1、seq()
用来生成一组数字的函数。
Usage：
## Default S3 method:
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 1 unless just
from ： is supplied as an unnamed argument.
by ： number: increment of the sequence.
length.out ：desired length of the sequence. A non-negative number, which for seq and
seq.int ： will be rounded up if fractional.
along.with ： take the length from the length of this argument.
Typical usages are
seq(from, to)
seq(from, to, by= )
seq(from, to, length.out= )
seq(along.with= )
seq(from)
seq(length.out= )
Examples：
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)