R help - Mar 2006 - is there a way to let R do smart matrix-vector operation?

Hi all,

I want to substract vector B from A's each column... how can R do that
smartly without a loop?
> A=matrix(c(2:7), 2, 3)
> A
     [,1] [,2] [,3]
[1,]    2    4    6
[2,]    3    5    7
> B=matrix(c(1, 2), 2, 1)
> B
     [,1]
[1,]    1
[2,]    2
> A-B
Error in A - B : non-conformable arrays

	[[alternative HTML version deleted]]

On Mon, 2006-03-06 at 15:10 -0800, Michael wrote:
> Hi all,
> 
> I want to substract vector B from A's each column... how can R do that
> smartly without a loop?
> 
> > A=matrix(c(2:7), 2, 3)
> > A
>      [,1] [,2] [,3]
> [1,]    2    4    6
> [2,]    3    5    7
> > B=matrix(c(1, 2), 2, 1)
> > B
>      [,1]
> [1,]    1
> [2,]    2
> > A-B
> Error in A - B : non-conformable arrays
> apply(A, 2, "-", B)
     [,1] [,2] [,3]
[1,]    1    3    5
[2,]    1    3    5


You can use apply() on column-wise operations such as this.

See ?apply for more information.

HTH,

Marc Schwartz

It's rarely necessary to have a vector in matrix form.  In this case, it
actually make things harder.  If B had been a vector, you would have gotten:
> A=matrix(c(2:7), 2, 3)
> B=matrix(c(1, 2), 2, 1)
> A - as.vector(B)
     [,1] [,2] [,3]
[1,]    1    3    5
[2,]    1    3    5

Andy

From: Michael
> 
> Hi all,
> 
> I want to substract vector B from A's each column... how can 
> R do that smartly without a loop?
> 
> > A=matrix(c(2:7), 2, 3)
> > A
>      [,1] [,2] [,3]
> [1,]    2    4    6
> [2,]    3    5    7
> > B=matrix(c(1, 2), 2, 1)
> > B
>      [,1]
> [1,]    1
> [2,]    2
> > A-B
> Error in A - B : non-conformable arrays
> 
> 	[[alternative HTML version deleted]]
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list 
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide! 
> http://www.R-project.org/posting-guide.html
> 
>

Well, I'm not sure what's "smart" and what's
"dumb." As Mark said, a
standard prescription for this sort of thing is to use apply() type calls --
but that actually is still using a loop, though generally very efficiently. 

If you want to avoid even that sort of hidden looping then try:


matrix(as.vector(A)-as.vector(B),nr=nrow(A))

If B is a vector, not a matrix, the as.vector(B) cast isn't necessary.


-- Bert Gunter
Genentech Non-Clinical Statistics
South San Francisco, CA
 
> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch 
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Michael
> Sent: Monday, March 06, 2006 3:11 PM
> To: R-help at stat.math.ethz.ch
> Subject: [R] is there a way to let R do smart matrix-vector operation?
> 
> Hi all,
> 
> I want to substract vector B from A's each column... how can R do that
> smartly without a loop?
> 
> > A=matrix(c(2:7), 2, 3)
> > A
>      [,1] [,2] [,3]
> [1,]    2    4    6
> [2,]    3    5    7
> > B=matrix(c(1, 2), 2, 1)
> > B
>      [,1]
> [1,]    1
> [2,]    2
> > A-B
> Error in A - B : non-conformable arrays
> 
> 	[[alternative HTML version deleted]]
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide! 
> http://www.R-project.org/posting-guide.html
>

Hi Michael, 

Try: 

  A-as.vector(B)

Cheers, 

Anders. 

On Monday 06 March 2006 01:10 pm, Michael wrote:
> Hi all,
>
> I want to substract vector B from A's each column... how can R do
> that smartly without a loop?
>
> > A=matrix(c(2:7), 2, 3)
> > A
>
>      [,1] [,2] [,3]
> [1,]    2    4    6
> [2,]    3    5    7
>
> > B=matrix(c(1, 2), 2, 1)
> > B
>
>      [,1]
> [1,]    1
> [2,]    2
>
> > A-B
>
> Error in A - B : non-conformable arrays
>
> 	[[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide!
> http://www.R-project.org/posting-guide.html

The following are nearly identical to what others have already
written but just in case:

A - c(B)

or

A - B[,1]

or if B were already a vector, b, in the first place, rather than a matrix:

b <- 1:2
A - b



On 3/6/06, Michael <comtech.usa at gmail.com>
wrote:
> Hi all,
>
> I want to substract vector B from A's each column... how can R do that
> smartly without a loop?
>
> > A=matrix(c(2:7), 2, 3)
> > A
>     [,1] [,2] [,3]
> [1,]    2    4    6
> [2,]    3    5    7
> > B=matrix(c(1, 2), 2, 1)
> > B
>     [,1]
> [1,]    1
> [2,]    2
> > A-B
> Error in A - B : non-conformable arrays
>
>        [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide!
http://www.R-project.org/posting-guide.html
>

One can always use the often-overlooked sweep():


 > a <- matrix(2:7,2,3)
 > b <- matrix(1:2,2,1)
 > sweep(a,1,b)
      [,1] [,2] [,3]
[1,]    1    3    5
[2,]    1    3    5
 >

best


rksh



On 6 Mar 2006, at 23:10, Michael wrote:
> Hi all,
>
> I want to substract vector B from A's each column... how can R do that
> smartly without a loop?
>
>> A=matrix(c(2:7), 2, 3)
>> A
>      [,1] [,2] [,3]
> [1,]    2    4    6
> [2,]    3    5    7
>> B=matrix(c(1, 2), 2, 1)
>> B
>      [,1]
> [1,]    1
> [2,]    2
>> A-B
> Error in A - B : non-conformable arrays
>
> 	[[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide! http://www.R-project.org/posting- 
> guide.html
--
Robin Hankin
Uncertainty Analyst
National Oceanography Centre, Southampton
European Way, Southampton SO14 3ZH, UK
  tel  023-8059-7743

On 06-Mar-06 Michael wrote:
> Hi all,
> 
> I want to substract vector B from A's each column... how can R do that
> smartly without a loop?
> 
>> A=matrix(c(2:7), 2, 3)
>> A
>      [,1] [,2] [,3]
> [1,]    2    4    6
> [2,]    3    5    7
>> B=matrix(c(1, 2), 2, 1)
>> B
>      [,1]
> [1,]    1
> [2,]    2
>> A-B
> Error in A - B : non-conformable arrays
What a lot of answers to this query!

Also this one (which I adopted following someone's useful
response to a query of mine a very long time ago -- sorry
I can't now trace who it was):

## Binary operators %+.%, %.+% to add vector
## or kx1 or 1xk matrix to cols or rows of a matrix
"%+.%"<-function(X,x){sweep(X,1,x,"+")} ## Adds to cols
"%.+%"<-function(X,x){sweep(X,2,x,"+")} ## Adds to rows

So A %+.% x would add the elements of x to cols of A,
or A %.+% y would add the elements of y to rows of A,
provided the length of x is the number of rows in A
or the length or y is the number of columns.

These operators can easily be extended to %-.%, %.-%,
%*.%, %.*%, %/.%, %./% (in an obvious notation).

I have the above operators set up in my .Rprofile (from
which the above code is copied). The only thing wrong with
them is that they are traps for typophiliacs (like me ... ).

Best wishes to all,
Ted.


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E-Mail: (Ted Harding) <Ted.Harding at nessie.mcc.ac.uk>
Fax-to-email: +44 (0)870 094 0861
Date: 07-Mar-06                                       Time: 08:34:03
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