我想编写trycatch代码来处理从web下载时出现的错误。
url <- c(
"http://stat.ethz.ch/R-manual/R-devel/library/base/html/connections.html",
"http://en.wikipedia.org/wiki/Xz")
y <- mapply(readLines, con=url)
这两个语句运行成功。下面，我创建一个不存在的Web地址：
url <- c("xxxxx", "http://en.wikipedia.org/wiki/Xz")
url[1]不存在。如何编写trycatch循环（函数），以便：当URL错误时，输出为：“web URL错误，无法获取”。
当URL错误时，代码不会停止，而是继续下载，直到URL列表结束？
最佳答案
那么：欢迎来到R世界；-）
干得好
设置代码
urls <- c(
"http://stat.ethz.ch/R-manual/R-devel/library/base/html/connections.html",
"http://en.wikipedia.org/wiki/Xz",
"xxxxx"
)
readUrl <- function(url) {
out <- tryCatch(
{
# Just to highlight: if you want to use more than one
# R expression in the "try" part then you'll have to
# use curly brackets.
# 'tryCatch()' will return the last evaluated expression
# in case the "try" part was completed successfully
message("This is the 'try' part")
readLines(con=url, warn=FALSE)
# The return value of `readLines()` is the actual value
# that will be returned in case there is no condition
# (e.g. warning or error).
# You don't need to state the return value via `return()` as code
# in the "try" part is not wrapped insided a function (unlike that
# for the condition handlers for warnings and error below)
},
error=function(cond) {
message(paste("URL does not seem to exist:", url))
message("Here's the original error message:")
message(cond)
# Choose a return value in case of error
return(NA)
},
warning=function(cond) {
message(paste("URL caused a warning:", url))
message("Here's the original warning message:")
message(cond)
# Choose a return value in case of warning
return(NULL)
},
finally={
# NOTE:
# Here goes everything that should be executed at the end,
# regardless of success or error.
# If you want more than one expression to be executed, then you
# need to wrap them in curly brackets ({...}); otherwise you could
# just have written 'finally=<expression>'
message(paste("Processed URL:", url))
message("Some other message at the end")
}
)
return(out)
}
应用代码
> y <- lapply(urls, readUrl)
Processed URL: http://stat.ethz.ch/R-manual/R-devel/library/base/html/connections.html
Some other message at the end
Processed URL: http://en.wikipedia.org/wiki/Xz
Some other message at the end
URL does not seem to exist: xxxxx
Here's the original error message:
cannot open the connection
Processed URL: xxxxx
Some other message at the end
Warning message:
In file(con, "r") : cannot open file 'xxxxx': No such file or directory
调查输出
> head(y[[1]])
[1] "<!DOCTYPE html PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">"
[2] "<html><head><title>R: Functions to Manipulate Connections</title>"
[3] "<meta http-equiv=\"Content-Type\" content=\"text/html; charset=utf-8\">"
[4] "<link rel=\"stylesheet\" type=\"text/css\" href=\"R.css\">"
[5] "</head><body>"
[6] ""
> length(y)
[1] 3
> y[[3]]
[1] NA
附加说明
试拍
tryCatch返回与执行expr相关联的值，除非出现错误或警告。在这种情况下，可以通过提供相应的处理函数（参见return(NA)中的参数error和warning来指定特定的返回值（参见上面的?tryCatch）。这些可以是已经存在的函数，但也可以在tryCatch()中定义它们（正如我上面所做的那样）。选择处理函数的特定返回值的含义
因为我们已经指定了在出错时返回
NA，所以y中的第三个元素是NA。如果我们选择NULL作为返回值，y的长度应该是2而不是3，因为lapply()只是“忽略”了NULL的返回值。还要注意，如果不通过return()指定显式返回值，则处理程序函数将返回NULL（即，在出现错误或警告条件的情况下）。“意外”警告信息
因为
warn=FALSE似乎没有任何效果，所以抑制警告的另一种方法是使用suppressWarnings(readLines(con=url))
而不是
readLines(con=url, warn=FALSE)
多个表达式
请注意，如果您将多个表达式用花括号括起来（正如我在
expr部分中所示），则还可以将它们放在“实际表达式部分”（参数tryCatch()或finally）中。关于r - 如何在R中编写trycatch，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/33733102/