括号 [ ] 和双括号 [[ ]] 用于访问列表或数据框元素的区别 [英] The difference between bracket [ ] and double bracket [[ ]] for accessing the elements of a list or dataframe

问题描述

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

What is the difference between the two, and when 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.

这篇关于括号 [ ] 和双括号 [[ ]] 用于访问列表或数据框元素的区别的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！