[R] matrix indices

[Prof Brian D Ripley]

On Thu, 15 Jul 1999, KAREN KOTSCHY wrote:

> Hi
> 
> Could somebody please explain the following to me:
> If x is a 10x10 matrix,
> typing x[3] prints all 10 values of column 3, although the length of 
> the vector =1. Why?
> x[,3] and x[[3]] both give all 10 values of column 3, length=10. 
> What is the difference between these two, actually?

I suspect that x is actually a data frame in your application, as matrices
do not behave that way:

> x <- matrix(1:100, 10, 10)
> x[3]
[1] 3
> x[[3]]
[1] 3
> x[, 3]
 [1] 21 22 23 24 25 26 27 28 29 30
> x <- as.data.frame(x)
> x[[3]]
 [1] 21 22 23 24 25 26 27 28 29 30
> x[3]
   V3
1  21
2  22
3  23
4  24
5  25
6  26
7  27
8  28
9  29
10 30
> length(x[3])
[1] 1

The reason is that a data frame is a list of columns: x[3] is a data frame
consisting of just one column: x[[3]] and x[, 3] give that column (as a
vector). You get length 1 as the length of the list.


-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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