如何将变量分配给numpy矩阵

Question

I want to make a function to generate rotational matrix R. Here is my code:

import numpy as np
def R(theta):
    a = np.cos(theta)
    b = -1*np.sin(theta)
    c = np.sin(theta)
    d = np.cos(theta)
    return np.matrix('a b; c d')

But it has error like this

raise TypeError("Invalid data string supplied: " + astr)
TypeError: Invalid data string supplied: a

Any suggestions?

Answer 1

使用.format将变量放入字符串形式

return np.matrix('{} {};{} {}'.format(a,b,c,d))

Answer 2

That matrix notation:

a = np.matrix('1 2; 3 4')

only works for literals, not variables; with scalar variables you'd use the bracket notation

np.matrix([[1, 2], [3, 4]])
np.matrix([[a, b], [c, d]])

This part, [[a, b], [c, d]] is an ordinary Python list (of lists). np.matrix(...) then takes this list and turns it into a numpy matrix.

But really, you should be using

np.array(...)

np.matrix was written to make things familiar to MATLAB visitors. Most numpy work is the array , not matrix .

Another point - a = np.cos(theta) works with theta is scalar or an array. But if an array, than a itself will be an array, not a scalar. You should be aware of that when constructing this rotation matrix.