在r续中使用上一个计算的行值 [英] Use previous calculated row value in r Continued
问题描述
我有一个data.table如下:
DT< - data.table(A = 1: 20,B = 1:20 * 10,C = 1:20 * 100)
DT
ABC
1:1 10 100
2:2 20 200
3:3 30 300
4:4 40 400
5:5 50 500
...
20:20 200 2000
我想要计算一个新列G,其第一个值为B列中前20行的平均值值,然后我想使用列G的第一行来帮助计算G的下一行值。
说出列B的前20行的平均值为105,G中下一行的公式为:DT $ G [2] = DT $ G [1] * 2,下一行为DT $ G [3] = DT $ G [2] 2。这意味着第一个值不应再次用于下一行,依此类推。
ABCG
1:1 10 100 105
2:2 20 200 210
3 :3 30 300 420
4:4 40 400 840
5:5 50 500 1680
...
20:20 200 2000 55050240
有任何想法吗?
解决方案您可以用一个小算术来做到这一点:
DT $ G < - 平均值:20])
DT $ G < - DT $ G * cumprod(rep(2,nrow(DT)))/ 2
或使用
data.table
语法,由@DavidArenburg提供:DT [,G:= mean(B [1:20])* cumprod(rep(2,.N))/ 2]
或从@Frank
DT $ G ; - cumprod(c(mean(head(DT $ B,20)),rep(2,nrow(DT)-1)))
I have a data.table that looks like this:
DT <- data.table(A=1:20, B=1:20*10, C=1:20*100) DT A B C 1: 1 10 100 2: 2 20 200 3: 3 30 300 4: 4 40 400 5: 5 50 500 ... 20: 20 200 2000
I want to be able to calculate a new column "G" that has the first value as the average of the first 20 rows in column B as the first value, and then I want to use the first row of column G to help calculate the next row value of G.
Say the Average of the first 20 rows of column B is 105, and the formula for the next row in G is: DT$G[2] = DT$G[1]*2, and the next row again is DT$G[3]=DT$G[2]*2. This means that the first value should not be used again in the next row and so forth.
A B C G 1: 1 10 100 105 2: 2 20 200 210 3: 3 30 300 420 4: 4 40 400 840 5: 5 50 500 1680 ... 20: 20 200 2000 55050240
Any ideas on this would be made?
解决方案You can do this with a little arithmetic:
DT$G <- mean(DT$B[1:20]) DT$G <- DT$G * cumprod(rep(2,nrow(DT)))/2
Or using
data.table
syntax, courtesy of @DavidArenburg:DT[ , G := mean(B[1:20]) * cumprod(rep(2, .N)) / 2]
or from @Frank
DT$G <- cumprod(c( mean(head(DT$B,20)), rep(2,nrow(DT)-1) ))
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