[R] division of col by the sum of the col

[peter dalgaard]

On 23 Sep 2014, at 23:30 , Rolf Turner <r.turner at auckland.ac.nz> wrote:

> On 24/09/14 03:10, carol white wrote:
>> Hi, If I want to divide the column of a matrix by the sum of the
>> column, should I loop over the columns or can I use apply family?
> 
> m1 <- apply(m,2,function(x){x/sum(x)})
> 
> should do what you want IIUYC.

It might, but there's a surprise if you do the same thing with rows.

The canonical construction is

sweep(M, 2, apply(M, 2, sum), "/")

although efficiency suggests replacing the apply() with colSums(M) nowadays.

> 
> cheers,
> 
> Rolf Turner
> 
> -- 
> Rolf Turner
> Technical Editor ANZJS
> 
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Peter Dalgaard, Professor,
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