Suppose you are given the following code:

x

Question

Suppose you are given the following code:

x <- 1:26

mat <- matrix( x , nrow=7 )

dim( mat )

What would be the second component of dim( mat )?

BlackTom AI · Accepted Answer

The question sets up an R vector and constructs a matrix from it. 
First, x <- 1:26 creates a numeric vector of length 26 containing 1, 2, ..., 26. 
Next, mat <- matrix(x, nrow=7) asks R to shape the data into a matrix with 7 rows. When filling a matrix by default, R uses column-major order and recycles elements if needed. 
Since there are 26 elements and 7 rows, the number of columns must be such that 7 * ncol >= 26. The smallest such integer is ncol = 4, giving a 7-by-4 matrix (7 rows and 4 columns) and leaving 2 extra slots to fill, so R recycles x[1] and x[2] for those positions. 
Therefore dim(mat) returns a vector c(7, 4). The question asks for the second component of dim(mat), which is 4. 
Note that if you expected a ratio 26/7 or an exact division, that would lead to confusion, but R always reports the matrix dimensions as chosen (nrow) and the derived ncol, with recycling only affecting the contents. 
In short, the size in the second dimension corresponds to the number of columns, which in this construction is four.

BTRY3010/BTRY5010/STSCI2200/STSCI5200 Pre - Discussion #11

Suppose you are given the following code: x <- 1:26 mat <- matrix( x , nrow=7 ) dim( mat ) What would be the second component of dim( mat )?

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