Robin Hankin <r.hankin <at> soc.soton.ac.uk> writes: : : Hi : : I have a two-column integer matrix like this: : : R> jj : : [,1] [,2] : [1,] -1 1 : [2,] -2 2 : [3,] -7 6 : [4,] -8 7 : [5,] -6 5 : [6,] -9 8 : [7,] -5 4 : [8,] 3 -3 : [9,] -10 9 : [10,] -4 3 : : I want a diagnostic that detects whether a row is a multiple of : the first row or not. In this case, this would be rows 1,2, and 8. : : How to do this nicely? Sometimes the first row has a zero, so the : method would have to work on : : [,1] [,2] : [1,] 0 1 : [2,] 1 1 : [3,] 0 8 : [4,] 0 -4 : [5,] 0 0 : [6,] 4 0 : > : : in which case rows 1,3,4,5 are multiples. It'd be nice to have : a solution that works for any number of columns.
If rowi is a multiple of row1 then crossprod(cbind(row1, rowi)) is singular so: apply(mat, 1, function(x) det(crossprod(cbind(x, mat[1,])))) == 0 If your matrix only has two columns then crossprod(x) is singular iff x is so you can eliminate the crossprod. ______________________________________________ [email protected] mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
