Selecting and building matrices

Selecting parts of matrices

We can pick out specific elements, rows or columns of a data frame (can be numbers and/or characters) or of a matrix (just numbers) as in the following examples:

> gala[2,3]
> gala[2,]
> gala[,3]

We can construct new vectors using the c() command:

> c(2,4,6)

and select particular columns out of the matrix:

> gala[,c(2,4,6)]

or take everything but certain columns:

> gala[,-c(2,4,6)]

Question Suppose I'm not interested in the variable Endemics and I'd like to exclude the last island, Wolf, because perhaps there is something atypical about it. How would you create a new dataset without this variable and case? (Tip: use dim(gala) to find the dimensions of the matrix)

Building matrices

Creating a vector of repeated elements is done using rep()

> rep(1,30)

and vectors and matrices can be bound together column-wise as in

> x <- cbind(rep(1,30),gala[,-c(1,2)])
> x

although

> cbind(1,gala[,-c(1,2)])

has the same effect. Other useful building commands are rbind() and seq() which is useful for creating sequences.

A few commands:

>t(A) would take Matrix Transpose

>solve(A) would take motrix inverse.