Another R tip. Use `seq_len()`

to avoid the backwards sequence trap.

Many R users use the “colon sequence” notation to build sequences. For example:

for(i in 1:5) { print(paste(i, i*i)) } #> [1] "1 1" #> [1] "2 4" #> [1] "3 9" #> [1] "4 16" #> [1] "5 25"

However, the colon notation can be unsafe as it does not properly handle the empty sequence case:

n <- 0 1:n #> [1] 1 0

Notice the above example built a reversed sequence, instead of an empty sequence.

This leads to the backwards sequence trap: writing code of the form “`1:length(x)`

” is often wrong. For example “`for(i in 1:length(x)) { statements involving x[[i]] }`

“, which will fail for length-zero `x`

.

To avoid this use `seq_len()`

or `seq_along()`

:

seq_len(5) #> [1] 1 2 3 4 5 n <- 0 seq_len(n) #> integer(0)

“`integer(0)`

” is a length zero sequence of integers (not a sequence containing the value zero).

### jmount

Data Scientist and trainer at Win Vector LLC. One of the authors of Practical Data Science with R.

I may be missing something, but I don’t understand why you call the result of 1:0 a bug. That produces exactly what I would expect it to.

The “:” operator can be thought of as shorthand for a call to the seq() function. This shorthand only accepts 2 of the arguments that seq() accepts—“from” and “to”—default values are used for the others. Note that seq(1,0) returns the same result that 1:0 does. To get an empty string, just issue seq(length.out=0).

Two R results that I mentioned:

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Thanks for your comment. I left a bit out which I have now put at the end of the article with an edit. Sorry about that.

The behavior does match

`help(`:`)`

, so it isn’t a bug on its own. But it is a key part of buggy for-loops such as this:From https://www.r-bloggers.com/writing-a-for-loop-in-r/:

The above errors out if

`foo`

has length 0. And the above form is unfortunately taught a lot. The fix I suggest is use`seq_len()`

. Yes, one can use`vapply()`

in this case.I wish I had said it in the original article, but code of the form

`1:length(x)`

is a very common (so common I forgot I had not mentioned it), and this the use of “`:`

” and is often wrong (not what the programmer actually intended).LikeLike

Agree that bug in the headline is misleading. Maybe replace “bug” with “situation”.

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Thanks! That is the idea that was not coming to me. I am going to say “trap” (still strong, but less misleading).

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That sounds good to me! Thanks for your articles.

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for(i in 1:5) {

+ print(paste(i, i*i))

+ }

produces-

[1] “1 1”

[1] “2 4”

[1] “3 9”

[1] “4 16”

[1] “5 25”

omitting the last line makes me worry that you wrote this in python :)

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Thank you. And sorry about that (now fixed).

And that is why I should have used RMarkdown instead of copying and pasting!

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Turns out we are all in good company:

(Quoted from the fortunes package.)

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