tryCatch和ggplot

Question

我使用tryCatch很多来捕获我的R代码中的潜在错误消息，但我今天遇到了一个奇怪的问题ggplot - 我真的不知道发生了什么事情（即使经过一段时间Google/SO搜索）。

简单实例与不完全ggplot呼叫（无添加层）：

> tryCatch(eval(parse(text = 'ggplot(mtcars, aes(x=hp, y=wt))')), error = function(e) e) 
Error: No layers in plot 

所以误差不会捕获。更：我得到R对象与ggplot类：

> str(tryCatch(eval(parse(text = 'ggplot(mtcars, aes(x=hp, y=wt))')), error = function(e) e)) 
List of 8 
$ data  :'data.frame': 32 obs. of 11 variables: 
    ..$ mpg : num [1:32] 21 21 22.8 21.4 18.7 18.1 14.3 24.4 22.8 19.2 ... 
    ..$ cyl : num [1:32] 6 6 4 6 8 6 8 4 4 6 ... 
    ..$ disp: num [1:32] 160 160 108 258 360 ... 
    ..$ hp : num [1:32] 110 110 93 110 175 105 245 62 95 123 ... 
    ..$ drat: num [1:32] 3.9 3.9 3.85 3.08 3.15 2.76 3.21 3.69 3.92 3.92 ... 
    ..$ wt : num [1:32] 2.62 2.88 2.32 3.21 3.44 ... 
    ..$ qsec: num [1:32] 16.5 17 18.6 19.4 17 ... 
    ..$ vs : num [1:32] 0 0 1 1 0 1 0 1 1 1 ... 
    ..$ am : num [1:32] 1 1 1 0 0 0 0 0 0 0 ... 
    ..$ gear: num [1:32] 4 4 4 3 3 3 3 4 4 4 ... 
    ..$ carb: num [1:32] 4 4 1 1 2 1 4 2 2 4 ... 
$ layers  : list() 
$ scales  :Reference class 'Scales' [package "ggplot2"] with 1 fields 
    ..$ scales: NULL 
    ..and 20 methods, of which 9 are possibly relevant: 
    .. add, clone, find, get_scales, has_scale, initialize, input, n, 
    .. non_position_scales 
$ mapping :List of 2 
    ..$ x: symbol hp 
    ..$ y: symbol wt 
$ options :List of 1 
    ..$ labels:List of 2 
    .. ..$ x: chr "hp" 
    .. ..$ y: chr "wt" 
$ coordinates:List of 1 
    ..$ limits:List of 2 
    .. ..$ x: NULL 
    .. ..$ y: NULL 
    ..- attr(*, "class")= chr [1:2] "cartesian" "coord" 
$ facet  :List of 1 
    ..$ shrink: logi TRUE 
    ..- attr(*, "class")= chr [1:2] "null" "facet" 
$ plot_env :<environment: R_GlobalEnv> 
- attr(*, "class")= chr "ggplot" 

任何想法捕捉到这些样的错误？

---

更新：我知道的一个事实，即上面的调用可能会在某些情况下，OK（像后来添加层）。但是，那么请考虑下面的例子（请注意，我们有一个错字有：t提供在y轴代替wt）：

> tryCatch(eval(parse(text = 'ggplot(mtcars, aes(x=hp, y=t)) + geom_point()')), error = function(e) e) 
Error in data.frame(x = c(110, 110, 93, 110, 175, 105, 245, 62, 95, 123, : 
    arguments imply differing number of rows: 32, 0 

我得到错误信息，但是这不是tryCatch抓获。

Answer 1

这是因为在通话不发生ggplot的错误，但在调用print.ggplot。

所以你tryCatch作品，但错误尝试打印时只是后来发生。这意味着你必须把tryCatch围绕print声明：

试试这个：

x <- tryCatch(eval(parse(text = 'ggplot(mtcars, aes(x=hp, y=wt))')), 
       error = function(e) e) 
tryCatch(print(x), error = function(e) e) 
<simpleError: No layers in plot> 

---

并为您的第二个例子：

x <- eval(parse(text = 'ggplot(mtcars, aes(x=hp, y=t)) + geom_point()')) 
tryCatch(print(x), error = function(e) e) 

<simpleError in data.frame(x = c(110, 110, 93, 110, 175, 105, 245, 62, 95, 123, 123, 180, 180, 180, 205, 215, 230, 66, 52, 65, 97, 150, 150, 245, 175, 66, 91, 113, 264, 175, 335, 109), y = structure(function (x) standardGeneric("t"), generic = structure("t", package = "base"), package = "base", group = list(), valueClass = character(0), signature = "x", default = structure(function (x) UseMethod("t"), target = structure("ANY", class = structure("signature", package = "methods"), .Names = "x", package = "methods"), defined = structure("ANY", class = structure("signature", package = "methods"), .Names = "x", package = "methods"), generic = structure("t", package = "base"), class = structure("derivedDefaultMethod", package = "methods")), skeleton = structure(function (x) UseMethod("t"), target = structure("ANY", class = structure("signature", package = "methods"), .Names = "x", package = "methods"), defined = structure("ANY", class = structure("signature", package = "methods"), .Names = "x", package = "methods"), generic = structure("t", package = "base"), class = structure("derivedDefaultMethod", package = "methods"))(x), class = structure("standardGeneric", package = "methods")),  PANEL = c(1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,  1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,  1L, 1L, 1L, 1L, 1L), check.names = FALSE, stringsAsFactors = TRUE): arguments imply differing number of rows: 32, 0>