R -如何通过指标向量得到多维数组的值

Let's say I have a multi-dimensional array called pi, and its number of dimensions isn't known until the runtime:

假设我有一个多维数组叫pi，它的维数直到运行时才知道:

dims <- rep(3, dim_count)
pi <- array(0, dims)

As you can see the dimension count depends on dim_count. How do I retrieve a value from the array when I have a vector of the indexes? For example when I have:

可以看到，维度计数取决于dim_count。当我有一个索引向量时，如何从数组中检索值?例如当我有:

dim_count <- 5
indexes <- c(1, 2, 3, 3, 3)

I want to retrieve

我想检索

pi[1, 2, 3, 3, 3]

Is there a short, effective and hopefully elegant way of doing this?

是否有一种简短、有效、有希望的优雅的方法来实现这一目标?

3 个解决方案

#1

Making use of a little known usage of [:

利用一些已知的用法[:

When indexing arrays by [ a single argument i can be a matrix with as many columns as there are dimensions of x; the result is then a vector with elements corresponding to the sets of indices in each row of i.

当通过[单个参数对数组进行索引时，我可以是一个矩阵，其列数与x的维数相等;结果是一个向量，其元素对应于i的每一行中的索引集。

you can simply do:

你可以做的:

pi[matrix(indexes, 1)]

#2

do.call("[",...) seems to work.

do.call(“(”,…)似乎工作。

indexes <- c(1,2,3,3,3)
pi[1,2,3,3,3] <- 17  ## so we know if we succeeded or not
do.call("[",c(list(pi),as.list(indexes)))

Note that your example wouldn't work -- your dimensions were all 3, but some of your index elements were >3 ...

注意，您的示例不起作用——您的维度都是3，但是您的一些索引元素是>3……

#3

do.call() is an option:

do.call()是一个选项:

dim_count <- 5
indexes <- c(1, 2, 2, 2, 3)
dims <- rep(3, dim_count)
pi <- array(seq_len(prod(dims)), dims)

do.call(`[`, c(list(x = pi), as.list(indexes)))

Which gives:

这使:

> do.call(`[`, c(list(x = pi), as.list(indexes)))
[1] 202
> pi[1, 2, 2, 2, 3]
[1] 202

The tricky bit is getting the list of arguments in the right format. pi should be the first argument to "[" (or named as argument x, see ?"["), whilst we want each element of indexes itself to be a component of the supplied list, not a vector within that list. Hence the convoluted c(list(x = pi), as.list(indexes)).

棘手的一点是要以正确的格式获取参数列表。pi应该是第一个到"["(或命名为参数x，参见?"[])的参数，同时我们希望索引本身的每个元素都是所提供列表的组件，而不是列表中的向量。因此出现了卷积c(list(x = pi)， as.list(indexes))。

An alternative way to construct the argument list which might be easier to follow is:

另一种构建参数列表的方法是:

ARGS <- vector("list", length = dim_count + 1)
ARGS[[1]] <- pi
ARGS[2:length(ARGS)] <- indexes
do.call("[", ARGS)

which gives

这给了

> do.call("[", ARGS)
[1] 202
> pi[1, 2, 2, 2, 3]
[1] 202

#1