Python 中的二维数组切片和替换值

Question

I have a 2D array, and its size is based on a variable. I would like to change the values of specific rows and columns based on an equation.

For example:

import numpy as np

size = 6
weight = np.ones([size, size])
weight[size / 6 * 2:size / 6 * 4 + 1, :] = 5

print(weight)

But I got an error:

TypeError: slice indices must be integers or None or have an __index__ method

I think size / 6 * 2 and size / 6 * 4 are integers.

How can I fix it?

And if I have to set size / 6 equal to float , for example, 7 / 6 , how can I write a code to make 7 / 6 into the closest integer to avoid the error shown above?

Answer 1

Python supports two types of division natively. Floor division is performed with // , and always results in an int . True division is performed with / and always results in a float .

You can check with the following:

>>> size = 6
>>> type(size / 6)
float
>>> type(size // 6)
int
>>> size / 6 == size // 6
True

So the simplest way is to write:

weight[size // 6 * 2:size // 6 * 4 + 1, :] = 5

Alternatively, you can make the cast explicit, although this is less efficient:

weight[int(size / 6) * 2:int(size / 6) * 4 + 1, :] = 5

By the way, if you want to do a ceiling divide, just add (6 - 1) = 5 and do floor divide:

weight[(size + 5) // 6 * 2:(size + 5) // 6 * 4 + 1, :] = 5

If you want to round, add 6 / 2 = 3.0 and do floor divide:

weight[(size + 3) // 6 * 2:(size + 3) // 6 * 4 + 1, :] = 5

And of course you can round explicitly:

weight[round(size / 6 * 2):round(size / 6 * 4) + 1, :] = 5

All of these options will work for values of size that are non-integer multiples of 6.