如何在 R 中编写 trycatch [英] How to write trycatch in R
问题描述
我想编写trycatch代码来处理从网络下载时的错误.
I want to write trycatch code to deal with error in downloading from the web.
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)
这两条语句运行成功.下面，我创建了一个不存在的网址:
These two statements run successfully. Below, I create a non-exist web address:
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:
url[1] does not exist. How does one write a trycatch loop (function) so that:
- 当网址错误时，输出将是:网址错误，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?
推荐答案
那么:欢迎来到 R 世界 ;-)
Well then: welcome to the R world ;-)
给你
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 inside 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
补充说明
tryCatch
tryCatch 返回与执行 expr 相关的值，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程序函数(参见参数 error 和 warning)来指定特定的返回值(参见上面的 return(NA)> 在 ?tryCatch 中).这些可以是已经存在的函数，但您也可以在 tryCatch() 中定义它们(就像我上面所做的那样).
tryCatch returns the value associated to executing expr unless there's an error or a warning. In this case, specific return values (see return(NA) above) can be specified by supplying a respective handler function (see arguments error and warning in ?tryCatch). These can be functions that already exist, but you can also define them within tryCatch() (as I did above).
选择处理函数的特定返回值的含义
因为我们已经指定在出错时应该返回NA，y 中的第三个元素是NA.如果我们选择 NULL 作为返回值，y 的长度将是 2 而不是 3 作为 lapply() 将简单地忽略"；返回 NULL 的值.另请注意，如果您未通过 return() 指定 explicit 返回值，则处理函数将返回 NULL(即，如果错误或警告条件).
As we've specified that NA should be returned in case of error, the third element in y is NA. If we'd have chosen NULL to be the return value, the length of y would just have been 2 instead of 3 as lapply() will simply "ignore" return values that are NULL. Also note that if you don't specify an explicit return value via return(), the handler functions will return NULL (i.e. in case of an error or a warning condition).
不受欢迎的"警告信息
由于 warn=FALSE 似乎没有任何效果，抑制警告的替代方法(在这种情况下并不是真正感兴趣)是使用
As warn=FALSE doesn't seem to have any effect, an alternative way to suppress the warning (which in this case isn't really of interest) is to use
suppressWarnings(readLines(con=url))
代替
readLines(con=url, warn=FALSE)
多个表达
请注意，您还可以在实际表达式部分"中放置多个表达式；(tryCatch() 的参数 expr)，如果将它们括在大括号中(就像我在 finally 部分中说明的那样).
Note that you can also place multiple expressions in the "actual expressions part" (argument expr of tryCatch()) if you wrap them in curly brackets (just like I illustrated in the finally part).
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