如何用R写trycatch

我想写trycatch代码来处理从网络下载的错误。

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，我创建了一个不存在的网址:

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:

  1. 当URL错误时，输出将是:“web URL错误，无法获取”。
  2. 当URL错误时，代码不会停止，而是继续下载，直到URL列表的末尾?
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小开

R使用函数来实现try-catch块:

语法看起来像这样:

result = tryCatch({
expr
}, warning = function(warning_condition) {
warning-handler-code
}, error = function(error_condition) {
error-handler-code
}, finally={
cleanup-code
})
在tryCatch()中有两个'条件'可以处理:'警告'和'错误'。在编写每个代码块时，需要理解的重要事情是执行状态和作用域。 @source < / p >
小开
最佳答案

那么好吧:欢迎来到R世界;-)

给你

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)
}

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

调查产出

> 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，除非有错误或警告。在这种情况下，具体的返回值(参见上面的return(NA))可以通过提供相应的处理函数来指定(参见?tryCatch中的参数error和warning)。这些函数可以是已经存在的函数，但你也可以在tryCatch()中定义它们(就像我上面所做的那样)。

选择处理程序函数的特定返回值的含义

因为我们已经指定了在发生错误时应该返回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的长度将只是2而不是3，因为lapply()将简单地“忽略”;返回NULL的值。还要注意，如果你没有通过return()指定y1返回值，处理函数将返回NULL(即在错误或警告条件下)。

“Undesired"警告消息

由于warn=FALSE似乎没有任何效果，另一种抑制警告的方法(在这种情况下并不真正感兴趣)是使用

suppressWarnings(readLines(con=url))

而不是

readLines(con=url, warn=FALSE)

多个表达式

注意，你也可以在“实际表达式部分”中放置多个表达式。(tryCatch()的参数expr)，如果你用花括号将它们括起来(就像我在finally部分中说明的那样)。

小开

由于我刚刚花了两天时间试图为一个irr函数求解tryCatch，我认为我应该分享我的智慧(以及所缺少的东西)。供您参考- irr是来自FinCal的一个实际函数，在这种情况下，在大数据集上的一些情况下会出现错误。

  1. 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)
}

  2. For the error (or warning) to work, you actually need to create a function. I originally for error part just wrote error = return(NULL) and ALL values came back null.

  3. Remember to create a sub-output (like my "out") and to return(out).

小开

下面是一个简单的例子:

# 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)
}
)
}

如果你还想捕获一个“警告”，只需添加warning=类似于error=部分。

小开

tryCatch的语法结构有点复杂。然而，一旦我们理解了组成一个完整的tryCatch调用的4个部分，就很容易记住了:

expr: [要求]要计算的R代码

错误: [可选]如果在expr中计算代码时发生错误，应该运行什么

警告: [可选]如果在expr中计算代码时发生警告，应该运行什么

最后: [可选]在退出tryCatch调用之前应该运行的内容，无论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，一个简单的例子，计算一个值的对数可能是这样的:

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，运行三个案例:

有效案例

log_calculator(10)
# 2.30258509299405
# Successfully executed the log(x) call.
# All done, quitting.

一个“警告”案例

log_calculator(-10)
# Caught an warning!
# <simpleWarning in log(x): NaNs produced>
# All done, quitting.

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.

我已经写了一些我经常使用的有用的用例。在这里找到更多细节:https://rsangole.netlify.com/post/try-catch/

希望这对你有帮助。


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