R: How can I delete rows if an element in a row satisfies certain characteristic?

r delete-row
Akavall picture Akavall · Dec 7, 2011 · Viewed 23.1k times · Source

I am trying to figure out a way to delete rows of matrix if a cell in that row satisfies a certain characteristic. For example:

> mm <- matrix(c(1,2,3,2,3,4,1,2,3,4),5,2)
> mm
     [,1] [,2]
[1,]    1    4
[2,]    2    1
[3,]    3    2
[4,]    2    3
[5,]    3    4

I want to delete rows if the 1st column element in that row is 2. At the end I want this:

   [,1] [,2]
[1,]    1    4
[2,]    3    2
[3,]    3    4

How could I do this?

And what about a more general method if instead of deleting all rows who's first column element is 2, I needed to delete rows who's first column element corresponds to a set of numbers that are contained in a list? For example

delete_list <- c(2,3)

What is the best way to do this?

Thank You in advance.

Answer

nico picture nico · Dec 7, 2011

Just use

mm2 <- mm[mm[,1]!=2,]

This works because

mm[,1] != 2

returns

[1]  TRUE FALSE  TRUE FALSE  TRUE

and essentially you are using this boolean array to choose which rows to pick.

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