r - 如何在R中编写trycatch-6ren

r - 如何在R中编写trycatch

转载作者：行者123 更新时间：2023-12-03 04:00:35

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)

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。如何编写一个 trycatch 循环(函数)以便:

当网址错误时，输出将是:“网址错误，无法获取”。
当网址错误时，代码不会停止，而是继续下载，直到网址列表末尾？

最佳答案

那么: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

研究输出

> 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

附加说明

tryCatch

tryCatch 返回与执行 expr 相关的值，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))(请参阅参数 error 和 warning > 在 ?tryCatch 中)。这些可以是已经存在的函数，但您也可以在 tryCatch() 中定义它们(就像我上面所做的那样)。

选择处理函数的特定返回值的含义

由于我们已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，则 y 的长度将是 2 而不是 3 as lapply() 将简单地“忽略”返回值为 NULL 的值。另请注意，如果您未通过 return() 指定显式返回值，则处理函数将返回 NULL(即，如果错误或警告条件)。