User:Timothee Flutre/Notebook/Postdoc/2012/02/17: Difference between revisions

From OpenWetWare
Jump to navigationJump to search
(Autocreate 2012/02/17 Entry for User:Timothee_Flutre/Notebook/Postdoc)
 
(→‎Entry title: in R, subtract vector from columns of matrix)
Line 6: Line 6:
| colspan="2"|
| colspan="2"|
<!-- ##### DO NOT edit above this line unless you know what you are doing. ##### -->
<!-- ##### DO NOT edit above this line unless you know what you are doing. ##### -->
==Entry title==
==How to subtract a vector from a matrix in R==
* Insert content here...


In R, matrix algebra is very fast, so it's better to use it. But it's not always clear how to subtract a vector from the row of a matrix, or from its column, especially when the matrix is square.
* Before anything else, let's define our dummy data. Here is my matrix:
m <- matrix(1:4, nrow=2, ncol=2, byrow=TRUE)
m
      [,1] [,2]
[1,]    1    2
[2,]    3    4
And here is my vector:
v <- c(1, 5)
v
[1] 1 5
* First, I want to '''subtract v from each column of m'''. Here is what I want to obtain:
      [,1] [,2]
[1,]    0    1
[2,]  -2  -1
Internally, R represents a matrix as a vector concatenating the columns of the matrix. That's why the following simple command gives me what I want:
m - v
      [,1] [,2]
[1,]    0    1
[2,]  -2  -1
We could also have done it with [http://stat.ethz.ch/R-manual/R-patched/library/base/html/apply.html apply]:
apply(m, 2, "-", v)
      [,1] [,2]
[1,]    0    1
[2,]  -2  -1
Or with [http://stat.ethz.ch/R-manual/R-patched/library/base/html/sweep.html sweep]:
sweep(m, 1, v, "-")
      [,1] [,2]
[1,]    0    1
[2,]  -2  -1
Note that the parameter MARGIN in "apply" should be put to 2 to operate on the columns, while in "sweep" it should be put to 1...


<!-- ##### DO NOT edit below this line unless you know what you are doing. ##### -->
<!-- ##### DO NOT edit below this line unless you know what you are doing. ##### -->

Revision as of 15:33, 17 February 2012

Project name <html><img src="/images/9/94/Report.png" border="0" /></html> Main project page
<html><img src="/images/c/c3/Resultset_previous.png" border="0" /></html>Previous entry<html>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</html>Next entry<html><img src="/images/5/5c/Resultset_next.png" border="0" /></html>

How to subtract a vector from a matrix in R

In R, matrix algebra is very fast, so it's better to use it. But it's not always clear how to subtract a vector from the row of a matrix, or from its column, especially when the matrix is square.

  • Before anything else, let's define our dummy data. Here is my matrix:
m <- matrix(1:4, nrow=2, ncol=2, byrow=TRUE)
m
     [,1] [,2]
[1,]    1    2
[2,]    3    4

And here is my vector: 

v <- c(1, 5)
v
[1] 1 5
  • First, I want to subtract v from each column of m. Here is what I want to obtain:
     [,1] [,2]
[1,]    0    1
[2,]   -2   -1

Internally, R represents a matrix as a vector concatenating the columns of the matrix. That's why the following simple command gives me what I want: 

m - v
     [,1] [,2]
[1,]    0    1
[2,]   -2   -1

We could also have done it with apply: 

apply(m, 2, "-", v)
     [,1] [,2]
[1,]    0    1
[2,]   -2   -1

Or with sweep: 

sweep(m, 1, v, "-")
     [,1] [,2]
[1,]    0    1
[2,]   -2   -1

Note that the parameter MARGIN in "apply" should be put to 2 to operate on the columns, while in "sweep" it should be put to 1...


Retrieved from "https://openwetware.org/mediawiki/index.php?title=User:Timothee_Flutre/Notebook/Postdoc/2012/02/17&oldid=585778"