python - SciPy 中 10j 的值是多少？

Question

我正在学习 Python 和 SciPy。我遇到了以下两个表达式:

a = np.concatenate(([3], [0]*5, np.arange(-1, 1.002, 2/9.0)))

和

b = np.r_[3,[0]*5,-1:1:10j]

两个表达式输出相同的数组。我不明白第二个表达式中的 10j。它的值(value)是多少？非常感谢您的帮助。

Answer 1

这是创建 np.linspace 的简写.

根据 docs for np.r_ :

If slice notation is used, the syntax start:stop:step is equivalent to np.arange(start, stop, step) inside of the brackets. However, if step is an imaginary number (i.e. 100j) then its integer portion is interpreted as a number-of-points desired and the start and stop are inclusive. In other words start:stop:stepj is interpreted as np.linspace(start, stop, step, endpoint=1) inside of the brackets.

因此，对于您的特定情况，-1:1:10j 将导致步长为 (1 - (-1))/9 = 0.222222... 从而给出以下数组:

>>> np.r_[-1:1:10j]
array([-1.        , -0.77777778, -0.55555556, -0.33333333, -0.11111111,
        0.11111111,  0.33333333,  0.55555556,  0.77777778,  1.        ])

虽然这碰巧给您的答案与np.arange(-1, 1.002, 2/9.0)相同，但请注意arange 通常不是创建这样一个数组的好方法，因为在aranges 中使用非整数步长是bad idea :

When using a non-integer step, such as 0.1, the results will often not be consistent. It is better to use linspace for these cases.