R is a vector a 1 row matrix or 1 column matrix

Question

a <- c(1,2,3)
b <- c(2,4,6)

Then:

c = cbind(a,b)

In this case, I expect the result is a 1x6 matrix, but it turns out a 3x 2 matrix

d = rbind(a,b)

This turns out a 2x3 matrix. Why are the structures of a and b not consistent? What are the underlined rules here?

Answer 1

a and b are plain vectors (not arrays) and cbind treats plain vectors (and one dimensional arrays) as columns while rbind treats plain vectors (and one dimensional arrays) as rows.

If given 2 dimensional inputs then they do act in the way described in the question.

For example,

A13 <- matrix(1:3, 1)  # 1x3 matrix
B13 <- matrix(4:6, 1)  # 1x3 matrix

cbind(A13, B13)
##      [,1] [,2] [,3] [,4] [,5] [,6]
## [1,]    1    2    3    4    5    6

rbind(A13, B13)
##      [,1] [,2] [,3]
## [1,]    1    2    3
## [2,]    4    5    6

A31 <- matrix(1:3, 3)  # 3x1 matrix
B31 <- matrix(4:6, 3)  # 3x1 matrix

rbind(A31, B31)
##      [,1]
## [1,]    1
## [2,]    2
## [3,]    3
## [4,]    4
## [5,]    5
## [6,]    6

cbind(A31, B31)
##      [,1] [,2]
## [1,]    1    4
## [2,]    2    5
## [3,]    3    6