lapply和do.call有什么区别？ [英] What's the difference between lapply and do.call?

问题描述

我最近在学习R，并被两个函数混淆： lapply 和 do.call 。看起来它们与Lisp中的 map 函数类似。但为什么有两个名称不同的函数呢？为什么R不使用一个名为 map ？

解决方案

有一个名为 Map 的函数，可能类似于其他语言的地图：

• lapply 返回与X相同长度的列表，其中每个元素都是将FUN应用于X的对应元素的结果。




• do.call 构造并执行函数调用，从一个名称或一个函数以及要传递给的参数列表
  

• Map 将函数应用于给定向量的相应元素... Map 是对 mapply 的简单包装，它不会试图简化结果，类似于Common Lisp的mapcar（参数被回收，然而）。未来的版本可能允许对结果类型进行一些控制。





$ ol b $ b
• Map 是 mapply


• code> lapply 是 mapply


• 的一个特例，因此在许多情况下，Map 和 lapply 类似。
  

  例如，这是 lapply ：

    lapply（虹膜，class）
   $ Sepal.Length 
   [1]numeric
   
   $ Sepal.Width 
   [1]numeric
   
   $ Petal.Length 
   [1]numeric
   
   $ Petal.Width 
   [1]numeric
   
   $物种
   [1]factor
    

  同样使用 Map $ b $ pre code $ Map $ {
  $ Sepal.Length
  [1] numeric
  
  $ Sepal.Width
  [1]numeric
  
  $ Petal.Length
  [1]numeric
  
  $ Petal.Width
  [1]numeric
  
  $物种
  [1]factor
  

  
  
  

  ---

  
  
  

  do.call 将函数作为输入并将其他参数泼溅到函数中。例如，它被广泛用于将列表组装成更简单的结构（通常使用 rbind 或 cbind ）。例如：
  
  

    x < -  lapply（iris，class） 
   do.call（c，x）
   Sepal.Length Sepal.Width Petal.Length Petal.Width种类
  数字数字数字数字因子
    

  
  

  I'm learning R recently and confused by two function: lapplyand do.call. It seems that they're just similar to map function in Lisp. But why are there two functions with such a different name? Why doesn't R just use a function called map?

  解决方案

  There is a function called Map that may be similar to map in other languages:

  • lapply returns a list of the same length as X, each element of which is the result of applying FUN to the corresponding element of X.

  • do.call constructs and executes a function call from a name or a function and a list of arguments to be passed to it.

  • Map applies a function to the corresponding elements of given vectors... Map is a simple wrapper to mapply which does not attempt to simplify the result, similar to Common Lisp's mapcar (with arguments being recycled, however). Future versions may allow some control of the result type.

  ---

  1. Map is a wrapper around mapply
  2. lapply is a special case of mapply
  3. Therefore Map and lapply will be similar in many cases.

  For example, here is lapply:

  lapply(iris, class)
  $Sepal.Length
  [1] "numeric"
  
  $Sepal.Width
  [1] "numeric"
  
  $Petal.Length
  [1] "numeric"
  
  $Petal.Width
  [1] "numeric"
  
  $Species
  [1] "factor"
  

  And the same using Map:

  Map(class, iris)
  $Sepal.Length
  [1] "numeric"
  
  $Sepal.Width
  [1] "numeric"
  
  $Petal.Length
  [1] "numeric"
  
  $Petal.Width
  [1] "numeric"
  
  $Species
  [1] "factor"
  

  ---

  do.call takes a function as input and splatters its other arguments to the function. It is widely used, for example, to assemble lists into simpler structures (often with rbind or cbind).

  For example:

  x <- lapply(iris, class)
  do.call(c, x)
  Sepal.Length  Sepal.Width Petal.Length  Petal.Width      Species 
     "numeric"    "numeric"    "numeric"    "numeric"     "factor" 
  

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