Note that the result from our for loop is MOST decidedly not what we want. Possibly not even what we could have dreamed we could get.

Take care when NULL can be something that is assigned into a component of a list. Using lapply can be a good alternative.

8.1.57combining lists

Some people are pleasantly surprised that the c function works with lists. Then they go on to abuse it.

>xlis <- list(A=1:4, B=c('a', 'x'))

>c(xlis, C=6:5)

$A

[1] 1 2 3 4 $B

[1] "a" "x" $C1

[1] 6 $C2 [1] 5

Probably not what was intended. Try:

c(xlis, list(C=6:5))

8.1.58disappearing loop

Consider the loop:

for(i in 1:10) i

It is a common complaint that this loop doesn't work, that it doesn't do anything. Actually it works perfectly well. The problem is that no real action is involved in the loop. You probably meant something like:

for(i in 1:10) print(i)

Automatic printing of unassigned objects only happens at the top level.

8.1.59limited iteration

One of my favorite tricks is to only give the top limit of iteration rather than the sequence:

for(i in trials) f ... g

rather than

for(i in 1:trials) f ... g

Then I wonder why the results are so weird.

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8.1.60too much iteration

for(i in 1:length(x)) f ... g

is ne if x has a positive length. However, if its length is zero, then R wants to do two iterations. A safer idiom is:

for(i in seq along(x)) f ... g

or if you want to be compatible with S+:

for(i in seq(along=x)) f ... g

8.1.61wrong iterate

The for iterate can be from any vector. This makes looping much more general than in most other languages, but can allow some users to become confused:

nums <- seq(-1, 1, by=.01) ans <- NULL

for(i in nums) ans[i] <- i^2

This has two things wrong with it. You should recognize that we have tried (but failed) to visit Circle 2 (page 12) here, and the index on ans is not what the user is expecting. Better would be:

nums <- seq(-1, 1, by=.01) ans <- numeric(length(nums))

for(i in seq(along=nums)) ans[i] <- nums[i]^2

Even better, of course, would be to avoid a loop altogether. That is possible in this case, perhaps not in a real application.

8.1.62wrong iterate (encore)

A loop like:

for(i in 0:9) f this.x <- x[i]

...

does not do as intended. While C and some other languages index from 0, R indexes from 1. The unfortunate thing in this case is that an index of 0 is allowed in R, it just doesn't do what is wanted.

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8.1.63wrong iterate (yet again)

>nam <- c(4, 7)

>vec <- rep(0, length(nam))

>names(vec) <- nam

>for(i in nam) vec[i] <- 31

>vec

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00 NA 31 NA NA 31

8.1.64iterate is sacrosanct

In the following loop there are two uses of 'i'.

> for(i in 1:3) f

+cat("i is", i, "nn")

+i <- rpois(1, lambda=100)

+cat("end iteration", i, "nn")

+g

i is 1

end iteration 93 i is 2

end iteration 91 i is 3

end iteration 101

The i that is created in the body of the loop is used during that iteration but does not change the i that starts the next iteration. This is unlike a number of other languages (including S+).

This is proof that R is hard to confuse, but such code will de nitely confuse humans. So avoid it.

8.1.65wrong sequence

What was meant was either the second or third command, but mixing them together gets you the wrong result.

8.1.66empty string

Do not confuse

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character(0)

with

""

The rst is a vector of length zero whose elements would be character if it had any. The second is a vector of length one, and the element that it has is the empty string.

The result of nchar on the rst object is a numeric vector of length zero, while the result of nchar on the second object is 0|that is, a vector of length one whose rst and only element is zero.

>nchar(character(0))

numeric(0)

>nchar("")

[1]0

8.1.67NA the string

There is a missing value for character data. In normal printing (with quotes around strings) the missing value is printed as NA; but when quotes are not used, it is printed as <NA>. This is to distinguish it from the string 'NA':

>cna <- c('missing value'=NA, 'real string'='NA')

>cna

missing value real string NA "NA"

> noquote(cna)

missing value real string <NA> NA

NA the string really does happen. It is Nabisco in nance, North America in geography, and possibly sodium in chemistry. There are circumstances| particularly when reading data into R|where NA the string becomes NA the missing value. Having a name or dimname that is accidentally a missing value can be an unpleasant experience.

If you have missing values in a character vector, you may want to take some evasive action when operating on the vector:

>people <- c('Alice', NA, 'Eve')

>paste('hello', people)

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