r - tryCatch 和 ggplot

Question

我经常使用 tryCatch 来捕获 R 代码中潜在的错误消息，但今天我在使用 ggplot 时遇到了一个奇怪的问题 - 我真的不知道发生了什么(即使在 Google/SO 搜索上花费了一段时间)。

具有不完整的 ggplot 调用的简单示例(未添加图层):

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

因此错误未被捕获。更重要的是:我得到了一个带有 ggplot 类的 R 对象:

> 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"

有什么想法可以捕获此类错误吗？

<小时/>

更新:我知道上述调用在某些情况下可能没问题(例如稍后添加层)。但请考虑以下示例(请注意，我们有一个拼写错误:y 轴上提供了 t，而不是 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

我确实收到了错误消息，但 tryCatch 没有捕获该错误消息。

Answer 1

发生这种情况是因为错误不是发生在对 ggplot 的调用中，而是发生在对 print.ggplot 的调用中。

因此，您的 tryCatch 可以工作，但错误仅在稍后尝试打印时发生。这意味着您必须将 tryCatch 放在 print 语句周围:

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>

<小时/>

对于第二个例子:

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>