Question

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

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)]

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?

Answer 1

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

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