访问列表元素的[]和[[]]符号之间的区别 [英] The difference between [] and [[]] notations for accessing the elements of a list

问题描述

R提供了两种访问列表或数据元素的不同方法：框架 - [] 和 [[]] 操作符。

两者有什么区别？

解决方案

R语言定义适用于回答这些类型的问题： / p>

• http://cran.r-project.org/doc/manuals/R-lang.html#Indexing

R有三个基本的索引运算符，其语法由以下示例显示

 
x [i] 
x [i，j] 
x [[i]] 
x [[i，j]] 
x $ a 
x $a
 

对于向量和矩阵， [[形式很少被使用，尽管它们与[form（例如，它丢弃任何名称或dimnames属性，并且部分匹配用于字符索引）有一些轻微的语义差异）。当使用单个索引索引多维结构时， x [[i]] 或 x [i] 将返回 i 连续元素 x 。

对于列表，一般通常使用 [[来选择任何单个元素，而 [返回所选元素的列表。

[[ form]只允许使用整数或字符索引选择一个元素，而 允许通过向量进行索引。注意，对于列表，索引可以是向量，并且向量的每个元素依次应用于列表，所选择的组件，该组件的所选组件等等。结果仍然是一个单一的元素。

R provides two different methods for accessing the elements of a list or data.frame- the [] and [[]] operators.

What is the difference between the two? In what situations should I use one over the other?

解决方案

The R Language Definition is handy for answering these types of questions:

• http://cran.r-project.org/doc/manuals/R-lang.html#Indexing

R has three basic indexing operators, with syntax displayed by the following examples

    x[i]
    x[i, j]
    x[[i]]
    x[[i, j]]
    x$a
    x$"a"

For vectors and matrices the [[ forms are rarely used, although they have some slight semantic differences from the [ form (e.g. it drops any names or dimnames attribute, and that partial matching is used for character indices). When indexing multi-dimensional structures with a single index, x[[i]] or x[i] will return the ith sequential element of x.

For lists, one generally uses [[ to select any single element, whereas [ returns a list of the selected elements.

The [[ form allows only a single element to be selected using integer or character indices, whereas [ allows indexing by vectors. Note though that for a list, the index can be a vector and each element of the vector is applied in turn to the list, the selected component, the selected component of that component, and so on. The result is still a single element.

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