Dd *_* Pp 306 exception-handling r exception try-catch r-faq
我想编写trycatch代码来处理从网上下载时的错误.
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)
这两个语句成功运行.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:
Rap*_*ter 559
那么:欢迎来到R世界;-)
干得好
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)
}
> 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
> 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
试着抓
tryCatch返回与执行相关的值,expr除非出现错误或警告.在这种情况下,return(NA)可以通过提供相应的处理函数来指定特定的返回值(参见上文)(请参阅参数error和warningin ?tryCatch).这些可以是已经存在的函数,但您也可以在其中定义它们tryCatch()(如上所述).
选择处理函数的特定返回值的含义
正如我们已经指定的那样NA,如果出现错误应该返回,第三个元素y是NA.如果我们要选择NULL是返回值,长度y也只是一直2不是3因为lapply()将简单地"忽略"是返回值NULL.另请注意,如果未指定显式返回值via return(),则处理函数将返回NULL(即出现错误或警告条件).
"不受欢迎的"警告信息
由于warn=FALSE似乎没有任何影响,另一种抑制警告的方法(在这种情况下并不是真正感兴趣)是使用
suppressWarnings(readLines(con=url))
代替
readLines(con=url, warn=FALSE)
多个表达式
请注意,您也可以将多个表达式中的"实际表达部分"(争论expr的tryCatch()),如果你在大括号包起来(就像我在所示finally部分).
her*_*arn 60
R使用函数来实现try-catch块:
语法有点像这样:
result = tryCatch({
expr
}, warning = function(warning_condition) {
warning-handler-code
}, error = function(error_condition) {
error-handler-code
}, finally={
cleanup-code
})
在tryCatch()中,可以处理两个"条件":"警告"和"错误".编写每个代码块时要理解的重要事项是执行状态和范围. @资源
Rah*_*hul 34
tryCatch具有稍微复杂的语法结构。但是，一旦我们理解了构成一个完整的tryCatch调用的4个部分，如下所示，就很容易记住：
expr：[ 必需 ]要评估的R代码
错误：[ 可选 ]如果在评估expr中的代码时发生错误，应该怎么办
警告：[ 可选 ]如果在评估expr中的代码时发生警告，应该怎么办
最终：[ 可选 ]退出tryCatch调用之前应该执行什么操作，而不管expr是否成功运行，是否有错误或警告
tryCatch(
expr = {
# Your code...
# goes here...
# ...
},
error = function(e){
# (Optional)
# Do this if an error is caught...
},
warning = function(w){
# (Optional)
# Do this if an warning is caught...
},
finally = {
# (Optional)
# Do this at the end before quitting the tryCatch structure...
}
)
因此，一个用于计算值对数的玩具示例可能类似于：
log_calculator <- function(x){
tryCatch(
expr = {
message(log(x))
message("Successfully executed the log(x) call.")
},
error = function(e){
message('Caught an error!')
print(e)
},
warning = function(w){
message('Caught an warning!')
print(w)
},
finally = {
message('All done, quitting.')
}
)
}
现在，运行三种情况：
有效案例
log_calculator(10)
# 2.30258509299405
# Successfully executed the log(x) call.
# All done, quitting.
“警告”案
log_calculator(-10)
# Caught an warning!
# <simpleWarning in log(x): NaNs produced>
# All done, quitting.
“错误”案例
log_calculator("log_me")
# Caught an error!
# <simpleError in log(x): non-numeric argument to mathematical function>
# All done, quitting.
我写了一些我经常使用的有用用例。在此处查找更多详细信息：https : //rsangole.netlify.com/post/try-catch/
希望这会有所帮助。
Pau*_*aul 32
这是一个简单的例子:
# Do something, or tell me why it failed
my_update_function <- function(x){
tryCatch(
# This is what I want to do...
{
y = x * 2
return(y)
},
# ... but if an error occurs, tell me what happened:
error=function(error_message) {
message("This is my custom message.")
message("And below is the error message from R:")
message(error_message)
return(NA)
}
)
}
如果您还想捕获"警告",只需添加warning=类似于该error=部分.
小智 22
因为我只是失去了两天的生命,试图解决tryCatch的irr功能,我想我应该分享我的智慧(以及缺少的东西).FYI - irr是FinCal的实际功能,在这种情况下,在少数情况下在大型数据集中出现错误.
将tryCatch设置为函数的一部分.例如:
irr2 <- function (x) {
out <- tryCatch(irr(x), error = function(e) NULL)
return(out)
}
要使错误(或警告)起作用,您实际上需要创建一个函数.我最初的错误部分只是写了error = return(NULL),所有值都回来了null.
记得要创建一个子输出(比如我的"out")和return(out).