r - R 中 3D 数组的方差

Question

处理 3d 数组:

ar3d <- array(floor(100 * runif(100 * 20 * 30)), dim = c(100, 20, 30))

我很困惑为什么

length(apply(ar3d, MARGIN = 1, FUN = "sd"))
# [1] 100

同时

dim(apply(ar3d, MARGIN = 1, FUN = "var"))
# [1] 900 100

如何尽快计算 20 * 30 值之间的方差？

非常感谢

Answer 1

对于 var，如果您想获得标量，则应使用 var(c(x)) 而不是 var(x)相反，协方差矩阵。

当您输入 ?var 时，您会看到

var, cov and cor compute the variance of x and the covariance or correlation of x and y if these are vectors. If x and y are matrices then the covariances (or correlations) between the columns of x and the columns of y are computed.

你可以尝试一下

> ar3d <- array(floor(100 * runif(100 * 20 * 30)), dim = c(100, 20, 30))

> length(apply(ar3d, 1, \(x) var(c(x))))
[1] 100

> length(apply(ar3d, 1, var))
[1] 90000

> length(apply(ar3d, 1, sd))
[1] 100