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这次介绍一个大家可能会用到的函数，但是又与我们常用的函数有所区别。

lengths

一看，还以为是length函数，我们要介绍的函数后面多了一个s，专门用于计算列表list的长度，下面用例子来呈现我们的函数。

首先，我们先来介绍length函数，再来介绍lengths，最后对两个差异的地方进行对比，查看区别。

length

Get or set the length of vectors (including lists) and factors, and of any other R object for which a method has been defined

帮助文档内容：x是向量，因子，列表

> length(diag(4))  # = 16 (4 x 4)
[1] 16
> length(1:5)
[1] 5
> length(options())  # 12 or more
[1] 76
> length(y ~ x1 + x2 + x3)  # 3
[1] 3
> length(expression(x, {y <- x^2; y+2}, x^y))  # 3
[1] 3
> require(stats)
> fm1 <- lm(breaks ~ wool * tension, data = warpbreaks)
> length(fm1$call)      # 3, lm() and two arguments.
[1] 3
> length(formula(fm1))  # 3, ~ lhs rhs
[1] 3

lengths

Get the length of each element of a list or atomic vector (is.atomic) as an integer or numeric vector.

x为列表、向量

> require(stats)
> l <- split(airquality$Ozone, airquality$Month)
> l
$`5`
 [1]  41  36  12  18  NA  28  23  19   8  NA   7  16  11  14  18  14  34   6  30
[20]  11   1  11   4  32  NA  NA  NA  23  45 115  37

$`6`
 [1] NA NA NA NA NA NA 29 NA 71 39 NA NA 23 NA NA 21 37 20 12 13 NA NA NA NA NA
[26] NA NA NA NA NA

$`7`
 [1] 135  49  32  NA  64  40  77  97  97  85  NA  10  27  NA   7  48  35  61  79
[20]  63  16  NA  NA  80 108  20  52  82  50  64  59

$`8`
 [1]  39   9  16  78  35  66 122  89 110  NA  NA  44  28  65  NA  22  59  23  31
[20]  44  21   9  NA  45 168  73  NA  76 118  84  85

$`9`
 [1] 96 78 73 91 47 32 20 23 21 24 44 21 28  9 13 46 18 13 24 16 13 23 36  7 14
[26] 30 NA 14 18 20
> lengths(l,use.names = F)
[1] 31 30 31 31 30
> lengths(expression(u, v, 1+ 0:9))
[1] 1 1 3
> f <- c(rep(1, 3), rep(2, 6), 3)
> f
 [1] 1 1 1 2 2 2 2 2 2 3
> dates <- split(as.POSIXlt(Sys.time() + 1:10), f)
> dates
$`1`
[1] "2016-12-09 21:09:50 CST" "2016-12-09 21:09:51 CST"
[3] "2016-12-09 21:09:52 CST"

$`2`
[1] "2016-12-09 21:09:53 CST" "2016-12-09 21:09:54 CST"
[3] "2016-12-09 21:09:55 CST" "2016-12-09 21:09:56 CST"
[5] "2016-12-09 21:09:57 CST" "2016-12-09 21:09:58 CST"

$`3`
[1] "2016-12-09 21:09:59 CST"
> lengths(dates,use.names=F)
[1] 3 6 1

lengths 和 length

> f <- c(rep(1, 3), rep(2, 6), 3)
> f
 [1] 1 1 1 2 2 2 2 2 2 3
> dates <- split(as.POSIXlt(Sys.time() + 1:10), f)
> dates
$`1`
[1] "2016-12-09 21:09:50 CST" "2016-12-09 21:09:51 CST"
[3] "2016-12-09 21:09:52 CST"

$`2`
[1] "2016-12-09 21:09:53 CST" "2016-12-09 21:09:54 CST"
[3] "2016-12-09 21:09:55 CST" "2016-12-09 21:09:56 CST"
[5] "2016-12-09 21:09:57 CST" "2016-12-09 21:09:58 CST"

$`3`
[1] "2016-12-09 21:09:59 CST"
> lengths(dates,use.names=F)
[1] 3 6 1
> length(dates)
[1] 3

length得到的是list的个数，lengths是得到每个list下的个数。这个是计算时很重要。顿时感觉这个函数很神奇。哈哈哈！