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

arun smartpink111 at yahoo.com
Sat Jan 26 03:28:14 CET 2013

```Hi,
May be this helps:
Matrix[,colSums(diff(Matrix))!=0]
#     [,1] [,2] [,3]
#[1,]    1    5    5
#[2,]    2    4    1
#[3,]    3    3    4
#[4,]    4    2    3
#[5,]    5    1    2

A.K.

----- Original Message -----
From: Benjamin Ward (ENV) <B.Ward at uea.ac.uk>
To: "r-help at r-project.org" <r-help at r-project.org>
Cc:
Sent: Friday, January 25, 2013 6:17 PM
Subject: [R] Removal of columns from matrix where all values of the column are identical.

Hi all,

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

What I have written so far is a loop which will see if all values are the same, a bit of a hack since it just checks all values are equal to the first value of the column, if not, by definition the column cannot contain only one value/variable/character:

removals<-c()
for(i in 1:ncol(Matrix)){
if(all(Matrix[,i] == Matrix[[1,i]])){
removals<-append(removals, i)
}
}
new.Matrix <- Matrix[,-removals]

This works for matrices with numbers or characters.
My question is - is there a better or more efficient way of doing this, maybe with apply or something. My first thought was apply set to operate over all columns, but was unsure of the indexing and selecting columns to be deleted.

Thanks,

Ben W.

University of East Anglia (ENV): b.ward at uea.ac.uk
The Sainsbury Laboratory: ben.ward at sainsbury-laboratory.ac.uk

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