如何在R中写入trycatch

Question

我想写一些代码来处理从web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)

这两条语句运行成功。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，输出为：“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列表结束

Answer 1

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（）中有两个可以处理的“条件”：“警告”和“错误”。编写每个代码块时要了解的重要事情是执行状态和范围。@source

Answer 2

tryCatch的语法结构有点复杂。然而，一旦我们理解了构成如下所示的完整tryCatch调用的4个部分，就很容易记住了：

expr：[必需]待评估的R代码

错误：[可选]如果在计算exr中的代码时发生错误，应该运行什么

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

最后：[可选]在退出tryCatch调用之前应该运行什么，无论exr是否成功运行、有错误或有警告

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...
    }
)

因此，一个玩具示例，计算值的日志可能如下所示：

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.')
        }
    )    
}

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.

“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/

希望这有帮助。

Answer 3

那么：欢迎来到R世界；-）

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)
}

> 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

TryCatch

除非出现错误或警告，否则tryCatch将返回与执行expr相关的值。在这种情况下，可以通过提供相应的处理程序函数来指定特定的返回值（请参见上文中的参数“错误”和“警告”）。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）。

选择处理函数的特定返回值的含义

由于我们已经指定在发生错误时应返回NA，因此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lApplication（）将简单地“忽略”NULL的返回值。另请注意，如果您不通过返回（）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）。

“不希望的”警告信息

由于warn=FALSE（警告=假）似乎没有任何效果，抑制警告的另一种方法（在本例中，这并不重要）是使用

suppressWarnings(readLines(con=url))

而不是

readLines(con=url, warn=FALSE)

多个表达式

请注意，如果您将多个表达式包装在花括号中，您还可以将多个表达式放置在“实际表达式部分”（tryCatch（）的参数exr）中（就像我在最后部分）。