如何将数组的数组转换为矩阵? [英] How to convert an array of array into a matrix?

问题描述

假设我想将向量值函数phi应用于向量x:

Suppose I want to apply a vector-valued function phi to a vector x:

phi(x, d) = [x.^i for i=0:d]    # vector-valued function
x = rand(7)                     # vector
y = phi(x, 3)                   # should be matrix, but isn't

现在y应该是矩阵，但它是4-element Array{Array{Float64,1},1}，即数组数组.实际上，我希望y是一个矩阵. phi的实现是否错误?或如何转换?

Now y should be a matrix, but it is an 4-element Array{Array{Float64,1},1}, i.e. an array of arrays. Actually, I want y to be a matrix. Is the implementation of phi wrong? Or how do I convert it?

谢谢！

推荐答案

如前所述，您可以使用hcat(x...)连接数组x的数组，但是通常最好创建一个以其开头的矩阵.在这种情况下，您可以通过两种方式进行操作:

As you noted, you can concatenate an array of arrays x using hcat(x...), but it's usually better to create a matrix to begin with instead. Two ways that you can do it in this case:

1. 使用广播:

1. Using broadcasting:

phi(x, d) = x.^((0:d)')

只要x是向量，它将针对行矩阵(0:d)'进行广播.

As long as x is a vector, it will broadcast against the row matrix (0:d)'.

您可以通过转置x而不是范围0:d来获得转置结果.

You can get the transpose result by transposing x instead of the range 0:d.

使用二维数组理解:

phi(x, d) = [xi.^di for xi in x, di in 0:d]

只要x是可迭代的，它将起作用.如果x是n-d数组，则将其解释为好像先被展平了.

This will work as long as x is iterable. If x is an n-d array, it will be interpreted as if it were flattened first.

您可以通过切换理解变量的顺序来转置结果:

You can transpose the result by switching the order of the comprehension variables:

phi(x, d) = [xi.^di for di in 0:d, xi in x]

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