[R] Removal of columns from matrix where all values of the column are identical.

[Ben Bolker]

Benjamin Ward (ENV <B.Ward <at> uea.ac.uk> writes:

> I'd like to write a piece of code which will remove columns from a matrix, if
the column contains only one
> value, say every value in the column is a "3":
> 
> Matrix <- matrix(NA, nrow=5, ncol=4)
> Matrix[,1] <- c(1,2,3,4,5)
> Matrix[,2] <- c(3,3,3,3,3)
> Matrix[,3] <- c(5,4,3,2,1)
> Matrix[,4] <- c(5,1,4,3,2)
> 
>       [,1] [,2] [,3] [,4]
> [1,]    1    3    5    5
> [2,]    2    3    4    1
> [3,]    3    3    3    4
> [4,]    4    3    2    3
> [5,]    5    3    1    2

 I think you want Matrix[, apply(Matrix,2,function(x) { length(unique(x))>1 ) } ]

  The anonymous function tests whether there is more than one unique value.
apply() runs it on every column and returns a logical vector.  The [ , ]
indexing selects the corresponding columns ...