如何在R中编写trycatch

时间:2012-08-30 09:27:17

标签: r exception exception-handling 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:

  1. 当网址错误时,输出将是:&#34;网址错误,无法获取&#34;。
  2. 当网址错误时,代码不会停止,但会继续下载,直到网址列表结束?

5 个答案:

答案 0 :(得分:504)

那么:欢迎来到R世界;-)

你去吧

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

补充说明

<强> tryCatch

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来指定特定的返回值(请参阅上面的return(NA))(请参阅error中的参数warning?tryCatch)。这些可以是已经存在的函数,但您也可以在tryCatch()中定义它们(如上所述)。

选择处理程序函数的特定返回值的含义

正如我们已指定在出现错误时应返回NAy中的第三个元素为NA。如果我们选择NULL作为返回值,那么y的长度将只是2而不是3,因为lapply()将只是“忽略“返回NULL的值。另请注意,如果未通过return()指定显式返回值,则处理函数将返回NULL(即出现错误或警告情况)。

“不受欢迎”的警告信息

由于warn=FALSE似乎没有任何影响,因此使用

来抑制警告的替代方法(在这种情况下并不是真正令人感兴趣)
suppressWarnings(readLines(con=url))

而不是

readLines(con=url, warn=FALSE)

多个表达

请注意,如果用大括号括起来,也可以在“实际表达式部分”(expr的参数tryCatch())中放置多个表达式(就像我在{{1}中说明的那样)部分)。

答案 1 :(得分:59)

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。 @source

答案 2 :(得分:28)

这里有一个简单的例子

# 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”,只需添加与warning=部分类似的error=

答案 3 :(得分:21)

由于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的实际函数,在这种情况下,在少数情况下在大型数据集中出现错误。

  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. 要使错误(或警告)起作用,您实际上需要创建一个函数。我最初为错误部分写了error = return(NULL),所有值都返回null。

  3. 请记住创建子输出(例如我的&#34; out&#34;)和return(out)

答案 4 :(得分:14)

tryCatch具有稍微复杂的语法结构。但是,一旦我们理解构成一个完整的tryCatch调用的4个部分(如下所示),就很容易记住:

expr :[必需]个待评估的R代码

错误:[可选]如果评估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调用之前应该运行什么,无论expr是否成功运行,是否出现错误或警告

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,作为一个玩具示例,用于计算值的对数可能看起来像:

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”案例

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.

我已经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。