Python 如何将两个向量相乘并得到一个矩阵？
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How to multiply two vector and get a matrix?
提问by Larry
In numpy operation, I have two vectors, let's say vector A is 4X1, vector B is 1X5, if I do AXB, it should result a matrix of size 4X5.
在 numpy 操作中，我有两个向量，假设向量 A 是 4X1，向量 B 是 1X5，如果我做 AXB，它应该产生一个大小为 4X5 的矩阵。
But I tried lot of times, doing many kinds of reshape and transpose, they all either raise error saying not aligned or return a single value.
但是我尝试了很多次，进行了多种重塑和转置，它们要么引发错误，提示未对齐要么返回单个值。
How should I get the output product of matrix I want?
我应该如何获得我想要的矩阵的输出产品？
采纳答案by Dietrich Epp
Normal matrix multiplication works as long as the vectors have the right shape. Remember that *in Numpy is elementwise multiplication, and matrix multiplication is available with numpy.dot()(or with the @operator, in Python 3.5)
只要向量具有正确的形状，普通矩阵乘法就可以工作。请记住，*在 Numpy 中是elementwise multiplication，并且矩阵乘法可用numpy.dot()（或使用@运算符，在 Python 3.5 中）
>>> numpy.dot(numpy.array([[1], [2]]), numpy.array([[3, 4]]))
array([[3, 4],
[6, 8]])
This is called an "outer product." You can get it using plain vectors using numpy.outer():
这被称为“外积”。您可以使用普通向量获得它numpy.outer()：
>>> numpy.outer(numpy.array([1, 2]), numpy.array([3, 4]))
array([[3, 4],
[6, 8]])
回答by max yi
If you are using numpy.
如果您使用 numpy.
First, make sure you have two vectors. For example, vec1.shape = (10, )and vec2.shape = (26, ); in numpy, row vector and column vector are the same thing.
首先，确保你有两个向量。例如，vec1.shape = (10, )和vec2.shape = (26, )；在 numpy 中，行向量和列向量是一回事。
Second, you do res_matrix = vec1.reshape(10, 1) @ vec2.reshape(1, 26) ;.
其次，你做res_matrix = vec1.reshape(10, 1) @ vec2.reshape(1, 26) ;。
Finally, you should have: res_matrix.shape = (10, 26).
最后，你应该有：res_matrix.shape = (10, 26)。
numpy documentation says it will deprecate np.matrix(), so better not use it.
numpy 文档说它会弃用np.matrix()，所以最好不要使用它。
回答by Serenity
Function matmul(since numpy 1.10.1) works fine:
函数matmul（自 numpy 1.10.1 起）工作正常：
import numpy as np
a = np.array([[1],[2],[3],[4]])
b = np.array([[1,1,1,1,1],])
ab = np.matmul(a, b)
print (ab)
print(ab.shape)
You have to declare your vectors right. The first has to be a list of lists of one number (this vector has to have columns in one row), and the second - a list of list (this vector has to have rows in one column) like in above example.
你必须正确声明你的向量。第一个必须是一个数字列表的列表（这个向量必须在一行中有列），第二个 - 一个列表列表（这个向量必须在一个列中有行），就像上面的例子一样。
Output:
输出：
[[1 1 1 1 1]
[2 2 2 2 2]
[3 3 3 3 3]
[4 4 4 4 4]]
(4, 5)
