选择上下两个N行 [英] Select N rows above and below match
问题描述
我想选择一个匹配项上下的N行。
我正在尝试以下命令:
mtcars [which(mtcars $ vs == 1)+ c(-1:1),]
它返回以下警告:
警告消息:
其中( mtcars $ vs == 1)+ c(-1:1):
个较长的对象长度不是较短对象个数的倍数
这似乎是一个简单的问题,但并不像想像的那么简单。
问题在于 which(mtcars $ vs == 1)
返回向量而不是单个值:
[1] 3 4 6 8 9 10 11 18 19 20 21 26 28 32
如果另一个向量 -1:1
(即 c(-1L,0L ,1L)
)添加到其中,适用于对不等长向量进行操作的常规R规则:回收规则说
任何短向量操作数都通过循环其值来扩展,直到
与任何其他操作数的大小匹配为止。
因此,较短的向量 -1:1
将被回收到的长度,其中(mtcars $ vs == 1 )
,即
rep(-1:1,length.out = length(which( mtcars $ vs == 1)))
[1] -1 0 1 -1 0 1 -1 0 1 -1 0 1 -1 0
因此,
的结果(mtcars $ vs == 1 )+ -1:1
是元素的明智之和两个向量的元素,其中较短的向量已被回收以匹配较长的向量的长度。
[1] 2 4 7 7 9 11 10 18 20 19 21 27 27 32
其中
此外,我们得到了
警告消息:
其中(mtcars $ vs == 1)+ -1:1:
较长的对象长度不是较短的对象长度的倍数
因为其中(mtcars $ vs == 1)
的长度为14而 -1:1
的长度为3。
使用 outer()
的解决方案
为了选择每个匹配行上方和下方的 N
行,我们需要将 -N:N
添加到每个由 which(mtcars $ vs == 1)返回的行号
:
outer(which(mtcars $ vs == 1),-1:1,`+` )
[,1] [,2] [,3]
[1,] 2 3 4
[2,] 3 4 5
[3, ] 5 6 7
[4,] 7 8 9
[5,] 8 9 10
[6,] 9 10 11
[7,] 10 11 12
[8,] 17 18 19
[9,] 18 19 20
[10,] 19 20 21
[11,] 20 21 22
[12,] 25 26 27
[13,] 27 28 29
[14,] 31 32 33
现在,我们有了一个包含所有行号的数组。不幸的是,它不能直接用于子集,因为它包含重复项并且 mtcars
中不存在行号。因此,结果必须先进行后处理,然后才能用于子集。
library(magrittr)#为清楚起见使用
rn<-external(which(mtcars $ vs == 1),-1:1,`+`)%>%
as.vector()%&%;%
unique()%>%
Filter(function(x)x [1< = x& x< = nrow(mtcars)],。)
rn
[1] 2 3 4 5 6 7 8 9 10 11 12 17 18 19 20 21 22 25 26 27 28 29 31 32
mtcars [rn,]
<块报价>
mpg cyl disp hp drat wt qsec vs am gear碳水化合物
马自达RX4 Wag 21.0 6 160.0 110 3.90 2.875 17.02 0 1 4 4
Datsun 710 22.8 4 108.0 93 3.85 2.320 18.61 1 1 4 1
Hornet 4 Drive 21.4 6 258.0 110 3.08 3.215 19.44 1 0 3 1
Hornet Sportabout 18.7 8 360.0 175 3.15 3.440 17.02 0 0 3 2
英勇的18.1 6 225.0 105 2.76 3.460 20.22 1 0 3 1
ster子360 14.3 8 360.0 245 3.21 3.570 15.84 0 0 3 4
Merc 240D 24.4 4 146.7 62 3.69 3.190 20.00 1 0 4 2
Merc 230 22.8 4 140.8 95 3.92 3.150 22.90 1 0 4 2
Merc 280 19.2 6 167.6 123 3.92 3.440 18.30 1 0 4 4
Merc 280C 17.8 6 167.6 123 3.92 3.440 18.90 1 0 4 4
奔驰450SE 16.4 8 275.8 180 3.07 4.070 17.40 0 0 3 3
克莱斯勒帝国14.7 8 440.0 230 3.23 5.345 17.42 0 0 3 4
菲亚特128 32.4 4 78.7 66 4.08 2.200 19.47 1 1 4 1
本田思域30.4 4 75.7 52 4.93 1.615 18.52 1 1 4 2
丰田卡罗拉33.9 4 71.1 65 4.22 1.835 19.90 1 1 4 1
丰田Corona 21.5 4 120.1 97 3.70 2.465 20.01 1 0 3 1
道奇挑战者15.5 8 318.0 150 2.76 3.520 16.87 0 0 3 2
庞蒂亚克火鸟19.2 8 400.0 175 3.08 3.845 17.05 0 0 3 2
菲亚特X1-9 27.3 4 79.0 66 4.08 1.935 18.90 1 1 4 1
保时捷914-2 26.0 4 120.3 91 4.43 2.140 16.70 0 1 5 2
莲花欧罗巴30.4 4 95.1 113 3.77 1.513 16.90 1 1 5 2
福特Pantera L 15.8 8 351.0 264 4.22 3.170 14.50 0 1 5 4
玛莎拉蒂宝来15.0 8 301.0 335 3.54 3.570 14.60 0 1 5 8
沃尔沃142E 21.4 4 121.0 109 4.11 2.780 18.60 1 1 4 2
I would like to select N rows above and below a match.
