by copies the tapply behavior:

> by(9, factor(1, levels=1:2), sum) factor(1, levels = 1:2): 1

[1] 9

------------------------------------------------------------

factor(1, levels = 1:2): 2 [1] NA

aggregate drops the empty cell:

> aggregate(9, list(factor(1, levels=1:2)), sum) Group.1 x

11 9

You can get the \right" answer for the empty cell via split and sapply:

> sapply(split(9, factor(1, levels=1:2)), sum) 1 2 9 0

This behavior depends on the default value of drop=FALSE in split.

8.1.49arithmetic that mixes matrices and vectors

To do matrix multiplication between a matrix and a vector you do:

xmat %*% yvec

or

yvec %*% xmat

R is smart enough to orient the vector in the way that makes sense. There is no need to coerce the vector to a matrix.

If you want to multiply each row of a matrix by the corresponding element of a vector, then do:

xmat * yvec

or

yvec * xmat

This works because of the order in which the elements of the matrix are stored in the underlying vector.

But what to do if you want to multiply each column by the corresponding element of a vector? If you do:

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xmat * yvec

R does not check that the length of yvec matches the number of columns of xmat and do the multiplication that you want. It does a multiplication that you don't want. There are a few ways to get your multiplication, among them are:

xmat * rep(yvec, each=nrow(xmat))

and

sweep(xmat, 2, yvec, '*')

The sweep function is quite general|make friends with it. The scale function can be useful for related problems.

8.1.50single subscript of a data frame or array

Be careful of the number of commas when subscripting data frames and matrices. It is perfectly acceptable to subscript with no commas|this treats the object as its underlying vector rather than a two dimensional object. In the case of a data frame, the underlying object is a list and the single subscript refers to the columns of the data frame. For matrices the underlying object is a vector with length equal to the number of rows times the number of columns.

8.1.51non-numeric argument

> median(x)

Error in median.default(x) : need numeric data

If you get an error like this, it could well be because x is a factor.

8.1.52round rounds to even

The round function rounds to even if it is rounding o an exact 5.

Some people are surprised by this. I'm surprised that they are surprised| rounding to even is the sensible thing to do. If you want a function that rounds up, write it yourself (possibly using the ceiling and floor functions, or by slightly increasing the size of the numbers).

Some times there is the surprise that an exact 5 is not rounded to even. This will be due to Circle 1 (page 9)|what is apparently an exact 5 probably isn't.

8.1.53creating empty lists

You create a numeric vector of length 500 with:

numeric(500)

So obviously you create a list of length 500 with:

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list(500)

Right?

No. A touch of nesse is needed:

vector('list', 500)

Note that this command hints at the fact that lists are vectors in some sense. When \vector" is used in the sense of an object that is not a list, it is really shorthand for \atomic vector".

8.1.54list subscripting

my.list <- list('one', rep(2, 2))

There is a di erence between

my.list[[1]]

and

my.list[1]

The rst is likely what you want|the rst component of the list. The second is a list of length one whose component is the rst component of the original list.

>my.list[[1]] [1] "one"

>my.list[1]

[[1]] [1] "one"

>is.list(my.list[[1]]) [1] FALSE

>is.list(my.list[1]) [1] TRUE

Here are some guidelines:

single brackets always give you back the same type of object { a list in this case.

double brackets need not give you the same type of object.

double brackets always give you one item.

single brackets can give you any number of items.

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8.1.55NULL or delete

If you have a list xl and you want component comp not to be there any more, you have some options. If comp is the index of the component in question, then the most transparent approach is:

xl <- xl[-comp]

In any case you can do:

xl[[comp]] <- NULL

or

xl[comp] <- NULL

The rst two work in S+ as well, but the last one does not|it has no e ect in S+.

If you want the component to stay there but to be NULL, then do:

xl[comp] <- list(NULL)

Notice single brackets, not double brackets.

8.1.56disappearing components

A for loop can drop components of a list that it is modifying.

>xl.in <- list(A=c(a=3, z=4), B=NULL, C=c(w=8), D=NULL)

>xl.out <- vector('list', 4); names(xl.out) <- names(xl.in)

>for(i in 1:4) xl.out[[i]] <- names(xl.in[[i]])

>xl.out # not right

$A

[1] "a" "z" $C

NULL $D

[1] "w"

>xl.out2 <- lapply(xl.in, names)

>xl.out2

$A

[1] "a" "z" $B

NULL $C

[1] "w" $D NULL

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