- htmlParse(url[i])

tableNodes <- getNodeSet(html.page, "//table")

x <- as.numeric(Nodes$Col1[i])

df = readHTMLTable(tableNodes[[x]], colClasses = c("character"),stringsAsFactors = FALSE)

#tryCatch(df) here.....no clue


assign(paste0("", A[i]), df)
}

Answer 1

如果您收到subscript out of bounds错误消息，则应确保使用较低的x。基于您在原始问题中发布的演示代码的tryCatch常规演示(尽管我不知道x和2是什么，但我已将Players替换为s):

> msg <- tryCatch(readHTMLTable(tableNodes[[2]], colClasses = c("character"),stringsAsFactors = FALSE), error = function(e)e)
> str(msg)
List of 2
 $ message: chr "error in evaluating the argument 'doc' in selecting a method for function 'readHTMLTable': Error in tableNodes[[2]] : subscript"| __truncated__
 $ call   : language readHTMLTable(tableNodes[[2]], colClasses = c("character"), stringsAsFactors = FALSE)
 - attr(*, "class")= chr [1:3] "simpleError" "error" "condition"
> msg$message
[1] "error in evaluating the argument 'doc' in selecting a method for function 'readHTMLTable': Error in tableNodes[[2]] : subscript out of bounds\n"
> grepl('subscript out of bounds', msg$message)
[1] TRUE