tryCatch和ggplot

Question

我使用tryCatch很多來捕獲我的R代碼中的潛在錯誤消息，但我今天遇到了一個奇怪的問題ggplot - 我真的不知道發生了什麼事情（即使經過一段時間Google/SO搜索）。

簡單實例與不完全ggplot呼叫（無添加層）：

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

所以誤差不會捕獲。更：我得到R對象與ggplot類：

> 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" 

任何想法捕捉到這些樣的錯誤？

---

更新：我知道的一個事實，即上面的調用可能會在某些情況下，OK（像後來添加層）。但是，那麼請考慮下面的例子（請注意，我們有一個錯字有：t提供在y軸代替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 

我得到錯誤信息，但是這不是tryCatch抓獲。

Answer 1

這是因爲在通話不發生ggplot的錯誤，但在調用print.ggplot。

所以你tryCatch作品，但錯誤嘗試打印時只是後來發生。這意味着你必須把tryCatch圍繞print聲明：

試試這個：

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> 

---

併爲您的第二個例子：

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>