向量/矩阵结尾的优雅索引 [英] Elegant indexing up to end of vector/matrix

问题描述

R是否可以说 - 我希望位置 i 中的所有索引到向量/矩阵的结尾？
说我想要从第3列开始的子矩阵。我现在只知道这个方式：

  A = matrix（rep（1：8，each = 5），nrow = 5） ＃只需生成一些示例矩阵... 
 
 A [，3：dim（A）[2]]＃从第3列开始获取子矩阵
  pre> 
 
 
但是我真的需要写这个丑陋的 dim（A）[2] ？没有什么优雅的方式怎么说从第3列开始？像 A [，3：] ？ （或 A [，3：...] ）？
解决方案
有时候，告诉R你不要 想要什么更容易。换句话说，使用负数索引从矩阵中排除列：
 
 
 
以下是两种产生相同结果的两种替代方法：
  A [， - （1：2）] 
 A [，-seq_len（2）] 
  pre> 
 
 
结果：
  [，1] [，2] [，3] [，4] [，5] [，6] 
 [1，] 3 4 5 6 7 8 
 [2，] 3 4 5 6 7 8 
 [3 ，] 3 4 5 6 7 8 
 [4，] 3 4 5 6 7 8 
 [5，] 3 4 5 6 7 8 
  
 
 
 
 
 
 
但要回答您的问题：使用 ncol 找到列数。 （同样地，有 nrow 来查找行数。）
  A [，3：ncol（A）] 
 
 [，1] [，2] [，3] [，4] [，5] [，6] 
 [1，] 3 4 5 6 7 8 
 [2，] 3 4 5 6 7 8 
 [3，] 3 4 5 6 7 8 
 [4，] 3 4 5 6 7 8 
 [5，] 3 4 5 6 7 8 
  
 
Is it possible in R to say - I want all indices from position i to the end of vector/matrix?
Say I want a submatrix from 3rd column onwards. I currently only know this way:
A = matrix(rep(1:8, each = 5), nrow = 5) # just generate some example matrix...

A[,3:dim(A)[2]] # get submatrix from 3rd column onwards
But do I really need to write that ugly dim(A)[2]? Isn't there any elegant way how to say "from the 3rd column onwards"? Something like A[,3:]? (or A[,3:...])?
解决方案
Sometimes it's easier to tell R what you don't want.  In other words, exclude columns from the matrix using negative indexing:

Here are two alternative ways that both produce the same results:
A[, -(1:2)]
A[, -seq_len(2)]
Results:
     [,1] [,2] [,3] [,4] [,5] [,6]
[1,]    3    4    5    6    7    8
[2,]    3    4    5    6    7    8
[3,]    3    4    5    6    7    8
[4,]    3    4    5    6    7    8
[5,]    3    4    5    6    7    8




But to answer your question as asked: Use ncol to find the number of columns.  (Similarly there is nrow to find the number of rows.)
A[, 3:ncol(A)]

     [,1] [,2] [,3] [,4] [,5] [,6]
[1,]    3    4    5    6    7    8
[2,]    3    4    5    6    7    8
[3,]    3    4    5    6    7    8
[4,]    3    4    5    6    7    8
[5,]    3    4    5    6    7    8


                        
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