如何在R中选择向量的顶部或底部n个元素？
顶部或底部元素的选择可以借助R中的头和尾函数来完成。当我们想了解向量中的数据或对部分数据进行某种计算时，这是必需的。
示例
考虑下面的向量，我们将使用头和尾通过使用正号和负号来选择这些向量中的顶部和底部元素。这些将以不同的方式选择元素。
> x<-letters[1:26] > x [1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s" [20] "t" "u" "v" "w" "x" "y" "z" > head(x,3) [1] "a" "b" "c" > head(x,5) [1] "a" "b" "c" "d" "e" > head(x,10) [1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" > tail(x,3) [1] "x" "y" "z" > tail(x,5) [1] "v" "w" "x" "y" "z" > tail(x,10) [1] "q" "r" "s" "t" "u" "v" "w" "x" "y" "z" > tail(x,-3) [1] "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s" "t" "u" "v" [20] "w" "x" "y" "z" > tail(x,-5) [1] "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s" "t" "u" "v" "w" "x" [20] "y" "z" > tail(x,-10) [1] "k" "l" "m" "n" "o" "p" "q" "r" "s" "t" "u" "v" "w" "x" "y" "z" > head(x,-3) [1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s" [20] "t" "u" "v" "w" > head(x,-5) [1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s" [20] "t" "u" > head(x,-10) [1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" > y<-1:50 > y [1] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 [26] 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 > head(y,5) [1] 1 2 3 4 5 > head(y,10) [1] 1 2 3 4 5 6 7 8 9 10 > tail(y,5) [1] 46 47 48 49 50 > tail(y,10) [1] 41 42 43 44 45 46 47 48 49 50 > head(y,-5) [1] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 [26] 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 > head(y,-10) [1] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 [26] 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 > tail(y,-5) [1] 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 [26] 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 > tail(y,-10) [1] 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 [26] 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 > set.seed(9) > z<-sample(letters[1:20],30,replace=TRUE) > z [1] "f" "s" "c" "l" "p" "l" "e" "r" "e" "j" "l" "c" "p" "i" "a" "f" "b" "o" "b" [20] "n" "s" "c" "n" "e" "i" "f" "k" "c" "k" "q" > head(z,3) [1] "f" "s" "c" > head(z,5) [1] "f" "s" "c" "l" "p" > head(z,10) [1] "f" "s" "c" "l" "p" "l" "e" "r" "e" "j" > tail(z,3) [1] "c" "k" "q" > tail(z,5) [1] "f" "k" "c" "k" "q" > tail(z,10) [1] "s" "c" "n" "e" "i" "f" "k" "c" "k" "q" > head(z,-3) [1] "f" "s" "c" "l" "p" "l" "e" "r" "e" "j" "l" "c" "p" "i" "a" "f" "b" "o" "b" [20] "n" "s" "c" "n" "e" "i" "f" "k" > head(z,-5) [1] "f" "s" "c" "l" "p" "l" "e" "r" "e" "j" "l" "c" "p" "i" "a" "f" "b" "o" "b" [20] "n" "s" "c" "n" "e" "i" > head(z,-10) [1] "f" "s" "c" "l" "p" "l" "e" "r" "e" "j" "l" "c" "p" "i" "a" "f" "b" "o" "b" [20] "n" > tail(z,-3) [1] "l" "p" "l" "e" "r" "e" "j" "l" "c" "p" "i" "a" "f" "b" "o" "b" "n" "s" "c" [20] "n" "e" "i" "f" "k" "c" "k" "q" > tail(z,-5) [1] "l" "e" "r" "e" "j" "l" "c" "p" "i" "a" "f" "b" "o" "b" "n" "s" "c" "n" "e" [20] "i" "f" "k" "c" "k" "q" > tail(z,-10) [1] "l" "c" "p" "i" "a" "f" "b" "o" "b" "n" "s" "c" "n" "e" "i" "f" "k" "c" "k" [20] "q"