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:
当URL错误时，输出将是:“web URL错误，无法获取”。 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?
当前回答
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
尝试捕捉
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(参见上面的return(NA))(请参阅?tryCatch中的参数error和warning)。这些函数可以是已经存在的函数，但也可以在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
正如我们已经指定的，在错误的情况下应该返回NA, 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”返回值为NULL。还要注意，如果你没有通过return()指定显式的返回值，处理函数将返回NULL(即在错误或警告条件的情况下)。
“不受欢迎”警告信息
由于warn=FALSE似乎没有任何效果，另一种抑制警告的方法(在本例中并不真正感兴趣)是使用
suppressWarnings(readLines(con=url))
而不是
readLines(con=url, warn=FALSE)
多个表达式
注意，如果将多个表达式用大括号括起来(就像我在最后一部分中演示的那样)，还可以在“实际表达式部分”(tryCatch()的参数expr)中放置多个表达式。
其他回答
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)
}
)
}
如果您还想捕获一个“警告”，只需添加warning=类似于error=部分。
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
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
尝试捕捉
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(参见上面的return(NA))(请参阅?tryCatch中的参数error和warning)。这些函数可以是已经存在的函数，但也可以在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
正如我们已经指定的，在错误的情况下应该返回NA, 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”返回值为NULL。还要注意，如果你没有通过return()指定显式的返回值，处理函数将返回NULL(即在错误或警告条件的情况下)。
“不受欢迎”警告信息
由于warn=FALSE似乎没有任何效果，另一种抑制警告的方法(在本例中并不真正感兴趣)是使用
suppressWarnings(readLines(con=url))
而不是
readLines(con=url, warn=FALSE)
多个表达式
注意，如果将多个表达式用大括号括起来(就像我在最后一部分中演示的那样)，还可以在“实际表达式部分”(tryCatch()的参数expr)中放置多个表达式。
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。
将tryCatch设置为函数的一部分。例如: ir2 <- function (x) { out <- tryCatch(irr(x)，错误=函数(e) NULL) 返回() ｝ 要使错误(或警告)生效，实际上需要创建一个函数。我原来的错误部分只是写了错误=返回(NULL)和所有值返回NULL。 记住创建一个子输出(如我的“out”)并返回(out)。
tryCatch的语法结构略复杂。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:[必需的]要计算的R个代码
error:[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
[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，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。更多详情请访问:https://rsangole.netlify.com/post/try-catch/
希望这对你有帮助。
