I have a two dimensional table with distances in a data.frame in R (imported from csv): 

           CP000036   CP001063      CP001368
CP000036      0           a            b
CP001063      a           0            c
CP001368      b           c            0

I'd like to "flatten" it. that I have one axes's value in the first col, and the other axes's value in the second col, and then the distance in the third col:

Genome1      Genome2       Dist
CP000036     CP001063       a
CP000036     CP001368       b
CP001063     CP001368       c

Above is ideal, but it would be completely fine to have repetition such that each cell in the input matrix has it's own row:

Genome1      Genome2       Dist
CP000036     CP000036       0
CP000036     CP001063       a
CP000036     CP001368       b
CP001063     CP000036       a
CP001063     CP001063       0
CP001063     CP001368       c
CP001368     CP000036       b
CP001368     CP001063       c
CP001368     CP001368       0

Here is an example 3x3 matrix, but my dataset I is much larger (about 2000x2000). I would do this in Excel, but I need ~3 million rows for the output, whereas Excel's maximum is ~1 million.

This question is very similar to "How to “flatten” or “collapse” a 2D Excel table into 1D?"1

r 2d dataframe data.table
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