如何在 R 中编写 trycatch
[英]How to write trycatch in R
问题描述
I want to write trycatch code to deal with error in downloading from the web.
我想编写trycatch代码来处理从网络下载时出现的错误。
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)
These two statements run successfully.这两个语句运行成功。 Below, I create a non-exist web address:下面，我创建了一个不存在的网址：
url <- c("xxxxx", "http://en.wikipedia.org/wiki/Xz")
url[1] does not exist. url[1]不存在。 How does one write a trycatch loop (function) so that:如何编写一个trycatch循环（函数），以便：
- When the URL is wrong, the output will be: "web URL is wrong, can't get".当 URL 错误时，输出将是：“web URL 错误，无法获取”。
- When the URL is wrong, the code does not stop, but continues to download until the end of the list of URLs?当URL错误时，代码不停止，而是继续下载，直到URL列表结束？
5 个解决方案
解决方案1
770 已采纳
Well then: welcome to the R world ;-)那么：欢迎来到 R 世界 ;-)
Here you go干得好
Setting up the code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 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)
}
Applying the code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
Investigating the output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
Additional remarks附加说明
tryCatch试着抓
tryCatch returns the value associated to executing expr unless there's an error or a warning. tryCatch返回与执行expr关联的值，除非出现错误或警告。 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 ).在这种情况下，可以通过提供相应的处理程序函数（参见?tryCatch参数error和warning ）来指定特定的返回值（参见上面的return(NA) ）。 These can be functions that already exist, but you can also define them within tryCatch() (as I did above).这些可以是已经存在的函数，但您也可以在tryCatch()定义它们（就像我上面所做的那样）。
The implications of choosing specific return values of the handler functions选择处理程序函数的特定返回值的含义
As we've specified that NA should be returned in case of error, the third element in y is NA .由于我们已经指定在出现错误时应返回NA ，因此y的第三个元素是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 .如果我们选择NULL作为返回值，则y的长度将仅为2而不是3因为lapply()将简单地“忽略”为NULL返回值。 Also note that if you don't specify an explicit return value via return() , the handler functions will return NULL (ie in case of an error or a warning condition).另请注意，如果您未通过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）。
"Undesired" warning message “不需要的”警告消息
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由于warn=FALSE似乎没有任何效果，因此抑制警告（在这种情况下并不是真正感兴趣的）的另一种方法是使用
suppressWarnings(readLines(con=url))
instead of代替
readLines(con=url, warn=FALSE)
Multiple expressions多种表达方式
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).请注意，如果将多个表达式括在大括号中（就像我在finally部分中说明的那样），您还可以在“实际表达式部分”（ tryCatch()参数expr tryCatch()放置多个表达式。
解决方案2
102 2018-12-20 21:56:08
tryCatch has a slightly complex syntax structure. tryCatch有一个稍微复杂的语法结构。 However, once we understand the 4 parts which constitute a complete tryCatch call as shown below, it becomes easy to remember:但是，一旦我们理解了构成完整 tryCatch 调用的 4 个部分，如下所示，就很容易记住了：
expr : [ Required ] R code(s) to be evaluated expr : [必需] 要评估的 R 代码
error : [ Optional ] What should run if an error occured while evaluating the codes in expr错误：[可选] 如果在评估 expr 中的代码时发生错误，应该运行什么
warning : [ Optional ] What should run if a warning occured while evaluating the codes in expr警告：[可选] 如果在评估 expr 中的代码时出现警告，应该运行什么
finally : [ Optional ] What should run just before quitting the tryCatch call, irrespective of if expr ran successfully, with an error, or with a warning finally : [可选] 在退出 tryCatch 调用之前应该运行什么，无论 expr 是否成功运行、有错误还是有警告
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...
}
)
Thus, a toy example, to calculate the log of a value might look like:因此，一个计算值的对数的玩具示例可能如下所示：
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.')
}
)
}
Now, running three cases:现在，运行三种情况：
A valid case一个有效的案例
log_calculator(10)
# 2.30258509299405
# Successfully executed the log(x) call.
# All done, quitting.
A "warning" case一个“警告”案例
log_calculator(-10)
# Caught an warning!
# <simpleWarning in log(x): NaNs produced>
# All done, quitting.
An "error" case一个“错误”案例
log_calculator("log_me")
# Caught an error!
# <simpleError in log(x): non-numeric argument to mathematical function>
# All done, quitting.
I've written about some useful use-cases which I use regularly.我写了一些我经常使用的有用用例。 Find more details here: https://rsangole.netlify.com/post/try-catch/在此处查找更多详细信息： https : //rsangole.netlify.com/post/try-catch/
Hope this is helpful.希望这是有帮助的。
解决方案3
80 2012-08-30 09:34:14
R uses functions for implementing try-catch block: R 使用函数来实现 try-catch 块：
The syntax somewhat looks like this: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
})
In tryCatch() there are two 'conditions' that can be handled: 'warnings' and 'errors'.在 tryCatch() 中有两个可以处理的“条件”：“警告”和“错误”。 The important thing to understand when writing each block of code is the state of execution and the scope.编写每个代码块时要了解的重要一点是执行状态和范围。 @source @来源
解决方案4
50 2017-04-13 00:25:10
Here goes a straightforward example :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)
}
)
}
If you also want to capture a "warning", just add warning= similar to the error= part.如果您还想捕获“警告”，只需添加类似于error=部分的warning= 。
解决方案5
31 2016-09-25 18:55:30
Since I just lost two days of my life trying to solve for tryCatch for an irr function, I thought I should share my wisdom (and what is missing).由于我刚刚花了两天时间试图解决 irr 函数的 tryCatch，我想我应该分享我的智慧（以及缺少的东西）。 FYI - irr is an actual function from FinCal in this case where got errors in a few cases on a large data set.仅供参考 - irr 是 FinCal 的一个实际函数，在这种情况下，在少数情况下在大型数据集上出现错误。
Set up tryCatch as part of a function.
将 tryCatch 设置为函数的一部分。 For example:例如：irr2 <- function (x) { out <- tryCatch(irr(x), error = function(e) NULL) return(out) }For the error (or warning) to work, you actually need to create a function.
为了使错误（或警告）起作用，您实际上需要创建一个函数。 I originally for error part just wrote error = return(NULL)and ALL values came back null.我最初为错误部分只写了error = return(NULL)并且所有值都返回空值。Remember to create a sub-output (like my "out") and to
return(out).请记住创建一个子输出（如我的“输出”）并return(out)。
解决方案6
0 2019-10-21 16:02:16
Thanks for the hints, helped a lot, especially the error-handling-part with 感谢您的提示，对您有所帮助，尤其是错误处理部分
error = function(e){ return(NA) } 错误=函数（e）{return（NA）}
This was exactly what i needed. 这正是我所需要的。
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