matrix() Help
On Tue, 2004-02-24 at 10:51, Jonathan Wang wrote:
I have a question related to matrix(). The code below randomly generates 3 Poisson numbers into a 3 by 1 matrix:
matrix1 <- matrix(rpois(3,lambda=2),nrow=3,ncol=1)
And I use list() to see what they are:
[,1]
[1,] 3
[2,] 1
[3,] 4
, which is what I had intended.
I then I want to randomly generate y Normal numbers into a 3 by 8
matrix. y in this case would be 3, 1, and 4; so the resulting matrix
should be such that the first row has three different Normal numbers;
the second row has 1 Normal number; and the third row has four
different Normal numbers. I have the following code to do that:
matrix2 <- matrix(rnorm(poisNums),nrow=3,ncol=8)
To my surprise, matrix2 does not look anything like what I had intended. Instead, it repeats the values in the first column (of the matrix2) eight times. What part of the code do I need to fix?
See the help for rnorm(), which says:
n: number of observations. If 'length(n) > 1', the length is
taken to be the number required.
Thus, by using rnorm(matrix1), you get 3 numbers back, which is the
length of matrix1. Since 3 is less than 24 (3 x 8), the vector from
rnorm is cycled to fill in the matrix.
Use lapply() and have the final result be a 'list', since the number of
values in each component will vary:
matrix1 <- matrix(rpois(3, lambda = 2), nrow = 3, ncol = 1)
matrix1
[,1] [1,] 5 [2,] 3 [3,] 1 list2 <- lapply(matrix1, rnorm)
list2
[[1]] [1] -1.186458639 1.096777044 -0.005344028 0.707310667 1.034107735 [[2]] [1] 0.2234804 -0.8787076 1.1629646 [[3]] [1] -2.000165 See ?lapply Watch out for situations where an element in matrix1 is a zero. You will get a numeric(0) as a result in that list element. HTH, Marc Schwartz