如何防止ifelse()将Date对象转换为数字对象

Question

我正在使用该函数ifelse()来操作日期向量.我期望结果是类Date,并且很惊讶地得到了一个numeric向量.这是一个例子:

dates <- as.Date(c('2011-01-01', '2011-01-02', '2011-01-03', '2011-01-04', '2011-01-05'))
dates <- ifelse(dates == '2011-01-01', dates - 1, dates)
str(dates)

这尤其令人惊讶,因为在整个向量上执行操作会返回一个Date对象.

dates <- as.Date(c('2011-01-01', '2011-01-02', '2011-01-03', '2011-01-04','2011-01-05'))
dates <- dates - 1
str(dates)

我应该使用其他功能来操作Date矢量吗？如果是这样,有什么功能？如果没有,我如何强制ifelse返回与输入相同类型的向量？

帮助页面ifelse表明这是一个功能,而不是一个错误,但我仍然在努力寻找一个解释我发现的令人惊讶的行为.

Answer 1

你可以用data.table::fifelse.

从data.table >= 1.12.3发行说明:"[ dplyr::if_else]有更严格的语义data.table::fifelse:ifelse和fifelse参数必须是相同的类型.这给出了一个不太令人惊讶的返回类型,并保留了像日期这样的S3向量".

library(data.table)
dates <- fifelse(dates == '2011-01-01', dates - 1, dates)
str(dates)
# Date[1:5], format: "2010-12-31" "2011-01-02" "2011-01-03" "2011-01-04" "2011-01-05"

Answer 2

它涉及到记录值的ifelse:

具有相同长度和属性(包括尺寸和" class")的矢量test和来自yes或的值的数据值no.答案的模式将从逻辑强制进行,以适应从中获取的任何值yes,然后从中获取任何值no.

归结为它的影响,ifelse使得因素失去了他们的水平,日期失去了他们的阶级,只有他们的模式("数字")恢复.试试这个:

dates[dates == '2011-01-01'] <- dates[dates == '2011-01-01'] - 1
str(dates)
# Date[1:5], format: "2010-12-31" "2011-01-02" "2011-01-03" "2011-01-04" "2011-01-05"

你可以创建一个safe.ifelse:

safe.ifelse <- function(cond, yes, no){ class.y <- class(yes)
                                  X <- ifelse(cond, yes, no)
                                  class(X) <- class.y; return(X)}

safe.ifelse(dates == '2011-01-01', dates - 1, dates)
# [1] "2010-12-31" "2011-01-02" "2011-01-03" "2011-01-04" "2011-01-05"

后来的注释:我看到Hadley if_else在数据整形包的magrittr/dplyr/tidyr复合体中构建了一个.

Answer 3

DWin的解释是现场.在我意识到我可以在ifelse声明之后简单地强制完课之后,我摆弄并与之斗争了一段时间:

dates <- as.Date(c('2011-01-01','2011-01-02','2011-01-03','2011-01-04','2011-01-05'))
dates <- ifelse(dates=='2011-01-01',dates-1,dates)
str(dates)
class(dates)<- "Date"
str(dates)

起初,这对我来说感觉有点"愚蠢".但是现在我只想把它作为从ifelse()获得的性能回报的一个小代价.此外,它仍然比循环更简洁.

Answer 4

建议的方法不适用于因子列.我想建议这种改进:

safe.ifelse <- function(cond, yes, no) {
  class.y <- class(yes)
  if (class.y == "factor") {
    levels.y = levels(yes)
  }
  X <- ifelse(cond,yes,no)
  if (class.y == "factor") {
    X = as.factor(X)
    levels(X) = levels.y
  } else {
    class(X) <- class.y
  }
  return(X)
}

顺便说一句:ifelse糟透了......强大的功能带来了很大的责任,即1x1矩阵和/或数字的类型转换(例如当它们应该被添加时)对我来说是好的,但ifelse中的这种类型转换显然是不需要的.我现在碰到多次ifelse的'bug',它只是在偷我的时间:-(

FW

Answer 5

@ fabian-werner提供的答案很棒,但是对象可以有多个类,而"factor"可能不一定是返回的第一个class(yes),所以我建议这个小修改来检查所有类属性:

safe.ifelse <- function(cond, yes, no) {
      class.y <- class(yes)
      if ("factor" %in% class.y) {  # Note the small condition change here
        levels.y = levels(yes)
      }
      X <- ifelse(cond,yes,no)
      if ("factor" %in% class.y) {  # Note the small condition change here
        X = as.factor(X)
        levels(X) = levels.y
      } else {
        class(X) <- class.y
      }
      return(X)
    }

我还向R开发团队提交了一个请求,要求添加一个记录的选项,让base :: ifelse()根据用户选择要保留的属性来保留属性.该请求是在这里:https://bugs.r-project.org/bugzilla/show_bug.cgi?id=16609 -它已经,理由是它一直是现在的方式标记为"WONTFIX"但我提供了一个后续论点,说明为什么简单的添加可能会为很多R用户带来麻烦.也许你的错误线程中的"+1"将鼓励R Core团队重新审视.

编辑:这是一个更好的版本,允许用户指定要保留的属性,"cond"(默认ifelse()行为),"是",根据上面代码的行为,或"否",对于"no"值的属性更好:

safe_ifelse <- function(cond, yes, no, preserved_attributes = "yes") {
    # Capture the user's choice for which attributes to preserve in return value
    preserved           <- switch(EXPR = preserved_attributes, "cond" = cond,
                                                               "yes"  = yes,
                                                               "no"   = no);
    # Preserve the desired values and check if object is a factor
    preserved_class     <- class(preserved);
    preserved_levels    <- levels(preserved);
    preserved_is_factor <- "factor" %in% preserved_class;

    # We have to use base::ifelse() for its vectorized properties
    # If we do our own if() {} else {}, then it will only work on first variable in a list
    return_obj <- ifelse(cond, yes, no);

    # If the object whose attributes we want to retain is a factor
    # Typecast the return object as.factor()
    # Set its levels()
    # Then check to see if it's also one or more classes in addition to "factor"
    # If so, set the classes, which will preserve "factor" too
    if (preserved_is_factor) {
        return_obj          <- as.factor(return_obj);
        levels(return_obj)  <- preserved_levels;
        if (length(preserved_class) > 1) {
          class(return_obj) <- preserved_class;
        }
    }
    # In all cases we want to preserve the class of the chosen object, so set it here
    else {
        class(return_obj)   <- preserved_class;
    }
    return(return_obj);

} # End safe_ifelse function

Answer 6

这不起作用的原因是因为,ifelse()函数将值转换为因子.一个很好的解决方法是在评估之前将其转换为字符.

dates <- as.Date(c('2011-01-01','2011-01-02','2011-01-03','2011-01-04','2011-01-05'))
dates_new <- dates - 1
dates <- as.Date(ifelse(dates =='2011-01-01',as.character(dates_new),as.character(dates)))

除了基础R之外,这不需要任何库.