[R] group by rows

[S Ellison]

> How is it possible to group rows of a matrix or a data frame by the same values
> of the first column?

If you mean _group_ as in SQL GROUP BY, use aggregate() with a count or summary statistic. 

If you mean _sort_, just to get similar values close together, use order()

For example, to sort by the first column of a matrix mm:
oo <- order(mm[,1])
mm[oo,] 

order() also takes multiple sort fields so can sort by several columns simultaneously (eg sort by first column and within that by third column etc).

S Ellison




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