I'm trying the command:
mtcars[which(mtcars$vs == 1) + c(-1:1), ]
It returns the follow warning:
Warning message: In which(mtcars$vs == 1) + c(-1:1): longer object length is not a multiple of shorter object length
This seems to be a simple question but is not as trivial as presumably expected.
The issue is that which(mtcars$vs == 1)
returns a vector rather than a single value:
[1] 3 4 6 8 9 10 11 18 19 20 21 26 28 32
If another vector -1:1
(which is c(-1L, 0L, 1L)
) is added to it, the normal R rules for operations on vectors of unequal lengths apply: The recycling rule says
Any short vector operands are extended by recycling their values until they match the size of any other operands.
Therefore the shorter vector -1:1
will be recycled to the length of which(mtcars$vs == 1)
, i.e.,
rep(-1:1, length.out = length(which(mtcars$vs == 1)))
[1] -1 0 1 -1 0 1 -1 0 1 -1 0 1 -1 0
Therefore, the result of
which(mtcars$vs == 1) + -1:1
is the element-wise sum of the elements of both vectors where the shorter vector has been recycled to match the length of the longer vector.
[1] 2 4 7 7 9 11 10 18 20 19 21 27 27 32
which is propably not what the OP has expected.
In addition, we get the
Warning message:
In which(mtcars$vs == 1) + -1:1 :
longer object length is not a multiple of shorter object length
because which(mtcars$vs == 1)
has length 14 and -1:1
has length 3.
Solution using outer()
In order to select the N
rows above and below each matching row, we need to add -N:N
to each row number returned by which(mtcars$vs == 1)
:
outer(which(mtcars$vs == 1), -1:1, `+`)
[,1] [,2] [,3]
[1,] 2 3 4
[2,] 3 4 5
[3,] 5 6 7
[4,] 7 8 9
[5,] 8 9 10
[6,] 9 10 11
[7,] 10 11 12
[8,] 17 18 19
[9,] 18 19 20
[10,] 19 20 21
[11,] 20 21 22
[12,] 25 26 27
[13,] 27 28 29
[14,] 31 32 33
Now, we have an array of all row numbers. Unfortunately, it cannot be used directly for subsetting because it contains duplicates and there are row numbers which do not exist in mtcars
. So the the result has to be "post-processed" before it can be used for subsetting.
library(magrittr) # piping used for clarity
rn <- outer(which(mtcars$vs == 1), -1:1, `+`) %>%
as.vector() %>%
unique() %>%
Filter(function(x) x[1 <= x & x <= nrow(mtcars)], .)
rn
[1] 2 3 4 5 6 7 8 9 10 11 12 17 18 19 20 21 22 25 26 27 28 29 31 32
mtcars[rn, ]
mpg cyl disp hp drat wt qsec vs am gear carb Mazda RX4 Wag 21.0 6 160.0 110 3.90 2.875 17.02 0 1 4 4 Datsun 710 22.8 4 108.0 93 3.85 2.320 18.61 1 1 4 1 Hornet 4 Drive 21.4 6 258.0 110 3.08 3.215 19.44 1 0 3 1 Hornet Sportabout 18.7 8 360.0 175 3.15 3.440 17.02 0 0 3 2 Valiant 18.1 6 225.0 105 2.76 3.460 20.22 1 0 3 1 Duster 360 14.3 8 360.0 245 3.21 3.570 15.84 0 0 3 4 Merc 240D 24.4 4 146.7 62 3.69 3.190 20.00 1 0 4 2 Merc 230 22.8 4 140.8 95 3.92 3.150 22.90 1 0 4 2 Merc 280 19.2 6 167.6 123 3.92 3.440 18.30 1 0 4 4 Merc 280C 17.8 6 167.6 123 3.92 3.440 18.90 1 0 4 4 Merc 450SE 16.4 8 275.8 180 3.07 4.070 17.40 0 0 3 3 Chrysler Imperial 14.7 8 440.0 230 3.23 5.345 17.42 0 0 3 4 Fiat 128 32.4 4 78.7 66 4.08 2.200 19.47 1 1 4 1 Honda Civic 30.4 4 75.7 52 4.93 1.615 18.52 1 1 4 2 Toyota Corolla 33.9 4 71.1 65 4.22 1.835 19.90 1 1 4 1 Toyota Corona 21.5 4 120.1 97 3.70 2.465 20.01 1 0 3 1 Dodge Challenger 15.5 8 318.0 150 2.76 3.520 16.87 0 0 3 2 Pontiac Firebird 19.2 8 400.0 175 3.08 3.845 17.05 0 0 3 2 Fiat X1-9 27.3 4 79.0 66 4.08 1.935 18.90 1 1 4 1 Porsche 914-2 26.0 4 120.3 91 4.43 2.140 16.70 0 1 5 2 Lotus Europa 30.4 4 95.1 113 3.77 1.513 16.90 1 1 5 2 Ford Pantera L 15.8 8 351.0 264 4.22 3.170 14.50 0 1 5 4 Maserati Bora 15.0 8 301.0 335 3.54 3.570 14.60 0 1 5 8 Volvo 142E 21.4 4 121.0 109 4.11 2.780 18.60 1 1 4 2
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