seq
Sequence Generation
Generate regular sequences. seq is a standard generic with a
default method. seq.int is a primitive which can be
much faster but has a few restrictions. seq_along and
seq_len are very fast primitives for two common cases.
- Keywords
- manip
Usage
seq(...)
"seq"(from = 1, to = 1, by = ((to - from)/(length.out - 1)), length.out = NULL, along.with = NULL, ...)
seq.int(from, to, by, length.out, along.with, ...)
seq_along(along.with)
seq_len(length.out)Arguments
- ...
- arguments passed to or from methods.
- from, to
- the starting and (maximal) end values of the
sequence. Of length
1unless justfromis supplied as an unnamed argument. - by
- number: increment of the sequence.
- length.out
- desired length of the sequence. A
non-negative number, which for
seqandseq.intwill be rounded up if fractional. - along.with
- take the length from the length of this argument.
Details
Numerical inputs should all be finite (that is, not infinite,
NaN or NA).
The interpretation of the unnamed arguments of seq and
seq.int is not standard, and it is recommended always to
name the arguments when programming.
seq is generic, and only the default method is described here.
Note that it dispatches on the class of the first argument
irrespective of argument names. This can have unintended consequences
if it is called with just one argument intending this to be taken as
along.with: it is much better to use seg_along in that
case.
seq.int is an internal generic which dispatches on
methods for "seq" based on the class of the first supplied
argument (before argument matching).
Typical usages are
seq(from, to) seq(from, to, by= ) seq(from, to, length.out= ) seq(along.with= ) seq(from) seq(length.out= )The first form generates the sequence
from, from+/-1, ..., to
(identical to from:to). The second form generates from, from+by, ..., up to the
sequence value less than or equal to to. Specifying to -
from and by of opposite signs is an error. Note that the
computed final value can go just beyond to to allow for
rounding error, but is truncated to to. (‘Just beyond’
is by up to $1e-10$ times abs(from - to).)
The third generates a sequence of length.out equally spaced
values from from to to. (length.out is usually
abbreviated to length or len, and seq_len is much
faster.)
The fourth form generates the integer sequence 1, 2, ...,
length(along.with). (along.with is usually abbreviated to
along, and seq_along is much faster.)
The fifth form generates the sequence 1, 2, ..., length(from)
(as if argument along.with had been specified), unless
the argument is numeric of length 1 when it is interpreted as
1:from (even for seq(0) for compatibility with S).
Using either seq_along or seq_len is much preferred
(unless strict S compatibility is essential).
The final form generates the integer sequence 1, 2, ...,
length.out unless length.out = 0, when it generates
integer(0).
Very small sequences (with from - to of the order of $10^{-14}$
times the larger of the ends) will return from.
For seq (only), up to two of from, to and
by can be supplied as complex values provided length.out
or along.with is specified. More generally, the default method
of seq will handle classed objects with methods for
the Math, Ops and Summary group generics.
seq.int, seq_along and seq_len are
primitive.
Value
seq.int and the default method of seq for numeric
arguments return a vector of type "integer" or "double":
programmers should not rely on which.seq_along and seq_len return an integer vector, unless
it is a long vector when it will be double.
References
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.
See Also
The methods seq.Date and seq.POSIXt.
Examples
library(base)
seq(0, 1, length.out = 11)
seq(stats::rnorm(20)) # effectively 'along'
seq(1, 9, by = 2) # matches 'end'
seq(1, 9, by = pi) # stays below 'end'
seq(1, 6, by = 3)
seq(1.575, 5.125, by = 0.05)
seq(17) # same as 1:17, or even better seq_len(17)
Community examples
seq(1, 5, len = 100)
n<-1000 x<- seq(1,n) sum(x)
# to generate the 10 first odd numbers seq.int(1,10, by = 2) # or c(1:10)[c(T,F)]
`seq_len()` creates a sequence from 1 to n. ```{r} seq_len(17) # same as 1:17 ``` Its main advantage is that it behaves more intuitively when n is zero. ```{r} seq_len(0) 1:0 # Not the same! ``` `seq_along()` creates a sequence from 1 to the length of the input. ```{r} seq_along(month.abb) ``` This is really useful for [`for`](https://www.rdocumentation.org/packages/base/topics/Control) loops. ```{r} for(i in seq_along(month.abb)) { print(month.abb[i]) } ``` `seq.int()` creates a sequence from one number to another. ```{r} seq.int(-1.75, 3.25) ``` You can pass a `by` argument to specify the step size. ```{r} seq.int(-1.75, 3.25, by = 0.5) ``` The `by` argument doesn't have to exactly divide the range of the sequence. ```{r} seq.int(-1.75, 3.25, by = pi / 4) ``` If you get the sign of the `by` argument wrong, `seq.int()` will throw an error. ```{r} tryCatch( seq.int(-1.75, 3.25, by = -1), error = print ) ``` Alternatively you can pass a `length.out` argument to specify the length of the output. ```{r} seq.int(-1.75, 3.25, length.out = 21) ``` Or you can specify an `along.with` argument to specify the length of the output in a different way. ```{r} seq.int(-1.75, 3.25, along.with = month.abb) ``` `seq()` combines the functionality of the three previously mentioned functions into one. By only remembering this one function, you save on brain-memory at a cost of some run-time performance and a more complicated interface. Pass only a single number, and `seq()` behaves like `seq_len()`. ```{r} seq(17) ``` But looks what happens when you pass it zero. ```{r} seq(0) ``` Pass only a vector, and it behaves like `seq_along()`. ```{r} seq(month.abb) ``` Multi-argument uses behave like `seq.int()`. ```{r} seq(-1.75, 3.25, by = 0.5) seq(-1.75, 3.25, length.out = 21) seq(-1.75, 3.25, along.with = month.abb) ```