一次从矩阵中选择特定元素 [英] Selecting specific elements from a matrix all at once

问题描述

有没有一种方法可以一次从矩阵中选择一堆预先指定的元素?具体来说，假设我有以下矩阵:

Is there a way I can select a bunch of prespecified elements all at once from a matrix? Specifically, suppose I have the following matrix:

      58        59        60       62        63        64
58  0.000000  3.772139  6.367721 8.978718 12.197210 13.401126
59  3.772139  0.000000  3.755554 5.935946  9.592700 11.664533
60  6.367721  3.755554  0.000000 5.999409  9.324764 11.991269
62  8.978718  5.935946  5.999409 0.000000  3.810169  6.762802
63 12.197210  9.592700  9.324764 3.810169  0.000000  3.796884
64 13.401126 11.664533 11.991269 6.762802  3.796884  0.000000

我要选择单元格[1,2]，[2,3]，[3,4]，[4,5]，[5,6].我意识到我可以按索引引用它们，在这种情况下，我可以运行:

I want to select cells [1,2], [2,3], [3,4], [4,5], [5,6]. I realize that I can reference them by index, in this case I can run:

mymatrix[c(2,9,16,23,30)]

但是，从稍后阅读代码中并不能很清楚地看出这一点.有没有一种方法可以一次输入所有的实际(行，列)引用?

However, this is not very clear from reading the code later. Is there a way I can enter the actual (row, column) reference all at once?

推荐答案

可以使用2个列矩阵进行索引.将那些行号和列号转换为有效的R对象(而不是Matlab样式的表达式)后:

Indexing can be done with 2 column matrices. After converting those row and column numbers to a valid R object (rather than Matlab-style expressions):

> idxs <- gsub("\\]",")", gsub("\\[", "c(",  "[1,2], [2,3], [3,4], [4,5] ,[5,6]") )
# I edited the string value that idxs returned:
> midx <- rbind( c(1,2), c(2,3), c(3,4), c(4,5) ,c(5,6) )
> mat <-  matrix(scan(), nrow=6)  
1:  0.000000  3.772139  6.367721 8.978718 12.197210 13.401126
7:   3.772139  0.000000  3.755554 5.935946  9.592700 11.664533
13:   6.367721  3.755554  0.000000 5.999409  9.324764 11.991269
19:   8.978718  5.935946  5.999409 0.000000  3.810169  6.762802
25:  12.197210  9.592700  9.324764 3.810169  0.000000  3.796884
31:  13.401126 11.664533 11.991269 6.762802  3.796884  0.000000
37: 
Read 36 items
> mat[midx]
[1] 3.772139 3.755554 5.999409 3.810169 3.796884

如果您的目标是索引可以更广泛地实现的超对角线，则:

If your goal were to index the super-diagonal that could be accomplished more generally:

> mat[col(mat)==row(mat)+1]
[1] 3.772139 3.755554 5.999409 3.810169 3.796884

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