[英]compute all pairwise differences within a vector in R
There are several posts on computing pairwise differences among vectors, but I cannot find how to compute all differences within a vector. 关于矢量之间的计算成对差异有几个帖子,但我找不到如何计算矢量中的所有差异。
Say I have a vector, v. 说我有一个矢量,v。
v<-c(1:4)
I would like to generate a second vector that is the absolute value of all pairwise differences within the vector. 我想生成第二个向量,它是向量中所有成对差异的绝对值。 Similar to:
相近:
abs(1-2) = 1
abs(1-3) = 2
abs(1-4) = 3
abs(2-3) = 1
abs(2-4) = 2
abs(3-4) = 1
The output would be a vector of 6 values, which are the result of my 6 comparisons: 输出将是6个值的向量,这是我的6次比较的结果:
output<- c(1,2,3,1,2,1)
Is there a function in R that can do this? R中有一个可以做到这一点的功能吗?
as.numeric(dist(v))
seems to work; 似乎工作; it treats
v
as a column matrix and computes the Euclidean distance between rows, which in this case is sqrt((xy)^2)=abs(xy)
它将
v
视为列矩阵并计算行之间的欧几里德距离,在本例中为sqrt((xy)^2)=abs(xy)
If we're golfing, then I'll offer c(dist(v))
, which is equivalent and which I'm guessing will be unbeatable. 如果我们正在打高尔夫球,那么我将提供
c(dist(v))
,这是相同的,我猜测它将是无与伦比的。
@AndreyShabalin makes the good point that using method="manhattan"
will probably be slightly more efficient since it avoids the squaring/square-rooting stuff. @AndreyShabalin指出,使用
method="manhattan"
可能会稍微提高效率,因为它避免了平方/平方根的东西。
Let's play golf 我们打高尔夫球吧
abs(apply(combn(1:4,2), 2, diff))
@Ben, yours is a killer! @Ben,你的杀手!
> system.time(apply(combn(1:1000,2), 2, diff))
user system elapsed
6.65 0.00 6.67
> system.time(c(dist(1:1000)))
user system elapsed
0.02 0.00 0.01
> system.time({
+ v <- 1:1000
+ z = outer(v,v,'-');
+ z[lower.tri(z)];
+ })
user system elapsed
0.03 0.00 0.03
Who knew that elegant (read understandable/flexible) code can be so slow. 谁知道优雅(易读易懂/灵活)的代码可能会很慢。
A possible solution is: 可能的解决方案是:
z = outer(v,v,'-');
z[lower.tri(z)];
[1] 1 2 3 1 2 1
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