matrix() Help

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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

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.
For sure you used print(), but not list() ...
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?
You don't want to use a matrix here, but a list:

list2 <- apply(matrix1, 1, rnorm)

Please read "An Introduction to R" and learn a bit more on data 
structures in R ...

Uwe Ligges
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